0000875045-21-000010 | 10-K | XBRL Viewer

Product Revenues

Revenues by product are summarized as follows:


	For the Years Ended December 31,
	2020						2019						2018
(In millions)	United States		Rest of World		Total		United States		Rest of World		Total		United States		Rest of World		Total
Multiple Sclerosis (MS):
Fumarate*	$	2,742.0	$	1,163.4	$	3,905.4	$	3,312.0	$	1,126.2	$	4,438.2	$	3,253.2	$	1,020.9	$	4,274.1
Interferon**	1,273.5		604.0		1,877.5		1,426.6		675.2		2,101.8		1,668.3		694.7		2,363.0
TYSABRI	1,096.8		849.3		1,946.1		1,041.8		850.4		1,892.2		1,025.0		839.0		1,864.0
FAMPYRA	—		103.1		103.1		—		97.1		97.1		—		92.7		92.7
ZINBRYTA	—		—		—		—		—		—		—		1.4		1.4
Subtotal: MS	5,112.3		2,719.8		7,832.1		5,780.4		2,748.9		8,529.3		5,946.5		2,648.7		8,595.2

Spinal Muscular Atrophy:
SPINRAZA	787.8		1,264.3		2,052.1		933.4		1,163.6		2,097.0		854.0		870.2		1,724.2

Biosimilars:
BENEPALI	—		481.6		481.6		—		486.2		486.2		—		485.2		485.2
IMRALDI	—		216.3		216.3		—		184.0		184.0		—		16.7		16.7
FLIXABI	—		97.9		97.9		—		68.1		68.1		—		43.2		43.2
Subtotal: Biosimilars	—		795.8		795.8		—		738.3		738.3		—		545.1		545.1

Other:
FUMADERM	—		12.2		12.2		—		15.2		15.2		—		22.3		22.3

Total product revenues	$	5,900.1	$	4,792.1	$	10,692.2	$	6,713.8	$	4,666.0	$	11,379.8	$	6,800.5	$	4,086.3	$	10,886.8

*Fumarate includes TECFIDERA and VUMERITY. VUMERITY became commercially available in the U.S. in November 2019.

**Interferon includes AVONEX and PLEGRIDY.

We recognized revenues from two wholesalers accounting for 30.5% and 15.3% of gross product revenues in 2020, 30.0% and 17.2% of gross product revenues in 2019 and 32.0% and 18.4% of gross product revenues in 2018, respectively.

As of December 31, 2020, two wholesale distributors individually accounted for approximately 21.1% and 8.5% of net accounts receivable associated with our product sales, as compared to 24.1% and 13.9% as of December 31, 2019, respectively.

An analysis of the change in reserves for discounts and allowances is summarized as follows:


	December 31, 2020
(In millions)	Discounts		Contractual Adjustments		Returns		Total
Beginning balance	$	131.1	$	1,027.3	$	40.5	$	1,198.9
Current provisions relating to sales in current year	774.7		3,308.8		19.0		4,102.5
Adjustments relating to prior years	(1.0)		(54.0)		1.3		(53.7)
Payments/returns relating to sales in current year	(635.1)		(2,426.1)		—		(3,061.2)
Payments/returns relating to sales in prior years	(128.3)		(763.0)		(19.2)		(910.5)
Ending balance	$	141.4	$	1,093.0	$	41.6	$	1,276.0


	December 31, 2019
(In millions)	Discounts		Contractual Adjustments		Returns		Total
Beginning balance	$	127.8	$	888.8	$	34.7	$	1,051.3
Current provisions relating to sales in current year	666.2		3,011.5		20.9		3,698.6
Adjustments relating to prior years	0.3		(54.1)		5.5		(48.3)
Payments/returns relating to sales in current year	(535.5)		(2,242.9)		(0.2)		(2,778.6)
Payments/returns relating to sales in prior years	(127.7)		(576.0)		(20.4)		(724.1)
Ending balance	$	131.1	$	1,027.3	$	40.5	$	1,198.9


	December 31, 2018
(In millions)	Discounts		Contractual Adjustments		Returns		Total
Beginning balance	$	109.6	$	606.0	$	46.0	$	761.6
Current provisions relating to sales in current year	679.3		2,686.7		23.1		3,389.1
Adjustments relating to prior years	(0.3)		(10.0)		(1.8)		(12.1)
Payments/returns relating to sales in current year	(551.7)		(1,887.6)		(1.1)		(2,440.4)
Payments/returns relating to sales in prior years	(109.1)		(506.3)		(31.5)		(646.9)
Ending balance	$	127.8	$	888.8	$	34.7	$	1,051.3

The total reserves above, which are included in our consolidated balance sheets, are summarized as follows:


	As of December 31,
(In millions)	2020		2019
Reduction of accounts receivable	$	195.4	$	197.8
Component of accrued expenses and other	1,080.6		1,001.1
Total revenue-related reserves	$	1,276.0	$	1,198.9

Revenues from Anti-CD20 Therapeutic Programs

Revenues from anti-CD20 therapeutic programs are summarized in the table below. For purposes of this footnote, we refer to RITUXAN and RITUXAN HYCELA collectively as RITUXAN.


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	$	1,080.2	$	1,542.4	$	1,431.9
Other revenues from anti-CD20 therapeutic programs	897.6		748.0		548.3
Total revenues from anti-CD20 therapeutic programs	$	1,977.8	$	2,290.4	$	1,980.2

Approximately 14.7%, 15.9% and 14.7% of our total revenues in 2020, 2019 and 2018, respectively, were derived from our collaboration arrangements with Genentech. For additional information on our collaboration arrangements with Genentech, please read Note 18, Collaborative and Other Relationships, to these consolidated financial statements.

Other Revenues

Other revenues are summarized as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Revenues from collaborative and other relationships:
Revenues earned under our technical development agreement, manufacturing service agreements and royalty revenues on biosimilar products with Samsung Bioepis	$	20.9	$	106.2	$	96.4
Other revenues from collaborative and other relationships	0.7		—		(8.6)
Other royalty and corporate revenues:
Royalty	33.9		17.0		38.7
Other corporate	719.1		584.5		459.4
Total other revenues	$	774.6	$	707.7	$	585.9

Other corporate revenues primarily reflect amounts earned under contract manufacturing agreements with our strategic customers, including Bioverativ Inc. (Bioverativ). During the years ended December 31, 2020, 2019 and 2018, we recognized $48.6 million, $383.2 million and $206.7 million, respectively, in revenues under the manufacturing and supply agreement with Bioverativ entered into in connection with the spin-off of our hemophilia business.

During the third quarter of 2019, we amended our agreement with a contract manufacturing customer pursuant to which we licensed certain of our manufacturing-related intellectual property to the customer. In the second quarter of 2020, the customer received regulatory approval for its product that is being manufactured using certain of our manufacturing-related intellectual property. As a result we are entitled to $500.0 million in a series of three payments. The first payment became due upon a regulatory approval of such product and was received during the second quarter of 2020. Subsequent payments are due on the first and second anniversaries of the regulatory approval.

Other corporate revenues for the year ended December 31, 2020, reflect $346.2 million related to the delivery of the license for certain of our manufacturing-related intellectual property under the amended agreement discussed above and the performance of manufacturing product supply services for such customer. We have allocated the remaining $153.8 million of the $500.0 million transaction price to the performance of manufacturing product supply services for the customer, which we expect to perform through 2026. The value allocated to the manufacturing services was based on expected demand for supply and the fair value of comparable manufacturing and development services.

For additional information on our collaboration arrangements with Samsung Bioepis, please read Note 18, Collaborative and Other Relationships, to these consolidated financial statements.

The entire disclosure of revenue from contract with customer to transfer good or service and to transfer nonfinancial asset. Includes, but is not limited to, disaggregation of revenue, credit loss recognized from contract with customer, judgment and change in judgment related to contract with customer, and asset recognized from cost incurred to obtain or fulfill contract with customer. Excludes insurance and lease contracts.

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Goodwill and Intangible Assets Disclosure [Abstract]

Inventory

12 Months Ended

Dec. 31, 2020

Inventory Disclosure [Abstract]

Inventory

The components of inventory are summarized as follows:


	As of December 31,
(In millions)	2020		2019
Raw materials	$	314.9	$	169.7
Work in process	544.5		460.0
Finished goods	209.2		174.5
Total inventory	$	1,068.6	$	804.2

Inventory amounts written down as a result of excess, obsolescence, unmarketability or other reasons are charged to cost of sales, and totaled $26.6 million, $52.2 million and $41.9 million for the years ended December 31, 2020, 2019 and 2018, respectively.

Divestiture of Hillerød, Denmark Manufacturing Operations

In addition, we sold to FUJIFILM approximately $41.8 million of raw materials that were remaining at the Hillerød facility on the closing date of this transaction. These materials were sold at cost, which approximates fair value.

For additional information on the divestiture of our Hillerød, Denmark manufacturing operations, please read Note 3, Divestitures, to these consolidated financial statements.

Intangible Assets and Goodwill

12 Months Ended

Dec. 31, 2020

Intangible Assets and Goodwill

Intangible Assets

Intangible assets, net of accumulated amortization, impairment charges and adjustments are summarized as follows:


		As of December 31, 2020						As of December 31, 2019
(In millions)	Estimated Life	Cost		Accumulated Amortization		Net		Cost		Accumulated Amortization		Net
Completed technology	4-28 years	$	7,394.3	$	(5,136.5)	$	2,257.8	$	7,379.3	$	(4,881.4)	$	2,497.9
In-process research and development	Indefinite until commercialization	762.5		—		762.5		965.5		—		965.5
Trademarks and trade names	Indefinite	64.0		—		64.0		64.0		—		64.0
Total intangible assets		$	8,220.8	$	(5,136.5)	$	3,084.3	$	8,408.8	$	(4,881.4)	$	3,527.4

Amortization and Impairments

Amortization and impairments of acquired intangible assets totaled $464.8 million, $489.9 million and $747.3 million for the years ended December 31, 2020, 2019 and 2018, respectively.

Amortization of acquired intangible assets, excluding impairment charges, totaled $255.1 million, $274.0 million and $381.2 million for the years ended December 31, 2020, 2019 and 2018, respectively. The decrease in amortization of acquired intangible assets, excluding impairment charges, over the three years was primarily due to a lower rate of amortization for acquired intangible assets.

For the year ended December 31, 2020, amortization and impairment of acquired intangible assets reflects the impact of a $115.0 million impairment charge related to BIIB111, which was obtained as part of our acquisition of NST, a $75.4 million impairment charge related to BIIB054 (cinpanemab) and a $19.3 million impairment charge related to one of our other IPR&D intangible assets.

For the year ended December 31, 2019, amortization and impairments of acquired intangible assets reflects the impact of a $215.9 million impairment charge related to certain IPR&D assets associated with the Phase 2b study of BG00011 (STX-100) for the potential treatment of idiopathic pulmonary fibrosis (IPF), which was discontinued in the third quarter of 2019.

For the year ended December 31, 2018, amortization and impairments of acquired intangible assets reflects the impact of a $189.3 million impairment charge related to certain IPR&D assets associated with our vixotrigine program, as discussed below, and a $176.8 million impairment charge related to our U.S. license to Forward Pharma A/S' (Forward Pharma) intellectual property, including Forward Pharma's intellectual property related to TECFIDERA.

Completed Technology

Completed technology primarily relates to our acquisition of all remaining rights to TYSABRI from Elan as well as other amounts related to our other marketed products and programs acquired through business combinations.

IPR&D Related to Business Combinations

IPR&D represents the fair value assigned to research and development assets that we acquired as part of a business combination and had not yet reached technological feasibility at the date of acquisition. Included in IPR&D balances are adjustments related to foreign currency exchange rate fluctuations. We review amounts capitalized as acquired IPR&D for impairment annually, as of October 31, and whenever events or changes in circumstances indicate to us that the carrying value of the assets might not be recoverable. The carrying value associated with our IPR&D assets as of December 31, 2020 and 2019, relates to the various IPR&D programs we acquired in connection with our acquisitions of NST and Convergence Pharmaceuticals Holdings Ltd. (Convergence). The majority of the balance relates to our acquisition of NST in June 2019 whereby we acquired IPR&D programs with an estimated fair value of approximately $585.0 million as of December 31, 2020. For additional information on our acquisition of NST, please read Note 2, Acquisitions, to these consolidated financial statements.

BIIB111

During the fourth quarter of 2020 we began experiencing third-party manufacturing delays that may impact our timeline for a potential filing of a Biologics License Application (BLA) for BIIB111 for regulatory approval by up to one year. In addition, we determined that forecasted costs associated with advancing the BIIB111 program through Phase 3 development and potential commercialization will exceed our original estimates. We reassessed the fair value of the program based on these changes in assumptions and determined that the program was partially impaired. We recognized an impairment charge of $115.0 million during the fourth quarter of 2020, which resulted in a reduction of the IPR&D asset from $480.0 million to $365.0 million.

BIIB054

In February 2021 we announced that we discontinued development of BIIB054 as a potential treatment of Parkinson's disease as our Phase 2 SPARK study did not meet its primary or secondary endpoints. Although we made this determination in February 2021, it was based on conditions that existed as of December 31, 2020. As a result, we recognized an impairment charge of approximately $75.4 million during the fourth quarter of 2020 to reduce the fair value of the related IPR&D intangible asset to zero.

The IPR&D impairment charges were included in amortization and impairment of acquired intangible assets and the gain resulting from the remeasurement of our contingent consideration obligation was recorded in (gain) loss on fair value remeasurement of contingent consideration in our consolidated statements of income. The fair value of the intangible assets and contingent consideration obligations were based on a probability-adjusted discounted cash flow calculation using Level 3 fair value measurements and inputs including estimated revenues, costs and probabilities of success.

Vixotrigine

In the periods since we acquired vixotrigine, there have been numerous delays in the initiation of Phase 3 studies for the potential treatment of TGN as we engaged with the U.S. Food and Drug Administration (FDA) regarding the design of the Phase 3 studies and awaited data and insights from mid-stage clinical trials of vixotrigine in other indications that have since been completed. The fair value of the TGN asset is not significantly in excess of carrying value. As of December 31, 2020, the carrying value associated with our vixotrigine IPR&D assets was $177.5 million.

Estimated Future Amortization of Intangible Assets

The estimated future amortization of finite-lived intangible assets for the next five years is expected to be as follows:


(In millions)	As of December 31, 2020
2021	$	205.0
2022	215.0
2023	215.0
2024	225.0
2025	220.0

Goodwill

The following table provides a roll forward of the changes in our goodwill balance:


	As of December 31,
(In millions)	2020		2019
Goodwill, beginning of year	$	5,757.8	$	5,706.4
Increase to goodwill	—		117.5
Elimination of goodwill allocated to Hillerød, Denmark manufacturing operations	—		(69.5)
Other	4.3		3.4
Goodwill, end of year	$	5,762.1	$	5,757.8

As of December 31, 2020, we had no accumulated impairment losses related to goodwill. Other includes adjustments related to foreign currency exchange rate fluctuations.

Fair Value Measurements

12 Months Ended

Dec. 31, 2020

Fair Value Disclosures [Abstract]

Fair Value Measurements

The tables below present information about our assets and liabilities that are regularly measured and carried at fair value and indicate the level within the fair value hierarchy of the valuation techniques we utilized to determine such fair value:


	As of December 31, 2020
(In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	626.9	$	—	$	626.9	$	—
Marketable debt securities:
Corporate debt securities	1,301.5		—		1,301.5		—
Government securities	627.1		—		627.1		—
Mortgage and other asset backed securities	122.4		—		122.4		—
Marketable equity securities	1,974.3		271.1		1,703.2		—
Derivative contracts	20.5		—		20.5		—
Plan assets for deferred compensation	28.2		—		28.2		—
Total	$	4,700.9	$	271.1	$	4,429.8	$	—
Liabilities:
Derivative contracts	$	217.2	$	—	$	217.2	$	—
Contingent consideration obligations	259.8		—		—		259.8
Total	$	477.0	$	—	$	217.2	$	259.8


	As of December 31, 2019
(In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	2,541.1	$	—	$	2,541.1	$	—
Marketable debt securities:
Corporate debt securities	1,695.1		—		1,695.1		—
Government securities	1,013.9		—		1,013.9		—
Mortgage and other asset backed securities	261.3		—		261.3		—
Marketable equity securities	337.5		7.9		329.6		—
Derivative contracts	43.8		—		43.8		—
Plan assets for deferred compensation	27.7		—		27.7		—
Total	$	5,920.4	$	7.9	$	5,912.5	$	—
Liabilities:
Derivative contracts	$	8.3	$	—	$	8.3	$	—
Contingent consideration obligations	346.1		—		—		346.1
Total	$	354.4	$	—	$	8.3	$	346.1

There have been no material impairments of our assets measured and carried at fair value during the years ended December 31, 2020 and 2019. In addition, there have been no changes in valuation techniques during the years ended December 31, 2020 and 2019. The fair value of Level 2 instruments classified as cash equivalents and marketable debt securities was determined through third-party pricing services. The fair value of Level 2 instruments classified as marketable equity securities represents our investments in Sangamo Therapeutics, Inc. (Sangamo) common stock, Denali Therapeutics Inc. (Denali) common stock and Sage Therapeutics, Inc. (Sage) common stock and are valued using an option pricing valuation model as the investments are each subject to certain holding period restrictions. For additional information on our investments in Sangamo, Denali and Sage common stock, please read Note 8, Financial Instruments, to these consolidated financial statements.

Our investments in marketable equity securities also include shares of Ionis Pharmaceuticals, Inc. (Ionis) common stock acquired in June 2018. Our shares of Ionis common stock were initially subject to certain holding period restrictions that have since expired. The fair value of this investment was a Level 1 measurement as of December 31, 2020. For additional information on our collaboration arrangements with Ionis, please read Note 18, Collaborative and Other Relationships, to these consolidated financial statements.

For a description of our validation procedures related to prices provided by third-party pricing services and our option pricing valuation model, please read Note 1, Summary of Significant Accounting Policies - Fair Value Measurements, to these consolidated financial statements.

The following table summarizes the significant unobservable inputs in the fair value measurement of our contingent consideration obligations as of December 31, 2020:

As of December 31, 2020

(In millions)

Fair Value

Valuation Technique

Unobservable Input

Range

Weighted Average

Liabilities:

Contingent consideration obligation

$259.8

Discounted cash flow

Discount rate

0.60%

Expected timing of achievement of development milestones

2021 to 2025

—

The weighted average discount rate was calculated based on the relative fair value of our contingent consideration obligations. In addition, we apply various probabilities of technological and regulatory success, ranging from 39.9% to certain probability, to the valuation models to estimate the fair values of our contingent consideration obligations.

Nonrecurring Fair Value Measurements

In addition to assets and liabilities that are recorded at fair value on a recurring basis, we record assets and liabilities at fair value on a nonrecurring basis as required by accounting principles generally accepted in the U.S. (U.S. GAAP). Generally, assets are recorded at fair value on a nonrecurring basis as a result of impairment charges.

The gains or losses on assets measured at fair value on a nonrecurring basis, are summarized as follows:


	As of December 31, 2020
(In millions)	Beginning Book Value		Impairment		Ending Book Value
BIIB111 intangible asset	$	480.0	$	(115.0)	$	365.0

For the year ended December 31, 2020, we recorded a partial impairment charge of $115.0 million related to BIIB111. For additional information, please read Note 6, Intangible Assets and Goodwill, to these consolidated financial statements.

Debt Instruments

The fair values of our debt instruments, which are Level 2 liabilities, are summarized as follows:


	As of December 31,
(In millions)	2020		2019
2.900% Senior Notes due September 15, 2020(1)	$	—	$	1,509.6
3.625% Senior Notes due September 15, 2022	1,054.1		1,038.9
4.050% Senior Notes due September 15, 2025	2,003.1		1,897.2
2.250% Senior Notes due May 1, 2030	1,557.2		—
5.200% Senior Notes due September 15, 2045	2,365.1		2,107.9
3.150% Senior Notes due May 1, 2050	1,536.4		—
Total	$	8,515.9	$	6,553.6

(1) Our 2.900% Senior Notes due September 15, 2020, were redeemed in full in May 2020 using the net proceeds from the issuance on April 30, 2020, of our senior unsecured notes for an aggregate principal amount of $3.0 billion. For additional information, please read Note 12, Indebtedness, to these consolidated financial statements.

The fair values of each of our series of Senior Notes were determined through market, observable and corroborated sources. For additional information related to our Senior Notes, please read Note 12, Indebtedness, to these consolidated financial statements.

Contingent Consideration Obligations

In connection with our acquisitions of Convergence and Biogen International Neuroscience GmbH (BIN), we agreed to make additional payments based upon the achievement of certain milestone events. The following table provides a roll forward of the fair values of our contingent consideration obligations, which includes Level 3 measurements:


	As of December 31,
(In millions)	2020		2019
Fair value, beginning of year	$	346.1	$	409.8
Changes in fair value	(86.3)		(63.7)
Payments and other	—		—
Fair value, end of year	$	259.8	$	346.1

As of December 31, 2020 and 2019, approximately $110.3 million and $197.7 million, respectively, of the fair value of our total contingent consideration obligations was reflected as a component of other long-term liabilities in our consolidated balance sheets with the remaining balance reflected as a component of accrued expenses and other.

For the year ended December 31, 2020, changes in the fair value of our contingent consideration obligations were primarily due to our discontinuing development of BIIB054 for the potential treatment of Parkinson's disease, resulting in a reduction of our contingent consideration obligations of $51.0 million as well as other changes in the

probability and the expected timing of the achievement of certain remaining developmental milestones, changes in the interest rates used to revalue our contingent consideration liabilities and the passage of time.

For the year ended December 31, 2019, changes in the fair value of our contingent consideration obligations were primarily due to the discontinuation of the Phase 2b study of BG00011 for the potential treatment of IPF resulting in a reduction of our contingent consideration obligations of $61.2 million as well as other changes in the probability and expected timing of achievement of certain developmental milestones, a decrease in interest rates used to revalue our contingent consideration liabilities and the passage of time.

The fair values of the contingent consideration liabilities were based on a probability-adjusted discounted cash flow calculation using Level 3 fair value measurements and inputs. For additional information on the valuation techniques and inputs utilized in the valuation of our financial assets and liabilities, please read Note 1, Summary of Significant Accounting Policies, to these consolidated financial statements.

Convergence Pharmaceuticals Holdings Limited

In connection with our acquisition of Convergence in February 2015 we recorded a contingent consideration obligation of $274.5 million. As of December 31, 2020 and 2019, the fair value of this contingent consideration obligation was $259.8 million and $244.6 million, respectively. Our most recent valuation was determined based upon net cash flow projections of $400.0 million, probability weighted and discounted using a rate of 0.6%, which is a measure of the credit risk associated with settling the liability.

Biogen International Neuroscience GmbH

In connection with our acquisition of BIN in December 2010 we recorded a contingent consideration obligation of $81.2 million. We discontinued further development of BIIB054 for the potential treatment of Parkinson's disease based on the results of a Phase 2 study of BIIB054. Additionally, during the third and fourth quarters of 2020 we discontinued other programs related to our acquisition of BIN for which we had immaterial contingent consideration obligations. As a result, the fair value of the contingent consideration obligations related to our acquisition of BIN has been adjusted to zero, resulting in a gain of $101.5 million for the year ended December 31, 2020.

Acquired IPR&D

The fair values of the acquired IPR&D assets were based on a probability-adjusted discounted cash flow calculation using Level 3 fair value measurements and inputs including estimated revenues and probabilities of success. These assets are tested for impairment annually until commercialization, after which time the acquired IPR&D will be amortized over its estimated useful life using the economic consumption method. In connection with our acquisition of BIN, we recognized a $110.9 million acquired IPR&D intangible asset. We discontinued further development of BIIB054 for the potential treatment of Parkinson's disease and recognized an impairment charge of $75.4 million during the fourth quarter of 2020 to reduce the fair value of the IPR&D intangible asset to zero. In connection with our acquisition of Stromedix Inc., we recognized a $219.2 million acquired IPR&D intangible asset. During the third quarter of 2019 we discontinued the Phase 2b study of BG00011 for the potential treatment of IPF and recognized an impairment charge of $215.9 million to reduce the fair value of the IPR&D intangible asset to zero. In connection with our acquisition of Convergence, we recognized a $424.6 million acquired IPR&D intangible asset. During the third quarter of 2018 we recognized impairment charges related to certain IPR&D assets associated with our vixotrigine program totaling $189.3 million. For additional information on our IPR&D intangible assets, including a discussion of our most significant assumptions, please read Note 6, Intangible Assets and Goodwill, to these consolidated financial statements.

Investments, All Other Investments [Abstract]

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Financial Instruments

12 Months Ended

Dec. 31, 2020

Financial Instruments

The following table summarizes our financial assets with maturities of less than 90 days from the date of purchase included in cash and cash equivalents in our consolidated balance sheets:


	As of December 31,
(In millions)	2020		2019
Commercial paper	$	61.1	$	384.4
Overnight reverse repurchase agreements	37.4		368.8
Money market funds	505.1		1,628.5
Short-term debt securities	23.3		159.4
Total	$	626.9	$	2,541.1

The carrying values of our commercial paper, including accrued interest, overnight reverse repurchase agreements, money market funds and our short-term debt securities approximate fair value due to their short-term maturities.

Our marketable equity securities gains (losses) are recorded in other income (expense), net in our consolidated statements of income. The following tables summarize our marketable debt and equity securities, classified as available for sale:


	As of December 31, 2020
(In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Corporate debt securities
Current	$	897.8	$	0.4	$	(0.2)	$	898.0
Non-current	402.5		1.1		(0.1)		403.5
Government securities
Current	380.6		0.1		—		380.7
Non-current	245.9		0.5		—		246.4
Mortgage and other asset backed securities
Current	0.2		—		—		0.2
Non-current	122.1		0.2		(0.1)		122.2
Total marketable debt securities	$	2,049.1	$	2.3	$	(0.4)	$	2,051.0
Marketable equity securities, current	$	70.6	$	15.9	$	—	$	86.5
Marketable equity securities, non-current	$	1,168.9	$	733.8	$	(14.9)	$	1,887.8


	As of December 31, 2019
(In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Corporate debt securities
Current	$	1,057.2	$	1.0	$	—	$	1,058.2
Non-current	633.9		3.0		—		636.9
Government securities
Current	502.9		0.4		—		503.3
Non-current	510.1		0.8		(0.3)		510.6
Mortgage and other asset backed securities
Current	0.7		—		—		0.7
Non-current	260.2		0.8		(0.4)		260.6
Total marketable debt securities	$	2,965.0	$	6.0	$	(0.7)	$	2,970.3
Marketable equity securities, non-current	$	218.4	$	132.1	$	(13.0)	$	337.5

Summary of Contractual Maturities: Available-for-Sale Debt Securities

The estimated fair value and amortized cost of our marketable debt securities available-for-sale by contractual maturity are summarized as follows:


	As of December 31, 2020				As of December 31, 2019
(In millions)	Estimated Fair Value		Amortized Cost		Estimated Fair Value		Amortized Cost
Due in one year or less	$	1,278.9	$	1,278.6	$	1,562.2	$	1,560.8
Due after one year through five years	722.6		721.3		1,234.5		1,230.4
Due after five years	49.5		49.2		173.6		173.8
Total marketable debt securities	$	2,051.0	$	2,049.1	$	2,970.3	$	2,965.0

The average maturity of our marketable debt securities available-for-sale as of December 31, 2020 and 2019, was approximately 11 months and 14 months, respectively.

Proceeds from Marketable Debt Securities

The proceeds from maturities and sales of marketable debt securities and resulting realized gains and losses are summarized as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Proceeds from maturities and sales	$	7,299.4	$	6,007.0	$	9,173.7
Realized gains	17.7		6.0		3.2
Realized losses	26.0		1.5		11.7

Realized losses for the year ended December 31, 2020, 2019 and 2018, primarily relate to sales of corporate bonds, agency mortgage-backed securities and other asset-backed securities.

Strategic Investments

As of December 31, 2020 and 2019, our strategic investment portfolio was comprised of investments totaling $2,024.6 million and $393.9 million, respectively, which are included in investments and other assets in our consolidated balance sheets.

Our strategic investment portfolio includes investments in equity securities of certain biotechnology companies, which are reflected within our disclosures included in Note 7, Fair Value Measurements, to these consolidated financial statements, venture capital funds where the underlying investments are in equity securities of certain biotechnology companies and non-marketable equity securities.

The increase in our strategic investment portfolio for the year ended December 31, 2020, was primarily due to our purchases of Sage, Denali and Sangamo common stock, as discussed below. These purchases were reflected as net cash flows used in investing activities within the consolidated statement of cash flows.

Sage Therapeutics, Inc.

In November 2020 we entered into a global collaboration and license agreement with Sage. In connection with the closing of this collaboration in December 2020 we purchased approximately 6.2 million shares of Sage common stock. This investment is classified as a Level 2 marketable equity security due to certain holding period restrictions and is remeasured each reporting period and carried at fair value. The effects of certain holding period restrictions on the investment are estimated using an option pricing valuation model. The most significant assumptions within the model are the term of the restrictions and the stock price volatility, which is based upon historical volatility of similar companies. We also use a constant maturity risk free-interest rate to match the remaining term of the restrictions on our investment in Sage common stock and a dividend yield of zero based upon the fact that Sage and similar companies generally have not historically granted cash dividends.

For additional information on our collaboration agreement with Sage, please read Note 18, Collaborative and Other Relationships, to these consolidated financial statements.

Denali Therapeutics Inc.

In August 2020 we entered into a collaboration and license agreement with Denali. As part of this collaboration we purchased approximately 13 million shares of Denali common stock in September 2020. This investment is classified as a Level 2 marketable equity security due to certain holding period restrictions and is remeasured each reporting period and carried at fair value. The effects of certain holding period restrictions on the investment are estimated using an option pricing valuation model. The most significant assumptions within the model are the term of the restrictions and the stock price volatility, which is based upon historical volatility of similar companies. We also use a constant maturity risk free-interest rate to match the remaining term of the restrictions on our investment in Denali common stock and a dividend yield of zero based upon the fact that Denali and similar companies generally have not historically granted cash dividends.

For additional information on our collaboration agreement with Denali, please read Note 18, Collaborative and Other Relationships, to these consolidated financial statements.

Sangamo Therapeutics, Inc.

In February 2020 we entered into a collaboration and license agreement with Sangamo. In connection with the closing of this transaction in April 2020 we purchased approximately 24 million shares of Sangamo common stock. This equity method investment will be remeasured each reporting period and carried at fair value due to our election of the fair value option. The effects of certain holding period restrictions on the investment are estimated using an option pricing valuation model. The most significant assumptions within the model are the term of the restrictions and the stock price volatility, which is based upon historical volatility of similar companies. We also use a constant maturity risk free-interest rate to match the remaining term of the restrictions on our investment in Sangamo common stock and a dividend yield of zero based upon the fact that Sangamo and similar companies generally have not historically granted cash dividends.

For additional information on our collaboration agreement with Sangamo, please read Note 18, Collaborative and Other Relationships, to these consolidated financial statements.

Samsung Bioepis

In June 2018 we exercised our option under our joint venture agreement with Samsung BioLogics to increase our ownership percentage in Samsung Bioepis from approximately 5.0% to approximately 49.9%. The share purchase transaction was completed in November 2018 and, upon closing, we paid 759.5 billion South Korean won ($676.6 million) to Samsung BioLogics.

As of December 31, 2020 and 2019, the carrying value of our investment in Samsung Bioepis totaled 673.8 billion South Korean won ($620.2 million) and 670.8 billion South Korean won ($580.2 million), respectively, which is classified as a component of investments and other assets within our consolidated balance sheets.

For additional information on our collaboration arrangements with Samsung Bioepis, please read Note 18, Collaborative and Other Relationships, to these consolidated financial statements.

Derivative Instruments

12 Months Ended

Dec. 31, 2020

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Derivative Instruments

Foreign Currency Forward Contracts - Hedging Instruments

Due to the global nature of our operations, portions of our revenues and operating expenses are recorded in currencies other than the U.S. dollar. The value of revenues and operating expenses measured in U.S. dollars is therefore subject to changes in foreign currency exchange rates. In order to mitigate these changes, we use foreign currency forward contracts to lock in exchange rates associated with a portion of our forecasted international revenues and operating expenses.

Foreign currency forward contracts in effect as of December 31, 2020 and 2019, had durations of 1 to 24 months and 1 to 15 months, respectively. These contracts have been designated as cash flow hedges and unrealized gains or losses on the portion of these foreign currency forward contracts that are included in the effectiveness test are reported in accumulated other comprehensive income (loss) (referred to as AOCI in the table below). Realized gains and losses of such contracts are recognized in revenues when the sale of product in the currency being hedged is recognized and in operating expenses when the expense in the currency being hedged is recorded. We recognize all cash flow hedge reclassifications from accumulated other comprehensive income (loss)

and fair value changes of excluded portions in the same line item in our consolidated statements of income that has been impacted by the hedged item.

The notional value of foreign currency forward contracts that were entered into to hedge forecasted revenues and operating expenses is summarized as follows:


	Notional Amount As of December 31,
(In millions)	2020		2019
Euro	$	2,979.1	$	1,892.4
British pound	250.6		—



Total foreign currency forward contracts	$	3,229.7	$	1,892.4

The pre-tax portion of the fair value of these foreign currency forward contracts that were included in accumulated other comprehensive income (loss) in total equity reflected net losses of $212.5 million as of December 31, 2020, net gains of $0.5 million as of December 31, 2019, and net gains of $27.3 million as of December 31, 2018. We expect the net losses of $212.5 million to be settled over the next 24 months, of which $175.2 million of these losses are expected to be settled over the next 12 months, with any amounts in accumulated other comprehensive income (loss) to be reported as an adjustment to revenues or operating expenses. We consider the impact of our and our counterparties’ credit risk on the fair value of the contracts as well as the ability of each party to execute its contractual obligations. As of December 31, 2020 and 2019, credit risk did not materially change the fair value of our foreign currency forward contracts.

The following table summarizes the effect of foreign currency forward contracts designated as hedging instruments in our consolidated statements of income:


For the Years Ended December 31,
Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)							Net Gains/(Losses) Recognized in Operating Income (in millions)
Location	2020		2019		2018		Location	2020		2019		2018
Revenues	$	18.3	$	118.6	$	(42.5)	Revenues	$	(9.9)	$	2.9	$	10.8
Operating expenses	3.3		(3.3)		0.2		Operating expenses	—		0.2		(0.1)

Interest Rate Contracts - Hedging Instruments

We have entered into interest rate lock contracts or interest rate swap contracts on certain borrowing transactions to manage our exposure to interest rate changes and to reduce our overall cost of borrowing.

Interest Rate Swap Contracts

In connection with the issuance of our 2.90% Senior Notes due September 15, 2020, we entered into interest rate swaps with an aggregate notional amount of $675.0 million, which were set to expire on September 15, 2020. The interest rate swap contracts were designated as hedges of the fair value changes in our 2.90% Senior Notes attributable to changes in interest rates. The carrying value of our 2.90% Senior Notes as of December 31, 2019, included approximately $2.3 million related to changes in the fair value of these interest rate swap contracts. In May 2020 we settled our interest rate swap contracts, in conjunction with our early redemption of our 2.90% Senior Notes, resulting in a gain of approximately $3.3 million for the year ended December 31, 2020, which was recorded as a component of interest expense in our consolidated statements of income.

Net Investment Hedges - Hedging Instruments

In February 2012 we entered into a joint venture agreement with Samsung BioLogics establishing an entity, Samsung Bioepis, to develop, manufacture and market biosimilar products. In June 2018 we exercised our option under our joint venture agreement to increase our ownership percentage in Samsung Bioepis from approximately 5.0% to approximately 49.9%. The share purchase transaction was completed in November 2018 and, upon closing, we paid 759.5 billion South Korean won ($676.6 million) to Samsung BioLogics. Our investment in the equity of Samsung Bioepis is exposed to the currency fluctuations in the South Korean won.

In order to mitigate these currency fluctuations between the U.S. dollar and South Korean won, we have entered into foreign currency forward contracts. Foreign currency forward contracts in effect as of December 31, 2020, had remaining durations of 10 months. These contracts have been designated as net investment hedges. We recognize changes in the spot exchange rate in accumulated other comprehensive income (loss). The pre-tax portion

of the fair value of these foreign currency forward contracts that were included in accumulated other comprehensive income (loss) in total equity reflected net losses of $21.2 million and $1.5 million as of December 31, 2020 and 2019, respectively. We exclude fair value changes related to the forward rate from our hedging relationship and will amortize the forward points in other income (expense), net in our consolidated statements of income over the term of the contract. The pre-tax portion of the fair value of the forward points that were included in accumulated other comprehensive income (loss) in total equity reflected gains of $0.2 million and $2.9 million as of December 31, 2020 and 2019, respectively.

The following table summarizes the effect of our net investment hedges in our consolidated financial statements:


For the Years Ended December 31,
Net Gains/(Losses) Recognized in Other Comprehensive Income (Effective Portion) (in millions)							Net Gains/(Losses) Recognized in Other Comprehensive Income (Amounts Excluded from Effectiveness Testing) (in millions)							Net Gains/(Losses) Recognized in Net Income (Amounts Excluded from Effectiveness Testing) (in millions)
Location	2020		2019		2018		Location	2020		2019		2018		Location	2020		2019		2018
Gains (losses) on net investment hedge	$	(35.1)	$	25.3	$	(3.8)	Gains (losses) on net investment hedge	$	4.5	$	3.3	$	—	Other income (expense)	$	2.9	$	7.0	$	1.5

For additional information on our collaboration arrangements with Samsung Bioepis, please read Note 18, Collaborative and Other Relationships, to these consolidated financial statements.

Foreign Currency Forward Contracts - Other Derivative Instruments

We also enter into other foreign currency forward contracts, usually with durations of one month or less, to mitigate the foreign currency risk related to certain balance sheet positions. We have not elected hedge accounting for these transactions.

The aggregate notional amount of these outstanding foreign currency contracts was $1,158.0 million and $793.8 million as of December 31, 2020 and 2019, respectively. Net gains of $30.1 million, net losses of $5.9 million and net gains of $2.0 million related to these contracts were recorded as a component of other income (expense), net for the years ended December 31, 2020, 2019 and 2018, respectively.

Summary of Derivative Instruments

While certain of our derivative instruments are subject to netting arrangements with our counterparties, we do not offset derivative assets and liabilities in our consolidated balance sheets. The amounts in the table below would not be substantially different if the derivative assets and liabilities were offset.

The following table summarizes the fair value and presentation in our consolidated balance sheets of our outstanding derivative instruments, including those designated as hedging instruments:


		As of December 31,
(In millions)	Balance Sheet Location	2020		2019
Cash Flow Hedging Instruments:
Asset derivative instruments	Other current assets	$	—	$	33.8
Liability derivative instruments	Accrued expenses and other	157.1		2.0
	Other long-term liabilities	35.7		1.7

Net Investment Hedging Instruments:
Asset derivative instruments	Other current assets	—		2.0
Liability derivative instruments	Accrued expenses and other	19.7		—

Fair Value Hedging Instruments
Liability derivative instruments	Accrued expenses and other	—		2.3


Other Derivative Instruments:
Asset derivative instruments	Other current assets	20.5		8.0
Liability derivative instruments	Accrued expenses and other	4.7		2.4

Property, Plant and Equipment

12 Months Ended

Dec. 31, 2020

Property, Plant and Equipment [Abstract]

Property, Plant and Equipment

Property, plant and equipment are recorded at historical cost, net of accumulated depreciation. Components of property, plant and equipment, net are summarized as follows:


	As of December 31,
(In millions)	2020		2019
Land	$	119.8	$	118.1
Buildings	1,025.3		835.0
Leasehold improvements	104.6		99.5
Machinery and equipment	1,027.8		844.5
Computer software and hardware	903.0		798.4
Furniture and fixtures	62.5		58.3
Construction in progress	1,950.8		2,084.4
Total cost	5,193.8		4,838.2
Less: accumulated depreciation	(1,782.3)		(1,590.9)
Total property, plant and equipment, net	$	3,411.5	$	3,247.3

Depreciation expense totaled $201.9 million, $190.6 million and $269.4 million for the years ended December 31, 2020, 2019 and 2018, respectively.

For the years ended December 31, 2020, 2019 and 2018, we capitalized interest costs related to construction in progress totaling approximately $65.2 million, $68.8 million and $54.0 million, respectively.

Solothurn, Switzerland Manufacturing Facility

In order to support our future growth and drug development pipeline, we are building a large-scale biologics manufacturing facility in Solothurn, Switzerland. We expect this facility to be partially operational during the first half of 2021. Upon completion, this facility will include 393,000 square feet related to a large-scale biologics manufacturing facility, 290,000 square feet of warehouse, utilities and support space and 51,000 square feet of administrative space. As of December 31, 2020 and 2019, we had approximately $1.8 billion and $1.9 billion, respectively, capitalized as construction in progress related to this facility. For the year ended December 31, 2020, we placed approximately $256.8 million of fixed assets in service related to this facility.

Divestiture of Hillerød, Denmark Manufacturing Operations

In August 2019 we completed the sale of all of the outstanding shares of our subsidiary that owned our biologics manufacturing operations in Hillerød, Denmark to FUJIFILM. This transaction included $631.5 million of property, plant and equipment, which was primarily comprised of $312.5 million for buildings and $287.3 million for machinery and equipment. For additional information on the divestiture of our Hillerød, Denmark manufacturing operations, please read Note 3, Divestitures, to these consolidated financial statements.

Leases

12 Months Ended

Dec. 31, 2020

Leases [Abstract]

Leases

We lease real estate, including laboratory and office space, and certain equipment.

Our leases have remaining lease terms ranging from less than one year to ten years. Certain leases include one or more options to renew, exercised at our sole discretion, with renewal terms that can extend the lease term from one year to six years.

In addition, we sublease certain real estate to third parties. Our sublease portfolio consists of operating leases, with remaining lease terms ranging from four years to eight years. Our subleases do not include an option to renew as they are coterminous with our operating leases.

All of our leases qualify as operating leases. The following table summarizes the presentation in our consolidated balance sheets of our operating leases:


		As of December 31,
(In millions)	Balance sheet location	2020		2019
Assets:
Operating lease assets	Operating lease assets	$	433.3	$	427.0

Liabilities
Current operating lease liabilities	Accrued expenses and other	$	83.2	$	73.6
Non-current operating lease liabilities	Long-term operating lease liabilities	402.0		412.7
Total operating lease liabilities		$	485.2	$	486.3

The following table summarizes the effect of lease costs in our consolidated statements of income:


		For the Years Ended December 31,
(In millions)	Income Statement Location	2020		2019
Operating lease cost	Research and development	$	5.2	$	6.7
	Selling, general and administrative	93.1		84.6
Variable lease cost	Research and development	1.1		1.2
	Selling, general and administrative	21.1		23.7
Sublease income	Selling, general and administrative	(24.2)		(25.6)
	Other (income) expense, net	(3.9)		(3.9)
Net lease cost		$	92.4	$	86.7

Variable lease cost primarily related to operating expenses, taxes and insurance associated with our operating leases. As these costs are generally variable in nature, they are not included in the measurement of the operating lease asset and related lease liability.

The minimum lease payments for the next five years and thereafter is expected to be as follows:


(In millions)	As of December 31, 2020
2021	$	95.8
2022	93.5
2023	82.0
2024	75.0
2025	56.9
Thereafter	129.8
Total lease payments	$	533.0
Less: interest	47.8
Present value of operating lease liabilities	$	485.2

The weighted average remaining lease term and weighted average discount rate of our operating leases are as follows:


	As of December 31,
	2020		2019
Weighted average remaining lease term in years	6.30		7.07
Weighted average discount rate	2.9	%	3.2	%

Supplemental disclosure of cash flow information related to our operating leases included in cash flows provided by operating activities in our consolidated statements of cash flows is as follows:


	As of December 31,
(In millions)	2020		2019
Cash paid for amounts included in the measurement of lease liabilities	$	100.2	$	93.8
Operating lease assets obtained in exchange for lease obligations	59.0		35.9

Indebtedness

12 Months Ended

Dec. 31, 2020

Debt Disclosure [Abstract]

Indebtedness

Our indebtedness is summarized as follows:


	As of December 31,
(In millions)	2020		2019
Current portion:
2.900% Senior Notes due September 15, 2020(1)	$	—	$	1,495.8
Current portion of notes payable	$	—	$	1,495.8
Non-current portion:
3.625% Senior Notes due September 15, 2022	$	997.9	$	996.6
4.050% Senior Notes due September 15, 2025	1,741.2		1,739.5
2.250% Senior Notes due May 1, 2030	1,491.1		—
5.200% Senior Notes due September 15, 2045	1,723.4		1,722.9
3.150% Senior Notes due May 1, 2050	1,472.6		—
Non-current portion of notes payable	$	7,426.2	$	4,459.0

2020 Senior Notes

On April 30, 2020, we issued senior unsecured notes for an aggregate principal amount of $3.0 billion (2020 Senior Notes) as of December 31, 2020, consisting of the following:

•$1.5 billion aggregate principal amount of 2.25% Senior Notes due May 1, 2030, valued at 99.973% of par; and

•$1.5 billion aggregate principal amount of 3.15% Senior Notes due May 1, 2050, valued at 99.174% of par.

Our 2020 Senior Notes are senior unsecured obligations and may be redeemed at our option at any time at 100.0% of the principal amount plus accrued interest and, until a specified period before maturity, a specified make-whole amount. Our 2020 Senior Notes contain a change-of-control provision that, under certain circumstances, may require us to purchase our 2020 Senior Notes at a price equal to 101.0% of the principal amount plus accrued and unpaid interest to the date of repurchase.

We incurred approximately $24.4 million of costs associated with this offering which have been recorded as a reduction to the carrying amount of the debt on our consolidated balance sheet. These costs will be amortized as additional interest expense using the effective interest rate method over the period from issuance through maturity. The discounts will be amortized as additional interest expense over the period from issuance through maturity using the effective interest rate method. Interest on our 2020 Senior Notes is payable May 1 and November 1 of each year, commencing November 1, 2020.

2015 Senior Notes

The following is a summary of our currently outstanding senior secured notes issued in 2015 (the 2015 Senior Notes) as of December 31, 2020:

•$1.0 billion aggregate principal amount of 3.625% Senior Notes due September 15, 2022, valued at 99.920% of par;

•$1.75 billion aggregate principal amount of 4.05% Senior Notes due September 15, 2025, valued at 99.764% of par; and

•$1.75 billion aggregate principal amount of 5.20% Senior Notes due September 15, 2045, valued at 99.294% of par.

The original costs associated with this offering of approximately $47.5 million have been recorded as a reduction to the carrying amount of the debt in our consolidated balance sheets. These costs along with the discounts will be amortized as additional interest expense using the effective interest rate method over the period from issuance through maturity.

Our 2015 Senior Notes are senior unsecured obligations and may be redeemed at our option at any time at 100.0% of the principal amount plus accrued interest and a specified make-whole amount. Our 2015 Senior Notes contain a change of control provision that may require us to purchase the notes at a price equal to 101.0% of the principal amount plus accrued and unpaid interest to the date of purchase under certain circumstances.

On September 15, 2015, we issued $1.5 billion aggregate principal amount of 2.90% Senior Notes due September 15, 2020, at 99.792% of par. Our 2.90% Senior Notes were senior unsecured obligations. In connection with the 2.90% Senior Notes, we entered into interest rate swap contracts where we received a fixed rate and paid a variable rate. In May 2020 we used the net proceeds from the sale of our 2020 Senior Notes to redeem our 2.90% Senior Notes prior to their maturity and recognized a net pre-tax charge of $9.4 million upon the extinguishment of these notes. This charge, which was recognized in interest expense in other income (expense), net in our consolidated statements of income for the year ended December 31, 2020, reflects the payment of a $12.7 million early call premium and the write off of remaining unamortized original debt issuance costs and discount balances, partially offset by a $3.3 million gain related to the settlement of the associated interest rate swap contracts. For additional information on our interest rate contracts, please read Note 9, Derivative Instruments, to these consolidated financial statements.

2020 Credit Facility

In January 2020 we entered into a $1.0 billion, five-year senior unsecured revolving credit facility under which we are permitted to draw funds for working capital and general corporate purposes. The terms of the revolving credit facility include a financial covenant that requires us not to exceed a maximum consolidated leverage ratio. This revolving credit facility replaced the revolving credit facility that we entered into in August 2015. As of December 31, 2020, we had no outstanding borrowings and were in compliance with all covenants under this facility.

Debt Maturity

The total gross payments due under our debt arrangements are as follows:


(In millions)	As of December 31, 2020
2021	$	—
2022	1,000.0
2023	—
2024	—
2025	1,750.0
2026 and thereafter	4,750.0
Total	$	7,500.0

The fair value of our debt is disclosed in Note 7, Fair Value Measurements, to these consolidated financial statements.

Equity

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Equity

Preferred Stock

We have 8.0 million shares of Preferred Stock authorized, of which 1.75 million shares are authorized as Series A, 1.0 million shares are authorized as Series X junior participating and 5.25 million shares are undesignated. Shares may be issued without a vote or action of shareholders from time to time in classes or series with the designations, powers, preferences and the relative, participating, optional or other special rights of the shares of

each such class or series and any qualifications, limitations or restrictions thereon as set forth in the instruments governing such shares. Any such Preferred Stock may rank prior to common stock as to dividend rights, liquidation preference or both, and may have full or limited voting rights and may be convertible into shares of common stock. No shares of Preferred Stock were issued and outstanding during 2020, 2019 and 2018.

Common Stock

The following table describes the number of shares authorized, issued and outstanding of our common stock as of December 31, 2020, 2019 and 2018:

As of December 31, 2020

As of December 31, 2019

As of December 31, 2018

(In millions)

Authorized

Issued

Outstanding

Authorized

Issued

Outstanding

Authorized

Issued

Outstanding

Common stock

1,000.0

176.2

152.4

1,000.0

198.0

174.2

1,000.0

221.0

197.2

Share Repurchases

In October 2020 our Board of Directors authorized a program to repurchase up to $5.0 billion of our common stock (2020 Share Repurchase Program). Our 2020 Share Repurchase Program does not have an expiration date. All share repurchases under our 2020 Share Repurchase Program will be retired. Under our 2020 Share Repurchase Program, we repurchased and retired approximately 1.6 million shares of our common stock at a cost of approximately $400.0 million during the year ended December 31, 2020.

In December 2019 our Board of Directors authorized a program to repurchase up to $5.0 billion of our common stock (December 2019 Share Repurchase Program), which was completed as of September 30, 2020. All shares repurchased under our December 2019 Share Repurchase Program were retired. Under our December 2019 Share Repurchase Program, we repurchased and retired approximately 16.7 million shares of our common stock at a cost of approximately $5.0 billion during the year ended December 31, 2020.

In March 2019 our Board of Directors authorized a program to repurchase up to $5.0 billion of our common stock (March 2019 Share Repurchase Program), which was completed as of March 31, 2020. All shares repurchased under our March 2019 Share Repurchase Program were retired. Under our March 2019 Share Repurchase Program, we repurchased and retired approximately 4.1 million and 14.7 million shares of our common stock at a cost of approximately $1.3 billion and $3.7 billion during the years ended December 31, 2020 and 2019, respectively.

In August 2018 our Board of Directors authorized a program to repurchase up to $3.5 billion of our common stock (2018 Share Repurchase Program), which was completed as of June 30, 2019. All share repurchases under our 2018 Share Repurchase Program were retired. Under our 2018 Share Repurchase Program, we repurchased and retired approximately 8.9 million and 4.3 million shares of our common stock at a cost of approximately $2.1 billion and $1.4 billion during the years ended December 31, 2019 and 2018, respectively.

In July 2016 our Board of Directors authorized a program to repurchase up to $5.0 billion of our common stock (2016 Share Repurchase Program), which was completed as of June 30, 2018. All share repurchases under our 2016 Share Repurchase Program were retired. Under our 2016 Share Repurchase Program, we repurchased and retired approximately 10.5 million shares of common stock at a cost of approximately $3.0 billion during the year ended December 31, 2018.

Amounts paid to repurchase shares in excess of their par value are allocated between additional paid-in capital and retained earnings, with payments in excess of our additional paid-in-capital balance recorded as a reduction to retained earnings.

Accumulated Other Comprehensive Income (Loss)

The following tables summarize the changes in accumulated other comprehensive income (loss), net of tax by component:


	December 31, 2020
(In millions)	Unrealized Gains (Losses) on Securities Available for Sale, net of tax		Unrealized Gains (Losses) on Cash Flow Hedges, net of tax		Gains (Losses) on Net Investment Hedge, Net of Tax		Unfunded Status of Postretirement Benefit Plans, net of tax		Currency Translation Adjustments		Total
Balance, December 31, 2019	$	4.2	$	7.8	$	25.1	$	(32.8)	$	(139.5)	$	(135.2)
Other comprehensive income (loss) before reclassifications	(9.3)		(165.0)		(30.7)		(33.5)		92.9		(145.6)
Amounts reclassified from accumulated other comprehensive income (loss)	6.5		(21.8)		(2.9)		—		—		(18.2)
Net current period other comprehensive income (loss)	(2.8)		(186.8)		(33.6)		(33.5)		92.9		(163.8)
Balance, December 31, 2020	$	1.4	$	(179.0)	$	(8.5)	$	(66.3)	$	(46.6)	$	(299.0)


	December 31, 2019
(In millions)	Unrealized Gains (Losses) on Securities Available for Sale, net of tax		Unrealized Gains (Losses) on Cash Flow Hedges, net of tax		Gains (Losses) on Net Investment Hedge, Net of Tax		Unfunded Status of Postretirement Benefit Plans, net of tax		Currency Translation Adjustments		Total
Balance, December 31, 2018	$	(4.0)	$	34.7	$	3.5	$	(31.3)	$	(243.3)	$	(240.4)
Other comprehensive income (loss) before reclassifications	11.8		88.1		28.6		(1.5)		103.8		230.8
Amounts reclassified from accumulated other comprehensive income (loss)	(3.6)		(115.0)		(7.0)		—		—		(125.6)
Net current period other comprehensive income (loss)	8.2		(26.9)		21.6		(1.5)		103.8		105.2
Balance, December 31, 2019	$	4.2	$	7.8	$	25.1	$	(32.8)	$	(139.5)	$	(135.2)


	December 31, 2018
(In millions)	Unrealized Gains (Losses) on Securities Available for Sale, net of tax		Unrealized Gains (Losses) on Cash Flow Hedges, net of tax		Gains (Losses) on Net Investment Hedge, Net of Tax		Unfunded Status of Postretirement Benefit Plans, net of tax		Currency Translation Adjustments		Total
Balance, December 31, 2017	$	(1.6)	$	(104.5)	$	—	$	(36.8)	$	(175.5)	$	(318.4)
Amount reclassified, net of tax, upon adoption of ASU 2016-01	1.5		—		—		—		—		1.5
Balance, January 1, 2018	(0.1)		(104.5)		—		(36.8)		(175.5)		(316.9)
Other comprehensive income (loss) before reclassifications	(10.6)		97.4		5.0		5.5		(67.8)		29.5
Amounts reclassified from accumulated other comprehensive income (loss)	6.7		41.8		(1.5)		—		—		47.0
Net current period other comprehensive income (loss)	(3.9)		139.2		3.5		5.5		(67.8)		76.5
Balance, December 31, 2018	$	(4.0)	$	34.7	$	3.5	$	(31.3)	$	(243.3)	$	(240.4)

The following table summarizes the amounts reclassified from accumulated other comprehensive income:


(In millions)	Income Statement Location	Amounts Reclassified from Accumulated Other Comprehensive Income
		For the Years Ended December 31,
		2020		2019		2018
Gains (losses) on securities available for sale	Other income (expense)	$	(8.2)	$	4.5	$	(8.5)
	Income tax benefit (expense)	1.7		(0.9)		1.8

Gains (losses) on cash flow hedges	Revenues	18.3		118.6		(42.5)
	Operating expenses	3.3		(3.3)		0.2
	Other income (expense)	0.3		0.3		0.3
	Income tax benefit (expense)	(0.1)		(0.6)		0.2

Gains (losses) on net investment hedge	Other Income (expense)	2.9		7.0		1.5

Total reclassifications, net of tax		$	18.2	$	125.6	$	(47.0)

Earnings per Share

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

Earnings per Share

Basic and diluted earnings per share are calculated as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Numerator:
Net income attributable to Biogen Inc.	$	4,000.6	$	5,888.5	$	4,430.7
Denominator:
Weighted average number of common shares outstanding	160.9		187.1		204.9
Effect of dilutive securities:

Time-vested restricted stock units	0.2		0.2		0.3
Market stock units	0.1		0.1		0.1
Performance stock units settled in stock	0.1		—		—
Dilutive potential common shares	0.4		0.3		0.4
Shares used in calculating diluted earnings per share	161.3		187.4		205.3

Amounts excluded from the calculation of net income per diluted share because their effects were anti-dilutive were insignificant.

Earnings per share for the years ended December 31, 2020, 2019 and 2018, reflects the repurchase of approximately 22.4 million shares, 23.6 million shares and 14.8 million shares of our common stock, respectively, under our share repurchase programs. For additional information on our share repurchase programs, please read Note 13, Equity, to these consolidated financial statements.

Share-Based Payments

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Share-Based Payments

Share-Based Compensation Expense

The following table summarizes share-based compensation expense included in our consolidated statements of income:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Research and development	$	80.0	$	77.1	$	75.8
Selling, general and administrative	131.3		148.3		105.8
Subtotal	211.3		225.4		181.6
Capitalized share-based compensation costs	(6.2)		(8.9)		(11.5)
Share-based compensation expense included in total cost and expenses	205.1		216.5		170.1
Income tax effect	(33.5)		(35.7)		(27.5)
Share-based compensation expense included in net income attributable to Biogen Inc.	$	171.6	$	180.8	$	142.6

The following table summarizes share-based compensation expense associated with each of our share-based compensation programs:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Market stock units	$	40.5	$	30.4	$	27.2
Time-vested restricted stock units	142.6		134.0		126.6
Cash settled performance units	(1.7)		0.7		7.8
Performance units	(0.1)		1.6		3.1
Performance stock units settled in stock	7.9		15.5		4.7
Performance stock units settled in cash	8.6		5.5		1.7
Employee stock purchase plan	13.5		11.5		10.5
NST stock options	—		26.2		—
Subtotal	211.3		225.4		181.6
Capitalized share-based compensation costs	(6.2)		(8.9)		(11.5)
Share-based compensation expense included in total cost and expenses	$	205.1	$	216.5	$	170.1

As of December 31, 2020, unrecognized compensation cost related to unvested share-based compensation was approximately $207.6 million, net of estimated forfeitures. We expect to recognize the cost of these unvested awards over a weighted-average period of 1.9 years.

Share-Based Compensation Plans

We have three share-based compensation plans pursuant to which awards are currently being made: (i) the Biogen Inc. 2006 Non-Employee Directors Equity Plan (2006 Directors Plan); (ii) the Biogen Inc. 2017 Omnibus Equity Plan (2017 Omnibus Equity Plan); and (iii) the Biogen Inc. 2015 Employee Stock Purchase Plan (2015 ESPP).

Directors Plan

In May 2006 our shareholders approved the 2006 Directors Plan for share-based awards to our directors. Awards granted from the 2006 Directors Plan may include stock options, shares of restricted stock, RSUs, stock appreciation rights and other awards in such amounts and with such terms and conditions as may be determined by a committee of our Board of Directors, subject to the provisions of the 2006 Directors Plan. We have reserved a total of 1.6 million shares of common stock for issuance under the 2006 Directors Plan. The 2006 Directors Plan provides that awards other than stock options and stock appreciation rights will be counted against the total number of shares reserved under the plan in a 1.5-to-1 ratio. In June 2015 our shareholders approved an amendment to extend the term of the 2006 Directors Plan until June 2025.

Omnibus Plan

In June 2017 our shareholders approved the 2017 Omnibus Equity Plan for share-based awards to our employees. Awards granted from the 2017 Omnibus Equity Plan may include stock options, shares of restricted stock, RSUs, performance shares, stock appreciation rights and other awards in such amounts and with such terms and conditions as may be determined by a committee of our Board of Directors, subject to the provisions of the 2017 Omnibus Equity Plan. Shares of common stock available for grant under the 2017 Omnibus Equity Plan consist of 8.0 million shares reserved for this purpose, plus shares of common stock that remained available for grant under the Biogen Idec Inc. 2008 Omnibus Equity Plan (2008 Omnibus Equity Plan) as of June 7, 2017, or that could again become available for grant if outstanding awards under the 2008 Omnibus Equity Plan as of June 7, 2017, are cancelled, surrendered or terminated in whole or in part. The 2017 Omnibus Equity Plan provides that awards other than stock options and stock appreciation rights will be counted against the total number of shares available under the plan in a 1.5-to-1 ratio.

We have not made any awards pursuant to the 2008 Omnibus Equity Plan since our shareholders approved the 2017 Omnibus Equity Plan, and do not intend to make any awards pursuant to the 2008 Omnibus Equity Plan in the future, except that unused shares under the 2008 Omnibus Equity Plan have been carried over for use under the 2017 Omnibus Equity Plan.

Stock Options

We currently do not grant stock options to our employees or directors. Outstanding stock options previously granted to our employees and directors generally have a 10-year term and vest over a period of between one and four years, provided the individual continues to serve at Biogen through the vesting dates. Options granted under all plans are exercisable at a price per share not less than the fair market value of the underlying common stock on the date of grant. As of December 31, 2020, all outstanding options were exercisable.

The following table summarizes our stock option activity:


	Shares	Weighted Average Exercise Price
Outstanding at December 31, 2019	12,000	$	58.46
Granted	—	—
Exercised	(12,000)	58.46
Cancelled	—	—
Outstanding at December 31, 2020	—	$	—

The total intrinsic values of options exercised in 2020, 2019 and 2018 totaled $2.9 million, $4.2 million and $4.0 million, respectively.

The following table summarizes the amount of tax benefit realized for stock options and cash received from the exercise of stock options:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Tax benefit realized for stock options	$	2.9	$	2.5	$	2.2
Cash received from the exercise of stock options	0.7		0.4		0.8

Market Stock Units (MSUs)

MSUs awarded to employees prior to 2014 vested in four equal annual increments beginning on the first anniversary of the grant date. Participants may ultimately earn between zero and 150.0% of the target number of units granted based on actual stock performance.

MSUs awarded to employees in 2014 and thereafter vest in three equal annual increments beginning on the first anniversary of the grant date, and participants may ultimately earn between zero and 200.0% of the target number of units granted based on actual stock performance.

The vesting of these awards is subject to the respective employee’s continued employment. The number of MSUs granted represents the target number of units that are eligible to be earned based on the attainment of certain market-based criteria involving our stock price. The number of MSUs earned is calculated at each annual

anniversary from the date of grant over the respective vesting periods, resulting in multiple performance periods. Accordingly, additional MSUs may be issued or currently outstanding MSUs may be cancelled upon final determination of the number of awards earned.

The following table summarizes our MSU activity:


	December 31, 2020
	Shares	Weighted Average Grant Date Fair Value
Unvested at December 31, 2019	183,000	$	378.09
Granted (1)	133,000	398.61
Vested	(82,000)	375.42
Forfeited	(33,000)	402.62
Unvested at December 31, 2020	201,000	$	388.98

(1) MSUs granted during 2020 include awards granted in conjunction with our annual awards made in February 2020 and MSUs granted in conjunction with the hiring of employees. These grants reflect the target number of shares eligible to be earned at the time of grant. MSUs granted in 2020 also reflect an adjustment based upon the final performance multiplier in relation to shares granted in 2019, 2018 and 2017.

We value grants of MSUs using a lattice model with a Monte Carlo simulation. This valuation methodology utilizes several key assumptions, the 30 calendar day average closing stock price on the date of grant for MSUs, expected volatility of our stock price, risk-free rates of return and expected dividend yield.

The assumptions used in our valuation are summarized as follows:


	For the Years Ended December 31,
	2020	2019	2018
Expected dividend yield	—%	—%	—%
Range of expected stock price volatility	37.8% - 44.1%	31.2% - 33.6%	27.5% - 32.4%
Range of risk-free interest rates	1.41% - 1.48%	2.46% - 2.53%	1.9% - 2.3%
30 calendar day average stock price on grant date	$257.83 - $325.40	$228.59 - $331.18	$279.47 - $346.76
Weighted-average per share grant date fair value	$398.61	$378.08	$378.85

The fair values of MSUs vested in 2020, 2019 and 2018 totaled $26.9 million, $32.5 million and $26.9 million, respectively.

Cash Settled Performance Units (CSPUs)

CSPUs awarded to employees vest in three equal annual increments beginning on the first anniversary of the grant date. The vesting of these awards is subject to the respective employee’s continued employment with such awards settled in cash. The number of CSPUs granted represents the target number of units that are eligible to be earned based on the attainment of certain performance measures established at the beginning of the performance period, which ends on December 31 of each year. Participants may ultimately earn between zero and 200.0% of the target number of units granted based on the degree of actual performance metric achievement. Accordingly, additional CSPUs may be issued or currently outstanding CSPUs may be cancelled upon final determination of the number of units earned. CSPUs are classified as liability awards and will be settled in cash based on the 30 calendar day average closing stock price through each vesting date, once the actual vested and earned number of units is known. Since no shares are issued, these awards do not dilute equity.

The following table summarizes our CSPU activity:


			Shares
Unvested at December 31, 2019			13,000
Granted			—
Vested			(13,000)
Forfeited			—
Unvested at December 31, 2020			—

The cash paid in settlement of CSPUs vested in 2020, 2019 and 2018 totaled $3.8 million, $10.6 million and $15.1 million, respectively.

Performance-vested Restricted Stock Units (PUs)

PUs are granted to certain employees in the form of RSUs that may be settled in cash or shares of our common stock at the sole discretion of the Compensation and Management Development Committee of our Board of Directors. These awards are structured and accounted for the same way as the CSPUs, and vest in three equal annual increments beginning on the first anniversary of the grant date. The number of PUs granted represents the target number of units that are eligible to be earned based on the attainment of certain performance measures established at the beginning of the performance period, which ends on December 31 of each year. Participants may ultimately earn between zero and 200.0% of the target number of units granted based on the degree of actual performance metric achievement. Accordingly, additional PUs may be issued or currently outstanding PUs may be cancelled upon final determination of the number of units earned. PUs settling in cash are based on the 30 calendar day average closing stock price through each vesting date once the actual vested and earned number of units is known.

The following table summarizes our PU activity:


			Shares
Unvested at December 31, 2019			11,000
Granted			—
Vested			(11,000)
Forfeited			—
Unvested at December 31, 2020			—

All PUs that vested in 2020, 2019 and 2018 were settled in cash totaling $3.4 million, $10.4 million and $17.0 million, respectively.

Performance Stock Units (PSUs)

PSUs Settled in Stock

During the first quarter of 2018 we began granting awards for performance-vested RSUs that will settle in stock. PSUs awarded to employees have a three-year performance period and vest on the third anniversary of the grant date. The vesting of these awards is subject to the respective employee’s continued employment. The number of PSUs granted represents the target number of units that are eligible to be earned based on the achievement of cumulative three-year performance measures established at the beginning of the performance period, which ends on December 31 of the third year of the performance period.

The following table summarizes our PSUs that settle in stock activity:


	Shares	Weighted Average Grant Date Fair Value
Unvested at December 31, 2019	111,000	$	316.39
Granted (1)	73,000	293.35
Vested	—	—
Forfeited	(29,000)	323.42
Unvested at December 31, 2020	155,000	$	304.19

(1) PSUs settled in stock granted in 2020 include awards granted in conjunction with our annual awards made in February 2020 and PSUs granted in conjunction with the hiring of employees. These grants reflect the target number of shares eligible to be earned at the time of grant.

PSUs Settled in Cash

During the first quarter of 2018 we began granting awards for performance-vested restricted stock units that will settle in cash. PSUs awarded to employees have three performance periods and vest on the third anniversary of the grant date. The vesting of these awards is subject to the respective employee’s continued employment. The number of PSUs granted represents the target number of units that are eligible to be earned based on the

achievement of three annual performance measures established when the performance objectives are defined, which will be at the beginning of each year and will end on December 31 of such year.

Participants may ultimately earn between zero and 200.0% of the target number of PSUs granted based on the degree of achievement of the applicable performance metric. Accordingly, additional PSUs may be issued or currently outstanding PSUs may be cancelled upon final determination of the number of units earned. PSUs are classified as liability awards and will be settled in cash based on the 30 calendar day average closing stock price through the vesting date, once the actual vested and earned number of PSUs is determined. Since no shares are issued, these awards do not dilute equity.

The following table summarizes our PSUs that settle in cash activity:


			Shares
Unvested at December 31, 2019			82,000
Granted (1)			63,000
Vested			(2,000)
Forfeited			(23,000)
Unvested at December 31, 2020			120,000

(1) PSUs settled in cash granted in 2020 include awards granted in conjunction with our annual awards made in February 2020 and PSUs granted in conjunction with the hiring of employees. These grants reflect the target number of shares eligible to be earned at the time of grant.

Time-Vested Restricted Stock Units (RSUs)

RSUs awarded to employees generally vest no sooner than one-third per year over three years on the anniversary of the date of grant, or upon the third anniversary of the date of the grant, provided the employee remains continuously employed with us, except as otherwise provided in the plan. Shares of our common stock will be delivered to the employee upon vesting, subject to payment of applicable withholding taxes. RSUs awarded to directors for service on our Board of Directors vest on the first anniversary of the date of grant, provided in each case that the director continues to serve on our Board of Directors through the vesting date. Shares of our common stock will be delivered to the director upon vesting and are not subject to any withholding taxes.

The following table summarizes our RSU activity:


	Shares	Weighted Average Grant Date Fair Value
Unvested at December 31, 2019	938,000	$	306.55
Granted (1)	620,000	318.87
Vested	(440,000)	304.45
Forfeited	(100,000)	314.46
Unvested at December 31, 2020	1,018,000	$	314.46

(1) RSUs granted in 2020 primarily represent RSUs granted in conjunction with our annual awards made in February 2020 and awards made in conjunction with the hiring of new employees. RSUs granted in 2020 also include approximately 10,000 RSUs granted to our Board of Directors.

RSUs granted in 2019 and 2018 had weighted average grant date fair values of $304.44 and $316.32, respectively.

The fair values of RSUs vested in 2020, 2019 and 2018 totaled $140.5 million, $131.5 million and $111.7 million, respectively.

Employee Stock Purchase Plan (ESPP)

In June 2015 our shareholders approved the 2015 ESPP. The maximum aggregate number of shares of our common stock that may be purchased under the 2015 ESPP is 6.2 million.

The following table summarizes our ESPP activity:


	For the Years Ended December 31,
(In millions, except share amounts)	2020		2019		2018
Shares issued under the 2015 ESPP	212,000		204,000		170,000
Cash received under the 2015 ESPP	$	48.6	$	40.4	$	40.5

Income Taxes

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Income Taxes

Income Tax Expense

Income before income tax expense and the income tax expense consist of the following:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Income before income taxes (benefit):
Domestic	$	3,290.0	$	4,725.3	$	3,877.0
Foreign	1,757.5		2,400.6		2,022.6
Total	$	5,047.5	$	7,125.9	$	5,899.6
Income tax expense (benefit):
Current:
Federal	$	647.0	$	947.4	$	1,131.8
State	41.2		59.1		45.5
Foreign	155.1		84.4		140.0
Total	843.3		1,090.9		1,317.3
Deferred:
Federal	$	(1,749.9)	$	1,143.9	$	(62.0)
State	(6.8)		(2.3)		(7.4)
Foreign	1,905.7		(1,074.5)		177.7
Total	149.0		67.1		108.3
Total income tax expense	$	992.3	$	1,158.0	$	1,425.6

2017 Tax Act

The Tax Cuts and Jobs Act of 2017 (2017 Tax Act) resulted in significant changes to the U.S. corporate income tax system. These changes include a federal statutory rate reduction from 35.0% to 21.0%, the elimination or reduction of certain domestic deductions and credits and limitations on the deductibility of interest expense and executive compensation. The 2017 Tax Act also transitions international taxation from a worldwide system to a modified territorial system, which has the effect of subjecting certain earnings of our foreign subsidiaries and collaborations to immediate U.S. taxation as GILTI or Subpart F income, includes a one-time mandatory deemed repatriation tax on accumulated foreign subsidiaries' previously untaxed foreign earnings (the Transition Toll Tax) and base erosion prevention measures on U.S. earnings and reduces the effective tax rate on income that comes from U.S. exports, called Foreign Derived Intangible Income. These changes became effective in 2018.

During the year ended December 31, 2018, we recognized a net reduction of $34.6 million in our estimated Transition Toll Tax, an expense of $12.7 million to remeasure our deferred tax balances, an expense of $135.8 million related to establishing deferred taxes for GILTI and an expense of $11.0 million to reflect other aspects of the 2017 Tax Act.

Transition Toll Tax

The 2017 Tax Act eliminated the deferral of U.S. income tax on the historical unrepatriated earnings by imposing the Transition Toll Tax. The Transition Toll Tax was assessed on our share of our foreign corporations' accumulated foreign earnings that were not previously taxed. Earnings in the form of cash and cash equivalents were taxed at a rate of 15.5% and all other earnings were taxed at a rate of 8.0%.

As of December 31, 2020 and 2019, we have accrued income tax liabilities of $697.0 million under the Transition Toll Tax. Of the amounts accrued as of December 31, 2020, $62.0 million is expected to be paid within one year. The Transition Toll Tax will be paid over an eight--year period, which started in 2018, and does not accrue

interest.

Unremitted Earnings

At December 31, 2020, we considered our earnings not to be permanently reinvested outside the U.S. and therefore recorded deferred tax liabilities associated with an estimate of the total withholding taxes expected as a result of our repatriation of earnings. Other than for earnings, we are permanently reinvested for book/tax basis differences of approximately $1.5 billion as of December 31, 2020, primarily arising through the impacts of purchase accounting. These permanently reinvested basis differences could reverse through sales of the foreign subsidiaries, as well as various other events, none of which were considered probable as of December 31, 2020. The residual U.S. tax liability, if these differences reverse, would be between $300.0 million and $400.0 million as of December 31, 2020.

Coronavirus Aid, Relief and Economic Security Act

In response to the COVID-19 pandemic, the Coronavirus Aid, Relief and Economic Security Act (CARES Act) was signed into law in the U.S. in March 2020. The CARES Act adjusted a number of provisions of the tax code, including the calculation and eligibility of certain deductions and the treatment of net operating losses and tax credits. The enactment of the CARES Act did not result in any material adjustments to our income tax provision for the year ended December 31, 2020, or to our net deferred tax assets as of December 31, 2020.

TECFIDERA

In June 2020 and September 2020 judgments were entered in favor of the defendants in the patent infringement proceedings relating to TECFIDERA Orange-Book listed patents pursuant to the Drug Price Competition and Patent Term Restoration Act of 1984, commonly known as the Hatch-Waxman Act, in West Virginia and Delaware. We have appealed the judgments in both actions. For additional information, please read Note 20, Litigation, to these consolidated financial statements.

Multiple TECFIDERA generic entrants are now in the U.S. market and have deeply discounted prices compared to TECFIDERA. The generic competition for TECFIDERA significantly reduced our TECFIDERA revenues during the year ended December 31, 2020, and is expected to have a substantial negative impact on our TECFIDERA revenues for as long as there is generic competition.

As of December 31, 2020, we have assessed the realizability of our deferred tax assets that are dependent on future expected sales of TECFIDERA in the U.S. and reduced the net value of certain deferred tax assets by approximately $1.7 billion and reduced the net value of deferred tax liabilities associated with GILTI and tax credits by approximately $1.6 billion. For the year ended December 31, 2020, the income tax expense associated with these reductions was approximately $90.3 million.

Deferred Tax Assets and Liabilities

Significant components of our deferred tax assets and liabilities are summarized as follows:


	As of December 31,
(In millions)	2020		2019
Deferred tax assets:
Tax credits	$	113.4	$	106.6
Inventory, other reserves and accruals	165.9		162.0
Intangibles, net	1,546.0		3,380.0
Net operating loss	2,080.3		130.4
Share-based compensation	23.3		23.8
Other	103.1		103.7
Valuation allowance	(1,753.9)		(1.1)
Total deferred tax assets	$	2,278.1	$	3,905.4
Deferred tax liabilities:
Purchased intangible assets	$	(396.2)	$	(350.3)
GILTI	(1,143.7)		(1,381.6)
Tax credits	(174.6)		(1,617.2)
Depreciation, amortization and other	(227.0)		(135.0)
Total deferred tax liabilities	$	(1,941.5)	$	(3,484.1)

The change in the valuation allowance between December 31, 2020 and 2019, was primarily related to the establishment of a valuation allowance against certain deferred tax assets, the realization of which is dependent on future sales of TECFIDERA in the U.S., as discussed above.

In addition to deferred tax assets and liabilities, we have recorded deferred charges related to intra-entity sales of inventory. As of December 31, 2020 and 2019, the total deferred charges were $142.2 million and $243.8 million, respectively.

Tax Rate

A reconciliation between the U.S. federal statutory tax rate and our effective tax rate is summarized as follows:


	For the Years Ended December 31,
	2020		2019		2018
Statutory rate	21.0	%	21.0	%	21.0	%
State taxes	0.7		0.8		0.6
Taxes on foreign earnings	(3.3)		(4.5)		(1.9)
Credits and net operating loss utilization	(1.2)		(1.1)		(0.9)
Purchased intangible assets	0.7		0.4		1.2
Divestiture of Denmark manufacturing operations	(0.4)		1.0		—
Internal reorganization of certain intellectual property rights	—		(2.1)		—
TECFIDERA impairment	1.8		—		—
GILTI	1.3		1.5		1.6

U.S. tax reform	—		—		2.1
Swiss tax reform	—		(0.8)		—


Other	(0.9)		0.1		0.5
Effective tax rate	19.7	%	16.3	%	24.2	%

Changes in Tax Rate

For the year ended December 31, 2020, our effective tax rate was primarily increased by the income tax expense related to the establishment of a valuation allowance against certain deferred tax assets, the realization of which is dependent on future sales of TECFIDERA in the U.S., as discussed above, and partially offset by the benefit recognized on the effective settlement of certain tax matters. Additionally, our 2019 effective tax rate benefited from

an internal reorganization of certain intellectual property rights and the enactment of a new taxing regime in the country and certain cantons of Switzerland, which we refer to as Swiss Tax Reform, partially offset by tax expense related to the divestiture of our Hillerød, Denmark manufacturing operations. Although we recognized a loss on the divestiture of our Hillerød, Denmark manufacturing operations, the divestiture required us to write-off certain deferred tax assets and resulted in a taxable gain in certain jurisdictions. For additional information on the divestiture of our Hillerød, Denmark manufacturing operations, please read Note 3, Divestitures, to these consolidated financial statements.

As a result of the 2019 internal reorganization of certain intellectual property rights, we recorded a deferred tax asset of $754.1 million and a deferred tax liability of $603.3 million as of December 31, 2019.

For the year ended December 31, 2019, as compared to 2018, the decrease in our effective tax rate was primarily due to the combination of the internal reorganization of certain intellectual property rights and the impact of the Swiss Tax Reform. This decrease was partially offset by a $68.9 million tax expense related to the divestiture of our subsidiary that owned our Hillerød, Denmark manufacturing operations. We also had a higher effective tax rate in 2018 resulting from the unfavorable effects of the 2017 Tax Act and our sale of inventory, the tax effect of which had been included within prepaid taxes at January 1, 2018, at a higher effective tax rate than the 2018 statutory tax rate.

Tax Attributes

As of December 31, 2020, we had net operating losses and general business credit carry forwards for U.S. federal income tax purposes of approximately $0.6 million and $1.3 million, respectively, which begin to expire in 2026. For U.S. state income tax purposes, we had research and investment credit carry forwards of approximately $142.4 million that begin to expire in 2022. For foreign income tax purposes, we had $18.0 billion of Swiss federal net operating loss carryforwards that begin to expire in 2025 and $16.8 billion of Swiss cantonal net operating loss carryforwards that begin to expire in 2025.

In assessing the realizability of our deferred tax assets, we have considered whether it is more likely than not that some portion or all of the deferred tax assets will not be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible. In making this determination, under the applicable financial reporting standards, we are allowed to consider the scheduled reversal of deferred tax liabilities, projected future taxable income and tax planning strategies. Based upon the level of historical taxable income and income tax liability and projections for future taxable income over the periods in which the deferred tax assets are utilizable, we believe it is more likely than not that we will realize the net benefits of the deferred tax assets of our wholly owned subsidiaries, net of the recorded valuation allowance. In the event that actual results differ from our estimates or we adjust our estimates in future periods, we may need to adjust or establish a valuation allowance, which could materially impact our consolidated financial position and results of operations.

Accounting for Uncertainty in Income Taxes

A reconciliation of the beginning and ending amount of our unrecognized tax benefits is summarized as follows:



(In millions)	2020		2019		2018
Beginning balance at January 1,	$	129.9	$	114.2	$	66.8
Additions based on tax positions related to the current period	1.5		5.3		0.5
Additions for tax positions of prior periods	51.7		17.2		58.7
Reductions for tax positions of prior periods	(63.6)		(10.3)		(13.6)
Statute expirations	(7.9)		(0.1)		(2.9)
Settlement refund (payment)	(35.9)		3.6		4.7
Ending balance ar December 31,	$	75.7	$	129.9	$	114.2

Our 2020 activity reflects the impact of the effective settlement of certain tax matters. We and our subsidiaries are routinely examined by various taxing authorities. We file income tax returns in various U.S. states and in U.S. federal and other foreign jurisdictions. With few exceptions, we are no longer subject to U.S. federal tax examination for years before 2017 or state, local or non-U.S. income tax examinations for years before 2012.

The U.S. Internal Revenue Service and other national tax authorities routinely examine our intercompany transfer pricing with respect to intellectual property related transactions and it is possible that they may disagree with one or more positions we have taken with respect to such valuations.

Included in the balance of unrecognized tax benefits as of December 31, 2020, 2019 and 2018, are $68.8 million, $122.7 million and $109.1 million (net of the federal benefit on state issues), respectively, of unrecognized tax benefits that, if recognized, would affect the effective income tax rate in future periods.

We recognize potential interest and penalties related to unrecognized tax benefits in income tax expense. In 2020, 2019 and 2018 we recognized a net interest and penalty expense of $1.0 million, $4.7 million and $2.2 million, respectively. We have accrued $21.2 million and $20.0 million for the payment of interest and penalties as of December 31, 2020 and 2019, respectively.

Federal and State Uncertain Tax Positions

It is reasonably possible that we will adjust the value of our uncertain tax positions related to certain transfer pricing, collaboration matters and other issues as we receive additional information from various taxing authorities, including reaching settlements with such authorities.

We estimate that it is reasonably possible that our gross unrecognized tax benefits, exclusive of interest, could decrease by up to approximately $25.0 million in the next 12 months as a result of various audit closures, settlements and expiration of the statute of limitations.

Accounting for Bioverativ Spin-off

On February 1, 2017, in connection with the spin-off of our hemophilia business, we distributed all of the then outstanding shares of Bioverativ common stock to Biogen shareholders pursuant to a separation agreement. In March 2018 Bioverativ was acquired by Sanofi S.A. (Sanofi) and is now an indirect wholly-owned subsidiary of Sanofi. The spin-off of our hemophilia business was intended to qualify for tax-free treatment to Biogen and its shareholders under the Internal Revenue Code. Our 2017 tax return position remains open to audit. Bioverativ and Sanofi agreed to indemnify us for certain potential liabilities that may arise.

Organization, Consolidation and Presentation of Financial Statements [Abstract]

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References + Details

Other Consolidated Financial Statement Detail

12 Months Ended

Dec. 31, 2020

Other Consolidated Financial Statement Detail

Supplemental Cash Flow Information

Supplemental disclosure of cash flow information for the years ended December 31, 2020, 2019 and 2018, is as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Cash paid during the year for:
Interest	$	272.7	$	244.2	$	243.2
Income taxes	906.7		1,064.5		1,007.1

Non-cash Operating, Investing and Financing Activity

In the fourth quarter of 2018 we accrued $300.0 million upon reaching $20.0 billion in total cumulative sales of FUMADERM and TECFIDERA (together, the Fumapharm Products), which was paid in the first quarter of 2019. These amounts, net of tax benefit, were accounted for as increases to goodwill in accordance with the accounting standard applicable to business combinations when we acquired Fumapharm AG.

In connection with the construction of our large-scale biologics manufacturing facility in Solothurn, Switzerland, we accrued charges related to processing equipment and engineering services of approximately $12.4 million and $50.0 million in our consolidated balance sheets as of December 31, 2020 and 2019, respectively. For additional information on the construction of our manufacturing facility in Solothurn, Switzerland, please read Note 10, Property, Plant and Equipment, to these consolidated financial statements.

Other Income (Expense), Net

Components of other income (expense), net, are summarized as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Interest income	$	42.0	$	120.0	$	112.5
Interest expense	(222.5)		(187.4)		(200.6)
Gain (loss) on investments, net	685.7		204.7		119.5
Foreign exchange gains (losses), net	(10.7)		(7.0)		(9.9)
Other, net	2.9		(47.0)		(10.5)
Total other income (expense), net	$	497.4	$	83.3	$	11.0

Gain (loss) on investments, net, as reflected in the table above, relate to debt securities, equity securities of certain biotechnology companies, venture capital funds where the underlying investments are in equity securities of certain biotechnology companies and non-marketable equity securities.

For the year ended December 31, 2020, net unrealized and realized gains on our holdings in equity securities were approximately $681.8 million and $12.1 million, respectively, compared to net unrealized and realized gains of $150.1 million and $50.0 million in 2019. The net unrealized gains recognized during the year ended December 31, 2020, primarily reflects an increase in the fair value of Denali and Sangamo common stock of approximately $703.9 million.

The following table summarizes our gain (loss) on investments, net that relates to our equity securities held as of December 31, 2020, 2019 and 2018:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Net gains (losses) recognized during the period on equity securities	$	693.9	$	200.1	$	127.9
Less: Net gains (losses) recognized on equity securities sold during the period and on capital distributions	12.1		50.0		(0.6)
Unrealized gains (losses) recognized during the period on equity securities	$	681.8	$	150.1	$	128.5

Accrued Expenses and Other

Accrued expenses and other consists of the following:


	As of December 31,
(In millions)	2020		2019
Revenue-related reserves for discounts and allowances	$	1,080.6	$	1,001.1
Collaboration expenses	389.9		281.6
Employee compensation and benefits	333.8		309.1
Royalties and licensing fees	218.5		220.9
Derivative liabilities	181.5		6.7
Current portion of contingent consideration obligations	149.6		148.4

Other	791.4		798.0
Total accrued expenses and other	$	3,145.3	$	2,765.8

Other Long-term Liabilities

Other long-term liabilities were $1,329.6 million and $1,348.9 million as of December 31, 2020 and 2019, respectively, and include accrued income taxes totaling $709.9 million and $803.3 million, respectively.

Collaborative and Other Relationships

12 Months Ended

Dec. 31, 2020

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Collaborative and Other Relationships

Collaborative and Other RelationshipsIn connection with our business strategy, we have entered into various collaboration agreements that provide us with rights to develop, produce and market products using certain know-how, technology and patent rights maintained by our collaborative partners. Terms of the various collaboration agreements may require us to make milestone payments upon the achievement of certain product research and development objectives and pay royalties on future sales, if any, of commercial products resulting from the collaboration.

Depending on the collaborative arrangement, we may record funding receivable or payable balances with our collaboration partners, based on the nature of the cost-sharing mechanism and activity within the collaboration. Our significant collaborative arrangements are discussed below.

Genentech, Inc. (Roche Group)

We have certain business and financial rights with respect to RITUXAN for the treatment of non-Hodgkin's lymphoma, CLL and other conditions; RITUXAN HYCELA for the treatment of non-Hodgkin's lymphoma and CLL; GAZYVA for the treatment of CLL and follicular lymphoma; OCREVUS for the treatment of PPMS and RMS; and other potential anti-CD20 therapies pursuant to our collaboration arrangements with Genentech, a wholly-owned member of the Roche Group. For purposes of this footnote, we refer to RITUXAN and RITUXAN HYCELA collectively as RITUXAN.

Our collaboration arrangements will continue in effect until we mutually agree to terminate the collaboration, except that if we undergo a change in control, as defined in our collaboration agreement, Genentech has the right to present an offer to buy the rights to RITUXAN and we must either accept Genentech’s offer or purchase Genentech’s rights on the same terms as its offer. Genentech will also be deemed concurrently to have purchased our rights to OCREVUS and any other collaboration anti-CD20 products in development in exchange for a royalty as well as our rights to GAZYVA in exchange for the compensation described in the collaboration arrangement. Our collaboration with Genentech was created through a contractual arrangement and not through a joint venture or other legal entity.

RITUXAN

Genentech and its affiliates are responsible for the worldwide manufacture of RITUXAN as well as all development and commercialization activities as follows:

U.S.

We have co-exclusively licensed our rights to develop, commercialize and market RITUXAN in the U.S.

Canada

We have co-exclusively licensed our rights to develop, commercialize and market RITUXAN in Canada.

GAZYVA

The Roche Group and its sub-licensees maintain sole responsibility for the development, manufacture and commercialization of GAZYVA in the U.S. We recognize our share of the development and commercialization expenses of GAZYVA as a reduction of our share of pre-tax profits in revenues from anti-CD20 therapeutic programs.

Commercialization of GAZYVA impacts our percentage of the co-promotion profits for RITUXAN, as summarized in the table below.

OCREVUS

In March 2017 the FDA approved OCREVUS for the treatment of RMS and PPMS. Pursuant to the terms of our collaboration arrangements with Genentech, we receive a tiered royalty on U.S. net sales from 13.5% and increasing up to 24.0% if annual net sales exceed $900.0 million. There will be a 50.0% reduction to these royalties if a biosimilar to OCREVUS is approved in the U.S.

In addition, we receive a gross 3.0% royalty on net sales of OCREVUS outside the U.S., with the royalty period lasting 11 years from the first commercial sale of OCREVUS on a country-by-country basis. OCREVUS has been approved for the treatment of RMS and PPMS in the E.U. and certain other countries.

The commercialization of OCREVUS does not impact the percentage of the co-promotion profits we receive for RITUXAN or GAZYVA. Genentech is solely responsible for development and commercialization of OCREVUS and funding future costs. Genentech cannot develop OCREVUS in CLL, non-Hodgkin's lymphoma or rheumatoid arthritis. OCREVUS royalty revenues were based on our estimates from third party and market research data of OCREVUS sales occurring during the corresponding period. Differences between actual and estimated royalty revenues will be adjusted for in the period in which they become known, which is generally expected to be the following quarter.

Profit-sharing Formulas

RITUXAN Profit Share

Our current pretax co-promotion profit-sharing formula for RITUXAN provides for a 30.0% share on the first $50.0 million of co-promotion operating profits earned each calendar year. Our share of annual co-promotion profits in excess of $50.0 million varies, as summarized in the table below, upon the following events:


Until GAZYVA First Non-CLL FDA Approval	40.0	%
After GAZYVA First Non-CLL FDA Approval until First GAZYVA Threshold Date	39.0	%
After First GAZYVA Threshold Date until Second GAZYVA Threshold Date	37.5	%
After Second GAZYVA Threshold Date	35.0	%

First Non-CLL GAZYVA FDA Approval means the FDA’s first approval of GAZYVA in an indication other than CLL.

First GAZYVA Threshold Date means the earlier of (i) the date of the First Non-CLL GAZYVA FDA approval if U.S. gross sales of GAZYVA for the preceding consecutive 12-month period were at least $150.0 million or (ii) the first day of the calendar quarter after the date of the First Non-CLL GAZYVA FDA Approval that U.S. gross sales of GAZYVA within any consecutive 12-month period have reached $150.0 million.

Second GAZYVA Threshold Date means the first day of the calendar quarter after U.S. gross sales of GAZYVA within any consecutive 12-month period have reached $500.0 million. The Second GAZYVA Threshold Date can be achieved regardless of whether GAZYVA has been approved in a non-CLL indication.

Our share of RITUXAN pre-tax profits in the U.S. in excess of $50.0 million for the years ended December 31, 2020, 2019 and 2018, was 37.5%.

In addition, should the FDA approve an anti-CD20 product other than OCREVUS or GAZYVA that is acquired or developed by Genentech and subject to the collaboration agreement, our share of the co-promotion operating profits would be between 30.0% and 37.5% based on certain events.

GAZYVA Profit Share

Our current pretax profit-sharing formula for GAZYVA provides for a 35.0% share on the first $50.0 million of operating profits earned each calendar year. Our share of annual profits in excess of $50.0 million varies, as summarized in the table below, upon the following events:


Until First GAZYVA Threshold Date	39.0	%
After First GAZYVA Threshold Date until Second GAZYVA Threshold Date	37.5	%
After Second GAZYVA Threshold Date	35.0	%

Our share of GAZYVA pre-tax profits in excess of $50.0 million for the years ended December 31, 2020, 2019 and 2018, was 37.5%.

In November 2017 the FDA approved GAZYVA in combination with chemotherapy, followed by GAZYVA alone, for people with previously untreated advanced follicular lymphoma.

Revenues from Anti-CD20 Therapeutic Programs

Revenues from anti-CD20 therapeutic programs are summarized as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA, including the reimbursement of selling and development expenses	$	1,080.2	$	1,542.4	$	1,431.9
Other revenues from anti-CD20 therapeutic programs	897.6		748.0		548.3
Total revenues from anti-CD20 therapeutic programs	$	1,977.8	$	2,290.4	$	1,980.2

Prior to regulatory approval, we record our share of the expenses incurred by the collaboration for the development of anti-CD20 products in research and development expense in our consolidated statements of income.

After an anti-CD20 product is approved, we record our share of the development expenses related to that product as a reduction of our share of pre-tax profits in revenues from anti-CD20 therapeutic programs.

Ionis Pharmaceuticals, Inc.

SPINRAZA

In January 2012 we entered into a collaboration and license agreement with Ionis pursuant to which we have an exclusive, worldwide license to develop and commercialize SPINRAZA for the treatment of SMA.

Under our agreement with Ionis, we make royalty payments to Ionis on annual worldwide net sales of SPINRAZA using a tiered royalty rate between 11.0% and 15.0%, which are recognized in cost of sales within our consolidated statements of income. Royalty cost of sales related to sales of SPINRAZA for the years ended December 31, 2020, 2019 and 2018, totaled $286.6 million, $293.0 million and $238.0 million, respectively.

2012 Ionis Agreement

In December 2012 we entered into an agreement with Ionis for the development and commercialization of up to three gene targets.

Under this agreement, Ionis is responsible for global development of any product candidate through the completion of a Phase 2 trial and we will provide advice on the clinical trial design and regulatory strategy. We have an option to license the product candidate until completion of the Phase 2 trial. If we exercise our option, we will pay a license fee of up to $70.0 million to Ionis and assume global development, regulatory and commercialization responsibilities. Ionis is eligible to receive up to $130.0 million in additional milestone payments upon the achievement of certain regulatory milestones as well as royalties on future sales if we successfully develop the product candidate after option exercise.

Upon entering into this agreement, we made an upfront payment of $30.0 million to Ionis and agreed to make potential additional payments, prior to licensing, of up to $10.0 million based on the development of the selected product candidate as well as a mark-up of the cost estimate of the Phase 1 and Phase 2 trials. During 2015 we recognized this $10.0 million developmental milestone upon the selection of BIIB080 (tau ASO), which is currently in Phase 1 development for the potential treatment of Alzheimer's disease.

In December 2019 we exercised our option with Ionis and obtained a worldwide, exclusive, royalty-bearing license to develop and commercialize BIIB080. In connection with the option exercise, we made a payment of $45.0 million to Ionis, which was recorded as research and development expense in our consolidated statements of income. Future payments may include additional milestone payments of up to $155.0 million and royalties on future sales in the low- to mid-teens if we successfully develop the product candidate after option exercise.

2018 Ionis Agreement

In June 2018 we closed a 10-year exclusive collaboration agreement with Ionis to develop novel antisense oligonucleotide (ASO) drug candidates for a broad range of neurological diseases (2018 Ionis Agreement) for a total payment of $1.0 billion, consisting of an upfront payment of $375.0 million and the purchase of approximately 11.5 million shares of Ionis common stock at a cost of $625.0 million.

Upon closing, we recorded $50.9 million of the $375.0 million upfront payment as prepaid services in our consolidated balance sheets and recognized the remaining $324.1 million as research and development expense in our consolidated statements of income. The amount recorded as prepaid services represented the value of the employee resources committed to the arrangement to provide research and discovery services over the term of the agreement.

The 11.5 million shares of Ionis common stock were purchased at a premium to their fair value at the transaction closing date. The premium consisted of acquiring the shares at a price above the fair value based on the trailing 10-day weighted-average close price prior to entering into the 2018 Ionis Agreement in April 2018 and the effect of certain holding period restrictions. We recorded an asset of $462.9 million in investments and other assets in our consolidated balance sheets reflecting the fair value of the Ionis common stock as of the purchase date and a charge of $162.1 million to research and development expense in our consolidated statements of income in the second quarter of 2018 reflecting the premium paid for the Ionis common stock.

Our investment in Ionis common stock is remeasured each reporting period. Changes in the fair value of our investment in Ionis common stock, including the effect of the holding period restrictions, are reflected in other income (expense), net in our consolidated statements of income. For additional information on the fair value of our investment in Ionis common stock, please read Note 7, Fair Value Measurements, to these consolidated financial statements.

We have the option to license therapies arising out of the 2018 Ionis Agreement and will be responsible for the development and commercialization of such therapies. We may pay development milestones to Ionis of up to $125.0 million or $270.0 million for each program, depending on the indication plus an annual license fee, as well as royalties on potential net commercial sales.

During the years ended December 31, 2020 and 2019, we incurred milestones of $11.3 million and $30.0 million, respectively, related to the advancement of neurological targets identified under the 2018 Ionis Agreement.

2017 SMA Collaboration Agreement

In December 2017 we entered into a collaboration agreement with Ionis to identify new ASO drug candidates for the potential treatment of SMA. Under this agreement, we have the option to license therapies arising out of this collaboration and will be responsible for their development and commercialization of such therapies.

Upon entering into this agreement, we made a $25.0 million upfront payment to Ionis and we may pay Ionis up to $260.0 million in additional development and regulatory milestone payments if new drug candidates advance to marketing approval. Upon commercialization, we may also pay Ionis up to $800.0 million in additional performance-based milestone payments and tiered royalties on potential net sales of such therapies.

2013 Long-term Strategic Research Agreement

In September 2013 we entered into a six-year research collaboration agreement with Ionis under which both companies collaborate to perform discovery level research and subsequent development and commercialization activities of antisense or other therapeutics for the potential treatment of neurological diseases. Under this agreement, Ionis performs research on a set of neurological targets identified within the agreement.

Ionis is eligible to receive milestone payments, license fees and royalty payments for all product candidates developed through this collaboration, with the specific amount dependent upon the modality of the product candidate advanced by us under the terms of the agreement.

For non-ALS antisense product candidates, Ionis is responsible for global development through the completion of a Phase 2 trial and we provide advice on the clinical trial design and regulatory strategy. For ALS antisense product candidates, we are responsible for global development, clinical trial design and regulatory strategy. We have an option to license a product candidate until completion of the Phase 2 trial. If we exercise our option, we will pay Ionis up to a $70.0 million license fee and assume global development, regulatory and commercialization responsibilities. Ionis could receive additional milestone payments upon the achievement of certain regulatory milestones of up to $130.0 million, plus additional amounts related to the cost of clinical trials conducted by Ionis under the collaboration, and royalties on future sales if we successfully develop the product candidate after option exercise.

In December 2018 we exercised our option with Ionis and obtained a worldwide, exclusive, royalty-bearing license to develop and commercialize BIIB067 (tofersen), an investigational treatment for ALS with superoxide dismutase 1 (SOD1) mutations. In connection with the option exercise, we made a payment of $35.0 million to Ionis, which was recorded as research and development in our consolidated statements of income. Future payments may include potential post-licensing milestone payments of up to $55.0 million and royalties in the low- to mid-teen percentages on potential annual worldwide net sales. We are solely responsible for the costs and expenses related to the development, manufacturing and commercialization of tofersen following the option exercise.

During the years ending December 31, 2020, 2019 and 2018, we incurred milestones of $28.0 million, $20.0 million and $18.0 million, respectively, related to the advancement of programs under this agreement, which were recorded as research and development expense in our consolidated statements of income.

Eisai Co., Ltd.

BAN2401 Collaboration

We have a collaboration agreement with Eisai to jointly develop and commercialize BAN2401, a monoclonal antibody that targets amyloid beta aggregates, and elenbecestat, the oral BACE (base amyloid cleaving enzyme) inhibitor, two Eisai product candidates for the potential treatment of Alzheimer's disease (the BAN2401 Collaboration). In September 2019 we and Eisai discontinued the global Phase 3 studies of elenbecestat in early Alzheimer's disease.

Eisai serves as the global operational and regulatory lead for BAN2401 and all costs, including research, development, sales and marketing expenses, are shared equally between us and Eisai. If BAN2401 receives marketing approval, we and Eisai will co-promote BAN2401 and share profits equally. In addition, the BAN2401 Collaboration provides both parties with certain rights and obligations in the event of a change in control of either party.

The BAN2401 Collaboration also provided Eisai with an option to jointly develop and commercialize aducanumab (Aducanumab Option) and an option to jointly develop and commercialize one of our anti-tau monoclonal antibodies (Anti-Tau Option). In October 2017 Eisai exercised its Aducanumab Option and we entered into a new collaboration agreement for the joint development and commercialization of aducanumab (the Aducanumab Collaboration Agreement).

Eisai may exercise the Anti-Tau Option after completion of the Phase 1 clinical trial of such anti-tau monoclonal antibody. If Eisai exercises its Anti-Tau Option, we will receive an upfront payment from Eisai and will be entitled to additional development and commercial milestone payments. Eisai has not yet exercised its Anti-Tau Option.

A summary of development and sales and marketing expenses related to the BAN2401 Collaboration is as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Total development expense incurred by the collaboration related to the advancement of BAN2401 and elenbecestat	$	219.3	$	348.7	$	232.0
Biogen's share of BAN2401 and elenbecestat development expense reflected in research and development expense in our consolidated statements of income	109.6		174.3		116.0
Total sales and marketing expense incurred by the collaboration	9.8		32.4		10.7
Biogen's share of BAN2401 and elenbecestat sales and marketing expense reflected in selling, general and administrative expense in our consolidated statements of income	4.9		16.2		5.4

Aducanumab Collaboration Agreement

Under the Aducanumab Collaboration Agreement, we and Eisai will co-promote aducanumab with a region-based profit split and we lead the ongoing development of aducanumab.

In March 2019, based on a pre-specified futility analysis, we discontinued the global Phase 3 trials, EMERGE and ENGAGE, designed to evaluate the efficacy and safety of aducanumab in patients with early Alzheimer's disease. A new analysis of a larger dataset from these trials, conducted in scientific collaboration with the FDA, showed that the Phase 3 EMERGE study met its pre-specified primary and secondary endpoints. In the first quarter of 2019, as a result of the decision to discontinue the Phase 3 EMERGE and ENGAGE trials following the futility analysis, we accrued and subsequently paid approximately $45.0 million related to the termination of various clinical trials and research and development contracts net of the expected 45.0% Eisai reimbursement of development costs incurred under the Aducanumab Collaboration Agreement. In October 2019 we and Eisai announced that we plan to pursue regulatory approval for aducanumab in the U.S. In July 2020 we completed the submission of a BLA for the approval of aducanumab to the FDA.

For the period through March 31, 2018, we were responsible for 100.0% of development costs incurred by the collaboration for the advancement of aducanumab (aducanumab development expense). For the period April 1, 2018 through December 31, 2018, Eisai reimbursed us for 15.0% of aducanumab development expense incurred and beginning January 1, 2019, is reimbursing us for 45.0% of aducanumab development expense incurred.

For the year ended December 31, 2020, we recognized net profit-sharing income of $33.8 million to reflect Eisai's 45.0% share of the $75.0 million milestone expense related to the submission of the BLA for the approval of aducanumab to the FDA.

Upon commercialization, both companies will co-promote aducanumab with a region-based profit split. We will receive a 55.0% share of the potential profits (losses) in the U.S., a 68.5% share of the potential profits (losses) in the European Union (E.U.) and a 20.0% share of the potential profits (losses) in Japan and Asia, excluding China and South Korea. The two companies will continue to share equally in the potential profits (losses) in rest of world markets. Sales and marketing expense incurred before commercialization are shared in proportion to the same region-based profit split that will be utilized to co-promote aducanumab.

A summary of development, sales and marketing and milestone expense related to the Aducanumab Collaboration Agreement is as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Total aducanumab development expense	$	152.0	$	179.4	$	264.8
Biogen's share of aducanumab development expense reflected in research and development expense in our consolidated statements of income	83.6		98.7		234.6
Total aducanumab sales and marketing expense incurred by the collaboration	353.0		27.4		50.6
Biogen's share of aducanumab sales and marketing expense reflected in selling, general and administrative expense in our consolidated statements of income	193.7		15.1		27.3
Total aducanumab collaboration third party milestone expense	75.0		—		—
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income	33.8		—		—

In addition, we and Eisai co-promote AVONEX, TYSABRI and TECFIDERA in Japan in certain settings and Eisai distributes AVONEX, TYSABRI, TECFIDERA and PLEGRIDY in India and other Asia-Pacific markets, excluding China.

UCB Pharma S.A.

We have a collaboration agreement with UCB Pharma S.A. (UCB) to jointly develop and commercialize dapirolizumab pegol, an anti-CD40L pegylated Fab, for the potential treatment of systemic lupus erythematosus and other future agreed indications. Either we or UCB may propose development of dapirolizumab pegol in additional indications. If the parties do not agree to add an indication as an agreed indication to the collaboration, we or UCB may, at the sole expense of the applicable party, pursue development in such excluded indication(s), subject to an opt-in right of the non-pursuing party after proof of clinical activity.

All costs incurred for agreed indications, including research, development, sales and marketing expenses, are shared equally between us and UCB. Upon marketing approval, we and UCB will co-promote dapirolizumab pegol and share profits equally. A summary of development expense related to the UCB collaboration agreement is as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Total UCB development expense	$	58.3	$	31.9	$	29.7
Biogen's share of UCB development expense reflected in research and development expense in our consolidated statements of income	29.2		16.0		14.9

Alkermes

In November 2017 we entered into an exclusive license and collaboration agreement with Alkermes Pharma Ireland Limited, a subsidiary of Alkermes plc (Alkermes), for VUMERITY, a novel fumarate for the treatment of RMS. In October 2019 the FDA approved VUMERITY in the U.S. for the treatment of RMS. In November 2019 VUMERITY became commercially available in the U.S.

Under this agreement, we received an exclusive, worldwide license to develop and commercialize VUMERITY and we pay Alkermes a royalty of 15.0% on worldwide net commercial sales of VUMERITY. Royalties payable on net commercial sales of VUMERITY are subject, under certain circumstances, to tiered minimum annual payment requirements for a period of five years following FDA approval.

Alkermes is eligible to receive royalties in the high-single digits to sub-teen double digits of annual net commercial sales upon successful development and commercialization of new product candidates, other than VUMERITY, developed under the exclusive license from Alkermes.

During the fourth quarter of 2019, following the FDA's approval of VUMERITY, we paid Alkermes $155.0 million in milestone payments, which were recorded in intangible assets in our consolidated balance sheets and will be amortized over the useful life of the product. For the years ended December 31, 2020, 2019 and 2018, we recorded $32.4 million, $53.5 million and $68.7 million, respectively, in research and development expense in our consolidated statements of income related to this collaboration.

Alkermes currently supplies VUMERITY to us pursuant to a supply agreement. In October 2019 we entered into a new supply agreement and amended our license and collaboration agreement with Alkermes. We have elected to initiate a technology transfer and, following a transition period, to manufacture VUMERITY or have VUMERITY manufactured by a third party we have engaged in exchange for paying an increased royalty rate to Alkermes on any portion of future worldwide net commercial sales of VUMERITY that is manufactured by us or our designee.

Bristol-Myers Squibb Company

In June 2017 we completed an exclusive license agreement with Bristol-Myers Squibb Company (BMS) for the development and potential commercialization of BIIB092 (gosuranemab), an antibody targeting tau, the protein that forms the deposits, or tangles, in the brain associated with Alzheimer's disease.

Under this agreement, we received worldwide rights to gosuranemab and are responsible for the full development and potential commercialization of gosuranemab in Alzheimer's disease and progressive supranuclear palsy (PSP).

In December 2019 we announced that the Phase 2 PASSPORT study investigating gosuranemab in individuals with PSP did not meet its primary endpoint. Based on these results, we discontinued development of gosuranemab in PSP and other primary tauopathies. We will continue our ongoing Phase 2 TANGO study of gosuranemab for mild cognitive impairment due to Alzheimer's disease or mild Alzheimer's disease, given differences in disease pathology.

Upon entering into this agreement, we made an upfront payment of $300.0 million to BMS and assumed all remaining obligations to the former shareholders of iPierian, Inc. (iPierian) related to BMS’s acquisition of iPierian in 2014. We may pay BMS up to $360.0 million in additional milestone payments, and potential royalties, and we may pay the former shareholders of iPierian up to $370.0 million in remaining milestone payments as well as potential royalties on net commercial sales.

For the years ended December 31, 2020, 2019 and 2018, we recorded $62.4 million, $144.0 million and $97.0 million, respectively, in research and development expense in our consolidated statements of income related to this agreement.

Acorda Therapeutics, Inc.

In June 2009 we entered into a collaboration and license agreement with Acorda Therapeutics, Inc. (Acorda) to develop and commercialize products containing fampridine, such as FAMPYRA, in markets outside the U.S. We are responsible for all regulatory activities and the future clinical development of related products in those markets.

Under this agreement, we pay tiered royalties based on the level of ex-U.S. net sales and we may pay potential milestone payments based on the successful achievement of certain regulatory and commercial milestones, which would be capitalized as intangible assets upon achievement of the milestones and amortized utilizing an economic consumption model. The next expected milestone of $15.0 million, due if ex-U.S. net sales reach $100.0 million over a period of four consecutive quarters, was recognized during the third quarter of 2020 and capitalized within intangible assets, net in our consolidated balance sheet. Royalty payments are recognized in cost of sales within our consolidated statements of income.

In connection with the collaboration and license agreement, we also entered into a supply agreement with Acorda for the commercial supply of FAMPYRA. This agreement is a sublicense arrangement of an existing agreement between Acorda and Alkermes Inc., who acquired Elan Drug Technologies, the original party to the license with Acorda.

For the years ending December 31, 2020, 2019 and 2018, total cost of sales related to royalties and commercial supply of FAMPYRA reflected in our consolidated statements of income were $44.5 million, $42.0 million and $36.5 million, respectively.

Sage Therapeutics, Inc.

In November 2020 we entered into a global collaboration and license agreement with Sage to jointly develop and commercialize zuranolone (SAGE-217) for the potential treatment of major depressive disorder, postpartum depression and other psychiatric disorders and SAGE-324 for the potential treatment of essential tremor and other neurological disorders.

In connection of the closing of this transaction in December 2020 we purchased $650.0 million of Sage common stock, or approximately 6.2 million shares at approximately $104.14 per share, which are subject to transfer restrictions. We recorded an asset in investments and other assets in our consolidated balance sheets to reflect the initial fair value of the Sage common stock acquired and a charge of approximately $209.0 million to research and development expense in our consolidated statements of income to reflect the premium paid for the Sage common stock. We also made an upfront payment of $875.0 million that was recorded as research and development expense.

We may also pay Sage development and commercial milestone payments that could total up to approximately $1.6 billion if all the specified milestones set forth in this agreement are achieved. Both companies will share equal responsibility and costs for development as well as profits and losses for commercialization in the U.S. Outside of the U.S., we are responsible for development and commercialization, excluding Japan, Taiwan and South Korea, with respect to zuranolone and will pay Sage potential tiered royalties in the high teens to low twenties.

Denali Therapeutics Inc.

In August 2020 we entered into a collaboration and license agreement with Denali to co-develop and co-commercialize Denali's small molecule inhibitors of leucine-rich repeat kinase 2 (LRRK2) for Parkinson's disease. In addition to the LRRK2 program, we also have an exclusive option to license two preclinical programs from Denali’s Transport Vehicle platform, including its Antibody Transport Vehicle (ATV): ATV enabled anti-amyloid beta program and a second program utilizing its Transport Vehicle technology. Further, we have a right of first negotiation on two additional Transport Vehicle-enabled therapeutics, should Denali decide to seek a collaboration for such programs.

As part of this collaboration we purchased approximately $465.0 million of Denali common stock in September 2020, or approximately 13 million shares at approximately $34.94 per share, which are subject to transfer restrictions. We recorded an asset in investments and other assets in our consolidated balance sheets to reflect the initial fair value of the Denali common stock acquired and a charge of approximately $41.3 million to research and development expense in our consolidated statements of income to reflect the premium paid for the Denali common stock. We also made an upfront payment of $560.0 million that was recorded as research and development expense.

We may also pay Denali development and commercial milestone payments that could total up to approximately $1.1 billion if the milestones related to the LRRK2 program are achieved. Under this agreement, both companies share responsibility and costs for global development based on specified percentages and we are responsible for commercialization and will pay Denali potential tiered royalties.

For the year ended December 31, 2020, we recorded $8.8 million in research and development expense in our consolidated statements of income related to this collaboration.

Sangamo Therapeutics, Inc.

In February 2020 we entered into a collaboration and license agreement with Sangamo to develop and commercialize ST-501 for tauopathies, including Alzheimer's disease; ST-502 for synucleinopathies, including Parkinson’s disease; a third neuromuscular disease target; and up to nine additional neurological disease targets to be identified and selected within a five-year period. The companies are leveraging Sangamo’s proprietary zinc finger protein technology delivered via adeno-associated virus to modulate the expression of key genes involved in neurological diseases.

In connection with the closing of this transaction in April 2020 we purchased $225.0 million of Sangamo common stock, or approximately 24 million shares at approximately $9.21 per share, which are subject to transfer restrictions. We recorded an asset in investments and other assets in our consolidated balance sheets to reflect the initial fair value of the Sangamo common stock acquired and a charge of approximately $83.0 million to research

and development expense in our consolidated statements of income to reflect the premium paid for the Sangamo common stock. We also made an upfront payment of $125.0 million that was recorded as research and development expense.

We may also pay Sangamo research, development, regulatory and commercial milestone payments that could total up to approximately $2.4 billion if we select all of the targets allowed under this agreement and all the specified milestones set forth in this agreement are achieved. Of this amount, up to $80.0 million relates to the selection of targets, $1.9 billion relates to the achievement of specified research, clinical development, regulatory and first commercial sale milestones and $380.0 million relates to the achievement of specified sales-based milestones if annual worldwide net sales of licensed products reach specified levels. In addition, we will pay Sangamo tiered royalties on potential net commercial sales of any products developed under this collaboration in the high single digit to double digit sub-teen percentages.

For the year ended December 31, 2020, we recorded $6.4 million in research and development expense in our consolidated statements of income related to this collaboration.

Other Research and Discovery Arrangements

These arrangements may include the potential for future milestone payments based on the achievement of certain clinical and commercial development payable over a period of several years.

Other

For the years ended December 31, 2020, 2019 and 2018, we recorded $92.1 million, $77.0 million and $48.6 million, respectively, as research and development expense in our consolidated statements of income related to other research and discovery related arrangements.

Samsung Bioepis Co., Ltd.

Joint Venture Agreement

In February 2012 we entered into a joint venture agreement with Samsung BioLogics establishing an entity, Samsung Bioepis, to develop, manufacture and market biosimilar products. Samsung BioLogics contributed 280.5 billion South Korean won (approximately $250.0 million) for an 85.0% ownership interest in Samsung Bioepis and we contributed 49.5 billion South Korean won (approximately $45.0 million) for the remaining 15.0% ownership interest. In June 2018 we exercised our option under our joint venture agreement to increase our ownership percentage in Samsung Bioepis from approximately 5.0%, which reflected the effect of previous equity financings in which we did not participate, to approximately 49.9%. The share purchase transaction was completed in November 2018 and, upon closing, we paid 759.5 billion South Korean won ($676.6 million) to Samsung BioLogics. As of December 31, 2020, our ownership percentage remained at approximately 49.9%.

We recognize our share of the results of operations related to our investment in Samsung Bioepis under the equity method of accounting one quarter in arrears when the results of the entity become available, which is reflected as equity in income (loss) of investee, net of tax in our consolidated statements of income. During 2015, as our share of losses exceeded the carrying value of our initial investment, we suspended recognizing additional losses. In the first quarter of 2019 we restarted recognizing our share of Samsung Bioepis' income (losses), and we began recognizing amortization on certain basis differences resulting from our November 2018 investment.

Upon investment, the equity method of accounting requires us to identify and allocate differences between the fair value of our investment and the carrying value of our interest in the underlying net assets of the investee. These basis differences are amortized over their economic life. The total basis difference was approximately $675.0 million and relates to inventory, developed technology, IPR&D and deferred tax balances. The basis differences related to inventory were amortized, net of tax, over their estimated useful lives of 1.5 years, and the basis differences related to developed technology and IPR&D for marketed products will be amortized, net of tax, over their estimated useful lives of 15 years.

The former chief executive officer (the incumbent chairman of the board) and the chief financial officer of our joint venture partner, Samsung BioLogics, is currently subject to ongoing criminal proceedings that we continue to monitor. While these proceedings could impact the operations of Samsung Bioepis and its business, we have assessed the value of our investment in Samsung Bioepis and continue to believe that the fair value of the investment is in excess of its net book value.

For the year ended December 31, 2020, we recognized net income on our investment of $5.3 million, reflecting our share of income totaling $45.3 million offset by amortization of basis differences totaling $40.0 million.

For the year ended December 31, 2019, we recognized net losses on our investment of $79.4 million, reflecting our share of losses totaling $1.2 million and amortization of basis differences totaling $78.2 million.

2019 Transaction

In December 2019 we completed a transaction with Samsung Bioepis and secured the exclusive rights to commercialize two potential ophthalmology biosimilar products, SB11, a proposed ranibizumab biosimilar referencing LUCENTIS, and SB15, a proposed aflibercept biosimilar referencing EYLEA, in major markets worldwide, including the U.S., Canada, Europe, Japan and Australia. Samsung Bioepis will be responsible for development and will supply both products to us.

In connection with this transaction, we made an upfront payment of $100.0 million to Samsung Bioepis in January 2020, of which $63.0 million was recorded as research and development expense in 2019 and $37.0 million was recorded as an intangible asset in 2019. Additionally, during the third quarter of 2020, we paid Samsung Bioepis a $15.0 million development milestone, which was included in research and development expense in our consolidated statements of income. We may pay Samsung Bioepis up to $195.0 million in additional development, regulatory and sales-based milestones.

We also acquired an option to extend the term of our 2013 commercial agreement for BENEPALI, IMRALDI and FLIXABI by an additional five years, subject to payment of an option exercise fee of $60.0 million, and obtained an option to acquire exclusive rights to commercialize these products in China.

2013 Commercial Agreement

In December 2013 we entered into an agreement with Samsung Bioepis to commercialize, over a 10-year term, 3 anti-tumor necrosis factor (TNF) biosimilar product candidates in Europe and in the case of BENEPALI, Japan. As discussed above, we have an option to extend this agreement by an additional five years. Under this agreement, we have made upfront and clinical development milestone payments totaling $46.0 million, which were recorded as research and development expense in our consolidated statements of income as the programs they relate to had not achieved regulatory approval. We also agreed to make additional milestone payments of $25.0 million upon regulatory approval in the E.U. for each of the three anti-TNF biosimilar product candidates. IMRALDI, an adalimumab biosimilar referencing HUMIRA, FLIXABI, an infliximab biosimilar referencing REMICADE, and BENEPALI, an etanercept biosimilar referencing ENBREL, received regulatory approval in the E.U. in August 2017, May 2016 and January 2016, respectively, and we capitalized the related milestone payments totaling $75.0 million as intangible assets, net in our consolidated balance sheets.

In April 2018 we and Samsung Bioepis announced an agreement with AbbVie Inc. (AbbVie) related to the commercialization of IMRALDI. Under the terms of the agreement, AbbVie granted us and Samsung Bioepis patent licenses for the use and sale of IMRALDI in Europe, on a country-by-country basis, and we make royalty payments to AbbVie on behalf of Samsung Bioepis. We began to recognize revenues on sales of IMRALDI to third parties in Europe in the fourth quarter of 2018.

We reflect revenues on sales of BENEPALI, IMRALDI and FLIXABI to third parties in product revenues, net in our consolidated statements of income and record the related cost of revenues and sales and marketing expenses in our consolidated statements of income to their respective line items when these costs are incurred. Royalty payments to AbbVie on sales of IMRALDI are recognized in cost of sales within our consolidated statements of income.

We share 50% of the profit or loss related to our commercial agreement with Samsung Bioepis, which is recognized in collaboration profit (loss) sharing in our consolidated statements of income. For the years ended December 31, 2020, 2019 and 2018, we recognized a net profit-sharing expense of $266.5 million, $241.6 million and $187.4 million, respectively, to reflect Samsung Bioepis' 50% sharing of the net collaboration profits.

Other Services

Simultaneous with the formation of Samsung Bioepis, we also entered into a technical development services agreement, a manufacturing agreement and a license agreement with Samsung Bioepis.

Under the technical development services agreement, we provided Samsung Bioepis technical development and technology transfer services, which included, but were not limited to, cell culture development, purification process development, formulation development and analytical development.

Under the manufacturing agreement, we manufacture clinical and commercial quantities of bulk drug substance of biosimilar products for Samsung Bioepis pursuant to contractual terms.

Following the divestiture of our Hillerød, Denmark manufacturing operations in August 2019, FUJIFILM assumed responsibility for the manufacture of clinical and commercial quantities of bulk drug substance of biosimilar products for Samsung Bioepis. We no longer recognize revenues for the manufacturing completed after the divestiture date under the manufacturing agreements with Samsung Bioepis. For additional information on the divestiture of our Hillerød, Denmark manufacturing operations, please read Note 3, Divestitures, to these consolidated financial statements.

Under the license agreement, we granted Samsung Bioepis an exclusive license to use, develop, manufacture and commercialize biosimilar products created by Samsung Bioepis using Biogen product-specific technology. In exchange, we receive single digit royalties on biosimilar products developed and commercialized by Samsung Bioepis.

For the years ended December 31, 2020, 2019 and 2018, we recognized $20.9 million, $106.2 million and $96.4 million, respectively, in revenues under the license, technical development services and manufacturing agreements, which is reflected in revenues from collaborative and other relationships, as a component of other revenues in our consolidated statements of income.

Amounts receivable from Samsung Bioepis related to the agreements discussed above were $5.1 million and $85.0 million as of December 31, 2020 and 2019, respectively. Amounts payable to Samsung Bioepis as of December 31, 2020, were $99.0 million. Amounts payable to Samsung Bioepis as of December 31, 2019, consisted of the $100.0 million upfront payment related to the transaction we completed in December 2019, as discussed above.

Investments in Variable Interest Entities	12 Months Ended
Investments in Variable Interest Entities	Dec. 31, 2020
Investments in Variable Interest Entities [Abstract]
Investments in Variable Interest Entities	Investments in Variable Interest Entities Consolidated Variable Interest Entities Our consolidated financial statements include the financial results of variable interest entities in which we are the primary beneficiary. The following are our significant variable interest entities. Neurimmune SubOne AG We have a collaboration and license agreement with Neurimmune SubOne AG (Neurimmune) for the development and commercialization of antibodies for the potential treatment of Alzheimer's disease, including aducanumab (as amended, the Neurimmune Agreement). We are responsible for the development, manufacturing and commercialization of all collaboration products. The Neurimmune Agreement is effective for the longer of the duration of certain patents relating to a licensed product or 12 years from the first commercial sale of a licensed product. We consolidate the results of Neurimmune as we determined that we are the primary beneficiary of Neurimmune because we have the power through the collaboration to direct the activities that most significantly impact the entity’s economic performance and we are required to fund 100.0% of the research and development costs incurred in support of the collaboration. In October 2017 we amended the terms of the Neurimmune Agreement and made a $150.0 million payment to Neurimmune in exchange for a 15.0% reduction in the previously negotiated royalty rates payable on products developed under the Neurimmune Agreement, including royalties payable on potential commercial sales of aducanumab. In May 2018 we made an additional $50.0 million payment to Neurimmune to further reduce the previously negotiated royalty rates payable on products developed under the Neurimmune Agreement, including royalties payable on potential commercial sales of aducanumab, by an additional 5.0%. Our royalty rates payable on products developed under the Neurimmune Agreement, including royalty rates payable on potential commercial sales of aducanumab, now range from the high single digits to sub-teens. As we consolidate the results of Neurimmune, we treated these payments as distributions and recognized them as charges to noncontrolling interests in the fourth quarter of 2017 and the second quarter of 2018, as applicable. Under the terms of the Neurimmune Agreement, we were required to pay Neurimmune a milestone payment of $75.0 million upon the regulatory filing with the FDA for the approval of aducanumab. During the second quarter of 2020, we paid Neurimmune $75.0 million upon the completed submission of the BLA for the approval of aducanumab to the FDA, which was recognized as a charge to noncontrolling interests for the year ended December 31, 2020. In addition, for the year ended December 31, 2020, we recognized net profit-sharing income of $33.8 million to reflect Eisai's 45.0% share of the $75.0 million milestone expense. Additionally, if aducanumab receives regulatory approval in the jurisdictions where we have submitted filings, we may pay up to approximately $200.0 million in milestones to Neurimmune in 2021, which includes $100.0 million if launched in the U.S., $50.0 million if launched in three or more countries within the E.U. and $50.0 million if launched in Japan. Milestones payable to Neurimmune are shared expenses under the Aducanumab Collaboration Agreement with Eisai. Research and development costs for which we reimburse Neurimmune are reflected in research and development expense in our consolidated statements of income. During the years ending December 31, 2020, 2019 and 2018, amounts reimbursed were immaterial. The assets and liabilities of Neurimmune are not significant to our consolidated financial position or results of operations as it is a research and development organization. We have provided no financing to Neurimmune other than contractually required amounts. Under the Aducanumab Collaboration Agreement, Eisai had an option to share in the benefit and cost associated with the royalty reductions discussed above; however, Eisai did not elect to share in the benefit and cost with respect to either the October 2017 or May 2018 royalty reductions, which will impact the amount of profits (losses) on potential commercial sales of aducanumab to be shared with Eisai. For additional information on our collaboration arrangements with Eisai, please read Note 18, Collaborative and Other Relationships, to these consolidated financial statements. Unconsolidated Variable Interest Entities We have relationships with various variable interest entities that we do not consolidate as we lack the power to direct the activities that significantly impact the economic success of these entities. These relationships include investments in certain biotechnology companies and research collaboration agreements. As of December 31, 2020 and 2019, the carrying value of our investments in certain biotechnology companies representing potential unconsolidated variable interest entities totaled $12.8 million and $22.7 million, respectively. Our maximum exposure to loss related to these variable interest entities is limited to the carrying value of our investments. We have also entered into research collaboration agreements with certain variable interest entities where we are required to fund certain development activities. These development activities are included in research and development expense in our consolidated statements of income as they are incurred. We have provided no financing to these variable interest entities other than previously contractually required amounts.

Litigation	12 Months Ended
Litigation	Dec. 31, 2020
Loss Contingency, Information about Litigation Matters [Abstract]
Litigation	Litigation We are currently involved in various claims and legal proceedings, including the matters described below. For information as to our accounting policies relating to claims and legal proceedings, including use of estimates and contingencies, please read Note 1, Summary of Significant Accounting Policies, to these consolidated financial statements. With respect to some loss contingencies, an estimate of the possible loss or range of loss cannot be made until management has further information, including, for example, (i) which claims, if any, will survive dispositive motion practice; (ii) information to be obtained through discovery; (iii) information as to the parties' damages claims and supporting evidence; (iv) the parties’ legal theories; and (v) the parties' settlement positions. The claims and legal proceedings in which we are involved also include challenges to the scope, validity or enforceability of the patents relating to our products, pipeline or processes and challenges to the scope, validity or enforceability of the patents held by others. These include claims by third parties that we infringe their patents. An adverse outcome in any of these proceedings could result in one or more of the following and have a material impact on our business or consolidated results of operations and financial position: (i) loss of patent protection; (ii) inability to continue to engage in certain activities; and (iii) payment of significant damages, royalties, penalties and/or license fees to third parties. Loss Contingencies Aducanumab Securities Litigation We and certain current and former officers are named as defendants in an action filed by a shareholder on November 13, 2020, in the U.S. District Court for the Central District of California and an action filed by a shareholder on January 5, 2021, in the U.S. District Court for the District of Massachusetts. Both actions allege violations of federal securities laws under 15 U.S.C §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5 and are seeking a declaration of the action as a class action and an award of damages, interest and attorneys' fees. An estimate of the possible loss or range of loss cannot be made at this time. No trial date has been set. IMRALDI Patent Litigation In September 2018 Fresenius Kabi Deutschland GmbH (Fresenius Kabi) commenced proceedings for damages and injunctive relief against Biogen France SAS in the Tribunal de Grande Instance de Paris, alleging that IMRALDI, the adalimumab biosimilar product of Samsung Bioepis UK Limited that Biogen has commercialized in Europe, infringes the French counterpart of European Patent No. 3 148 510 (the '510 Patent), which was issued in June 2018 and expires in May 2035. No hearing has been scheduled. In October 2018 Fresenius Kabi commenced preliminary injunction proceedings against Biogen (Denmark) Manufacturing ApS and Biogen Denmark A/S in Denmark's Maritime and Commercial High Court alleging infringement of Danish Utility Models. In June 2019 the Danish court denied Fresenius Kabi's request for a preliminary injunction. Fresenius Kabi has appealed that decision and was permitted to add a claim of infringement of the Danish counterpart of the ‘510 patent, and the appeal is pending. In July 2020 the Danish Patent Board of Appeal revoked the Danish Utility Models that were the subject of Fresenius Kabi’s October 2018 request for a preliminary injunction and Fresenius Kabi has appealed those revocations to Denmark’s Maritime and Commercial High Court. No hearing has been scheduled in that appeal. In June 2020 Fresenius Kabi commenced preliminary injunction proceedings against Biogen (Denmark) Manufacturing ApS and Biogen (Denmark) A/S in Denmark’s Maritime and Commercial High Court alleging infringement of another Danish Utility Model. A hearing has been scheduled for May 2021. In November 2018 Fresenius Kabi commenced infringement proceedings for damages and injunctive relief against Biogen GmbH in the Düsseldorf Regional Court relating to the German counterpart of the ‘510 Patent. A hearing has been set for August 2021. In July 2019 Gedeon Richter PLC (Gedeon Richter) commenced proceedings against Biogen GmbH in the Düsseldorf Regional Court alleging infringement of the German counterpart of European Patent No. 3 212 667 (the '667 Patent), which was issued in September 2018 and expires in October 2035, and seeking damages and injunctive relief. A hearing has been set for November 2021. An estimate of the possible loss or range of loss in the IMRALDI patent litigation described above cannot be made at this time. Qui Tam Litigation In July 2015 a qui tam action filed by Michael Bawduniak on behalf of the U.S. and certain states was unsealed by the U.S. District Court for the District of Massachusetts. The action alleges sales and promotional activities in violation of the federal False Claims Act and state law counterparts and seeks single and treble damages, civil penalties, interest, attorneys’ fees and costs. No trial date has been set. The U.S. has not made an intervention decision. An estimate of the possible loss or range of loss cannot be made at this time. Dispute with Former Convergence Shareholders In November and December 2019 Shareholder Representative Services LLC, on behalf of the former shareholders of Convergence, sent us correspondence asserting claims of $200.0 million for alleged breach of the contract under which we acquired Convergence. We dispute the claims. Dispute with Jacobs Switzerland GmbH Jacobs Switzerland GmbH, the general contractor for the construction of our large-scale biologics manufacturing facility in Solothurn, Switzerland, claimed additional payments were due for construction costs. We have reached an agreement in principle to resolve the claim. Other Matters Petition for Inter Partes Review In July 2018 Mylan Pharmaceuticals, Inc. (Mylan) filed a petition that was granted by the U.S. Patent Trial and Appeal Board (PTAB) for inter partes review of our U.S. Patent No. 8,399,514 (the '514 Patent). The '514 Patent includes claims covering treatment of MS with 480 mg of dimethyl fumarate per day as provided for in our TECFIDERA label. In February 2020 the PTAB issued a final written decision upholding the patentability of the ‘514 Patent and in April 2020 Mylan filed an appeal in the U.S. Court of Appeals for the Federal Circuit (the Federal Circuit), which is pending. Hatch-Waxman Act Litigation relating to TECFIDERA Orange-Book Listed Patents In 2017 to 2020, we filed patent infringement proceedings relating to TECFIDERA Orange-Book listed patents pursuant to the Drug Price Competition and Patent Term Restoration Act of 1984, commonly known as the Hatch-Waxman Act (the Delaware Actions), against Accord Healthcare Inc., Alkem Laboratories Ltd., Amneal Pharmaceuticals LLC, Cipla Limited, Graviti Pharmaceuticals Pvt. Ltd., Hetero USA, Inc., Lupin Atlantis Holdings SA, Macleods Pharmaceuticals, Ltd., MSN Laboratories Pvt. Ltd., Pharmathen S.A., Prinston Pharmaceutical Inc., Sandoz Inc., Shilpa Medicare Limited, Slayback Pharma LLC, Sun Pharmaceutical Industries, Ltd., Sun Pharmaceutical Industries, Inc., Sun Pharma Global FZE, Torrent Pharmaceuticals Ltd., TWi Pharmaceuticals, Inc., Windlas Healthcare Pvt. Ltd. and Zydus Pharmaceuticals (USA) Inc. (collectively, the Delaware Defendants) in the U.S. District Court for the District of Delaware (the Delaware Court) and against Mylan in the U.S. District Court for the Northern District of West Virginia (the West Virginia Court). On June 22, 2020, the West Virginia Court entered judgment for Mylan that the asserted claims of the ‘514 Patent are invalid for lack of written description. We appealed the judgment to the Federal Circuit and the appeal is pending. The Delaware Court entered judgment for the Delaware Defendants on the grounds that the judgment of the West Virginia Court applies to the Delaware Actions under principles of collateral estoppel. We have appealed the judgments and the appeal is pending. European Patent Office Oppositions In 2016 the European Patent Office (EPO) revoked our European Patent No. 2 137 537, which covers the treatment of MS with 480 mg of dimethyl fumarate as provided for in our TECFIDERA label. We have appealed to the Technical Boards of Appeal of the EPO and a hearing date has been set for January 2022. In March 2018 the EPO revoked Forward Pharma's European Patent No. 2 801 355, which expires in October 2025. Forward Pharma has filed an appeal to the Technical Boards of Appeal of the EPO and a hearing has been set for September 2021. TYSABRI Patent Revocation Matters In November 2017 Bioeq GMBH, affiliated with the Polpharma Group, brought an action in the Polish Patent Office seeking to revoke Polish Patent No. 215263 (the Polish '263 Patent), which corresponds to our European Patent No. 1 485 127 (the E.U. '127 Patent) and covers administration of natalizumab (TYSABRI) to treat MS. The Polish '263 Patent expires in February 2023. A hearing was held in January 2021 and a decision is pending. In August 2020 a related entity, Polpharma Biologics S.A., also brought an action seeking to revoke the Polish ‘263 Patent. No hearing has been set in this matter. Swiss Pharma International AG, also affiliated with the Polpharma Group, filed actions in the District Court of the Hague, Netherlands (January 2016), the German Patents Court (March 2016) and the Commercial Court of Rome (November 2017) seeking to invalidate the Dutch, German and Italian counterparts, respectively, of the E.U. '127 Patent, which also cover administration of natalizumab (TYSABRI) to treat MS and expire in February 2023. The Dutch and German counterparts were ruled invalid. The decision in the Dutch action was affirmed on appeal and the German appeal has been withdrawn. No hearing has been set in the Italian action. Annulment Proceedings in General Court of the European Union relating to TECFIDERA Pharmaceutical Works Polpharma SA (Polpharma) and Mylan Ireland Ltd. (Mylan Ireland) have each filed applications in the General Court of the European Union (Polpharma in October 2018 and Mylan Ireland in November 2020) seeking to annul decisions of the European Medicines Agency (EMA) refusing to validate Polpharma’s and Mylan Ireland’s respective applications to market a generic version of TECFIDERA. The EMA’s refusals were on the grounds that TECFIDERA benefits from regulatory data protection. Biogen and the European Commission were granted leave to intervene in support of the EMA in the case brought by Polpharma. That case was heard in July 2020 and we are awaiting a decision. We intend to seek leave to intervene in support of the EMA in the case brought by Mylan Ireland. No hearing has been set in that matter. Product Liability and Other Legal Proceedings We are also involved in product liability claims and other legal proceedings generally incidental to our normal business activities. While the outcome of any of these proceedings cannot be accurately predicted, we do not believe the ultimate resolution of any of these existing matters would have a material adverse effect on our business or financial condition.

Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Royalty Payments TYSABRI In 2013 we acquired from Elan full ownership of all remaining rights to TYSABRI that we did not already own or control. Under the acquisition agreement, we are obligated to make contingent payments to Elan of 18.0% on annual worldwide net commercial sales up to $2.0 billion and 25.0% on annual worldwide net commercial sales that exceed $2.0 billion. Royalty payments to Elan and other third parties are recognized as cost of sales in our consolidated statements of income. Elan was acquired by Perrigo Company plc (Perrigo) in December 2013 and Perrigo subsequently sold its rights to these payments to a third-party effective January 2017. SPINRAZA In 2016 we exercised our option to develop and commercialize SPINRAZA from Ionis. Under our agreement with Ionis, we make royalty payments to Ionis on annual worldwide net commercial sales of SPINRAZA using a tiered royalty rate between 11.0% and 15.0%, which are recorded as cost of sales in our consolidated statements of income. For additional information on our collaboration arrangements with Ionis, please read Note 18, Collaborative and Other Relationships, to these consolidated financial statements. VUMERITY In October 2019 the FDA approved VUMERITY for the treatment of RMS. Under our agreement with Alkermes, we make royalty payments to Alkermes on worldwide net commercial sales of VUMERITY using a royalty rate of 15.0%, which are recorded as cost of sales in our consolidated statements of income. Royalties payable on net commercial sales of VUMERITY are subject, under certain circumstances, to tiered minimum annual payment requirements for a period of five years following FDA approval. For additional information on our collaboration arrangement with Alkermes, please read Note 18, Collaborative and Other Relationships, to these consolidated financial statements. Contingent Consideration related to Business Combinations In connection with our acquisition of Convergence, we agreed to make additional payments based upon the achievement of certain milestone events. As the acquisition of Convergence occurred after January 1, 2009, we recognized the contingent consideration liabilities associated with this transaction at their fair value on the acquisition date and revalue the remaining obligations each reporting period. We may pay up to approximately $400.0 million in remaining milestones related to these acquisitions. Fumapharm AG In 2006 we acquired Fumapharm AG. As part of this acquisition we acquired the Fumapharm Products. We were required to make contingent payments to former shareholders of Fumapharm AG and holders of their rights based on the attainment of certain cumulative sales levels of Fumapharm Products and the level of total net sales of Fumapharm Products in the prior 12-month period, as defined in the acquisition agreement, until such time as the cumulative sales level reached $20.0 billion, at which time no further contingent payments were due. During the first quarter of 2019 we paid the final $300.0 million contingent payment as we achieved the $20.0 billion cumulative sales levels related to the Fumapharm Products in the fourth quarter of 2018. Contingent Development, Regulatory and Commercial Milestone Payments Based on our development plans as of December 31, 2020, we could trigger potential future milestone payments to third parties of up to approximately $10.2 billion, including approximately $1.9 billion in development milestones, approximately $1.3 billion in regulatory milestones and approximately $7.0 billion in commercial milestones, as part of our various collaborations, including licensing and development programs. Payments under these agreements generally become due and payable upon achievement of certain development, regulatory or commercial milestones. Because the achievement of these milestones was not considered probable as of December 31, 2020, such contingencies have not been recorded in our financial statements. Amounts related to contingent milestone payments are not considered contractual obligations as they are contingent on the successful achievement of certain development, regulatory or commercial milestones. If certain clinical and commercial milestones are met, we may pay up to $86.2 million in milestones in 2021 under our current agreements. Additionally, if aducanumab receives regulatory approval in the jurisdictions where we have submitted filings, we may pay up to $200.0 million in milestones to Neurimmune in 2021, which includes $100.0 million if launched in the U.S., $50.0 million if launched in three or more countries within the E.U. and $50.0 million if launched in Japan. Milestones payable to Neurimmune are shared expenses under the Aducanumab Collaboration Agreement with Eisai. During the second quarter of 2020 we paid Neurimmune $75.0 million upon the completed submission of the BLA for the approval of aducanumab to the FDA, which was recognized as a charge to noncontrolling interests for the year ended December 31, 2020. Other Funding Commitments As of December 31, 2020, we have several ongoing clinical studies in various clinical trial stages. Our most significant clinical trial expenditures are to CROs. The contracts with CROs are generally cancellable, with notice, at our option. We recorded accrued expenses of approximately $21.7 million in our consolidated balance sheet for expenditures incurred by CROs as of December 31, 2020. We have approximately $593.0 million in cancellable future commitments based on existing CRO contracts as of December 31, 2020. As part of the sale of our Hillerød, Denmark manufacturing operations to FUJIFILM, we provided FUJIFILM with certain minimum batch production commitment guarantees. There is a risk that the minimum contractual batch production commitments will not be met. Based upon current estimates we do not expect to incur an adverse commitment obligation associated with such guarantees. We developed this estimate using a probability-weighted estimate of future manufacturing activity and may further adjust this estimate based upon changes in business conditions, which may result in the increase or reduction of this adverse commitment obligation in subsequent periods. Tax Related Obligations We exclude liabilities pertaining to uncertain tax positions from our summary of contractual obligations as we cannot make a reliable estimate of the period of cash settlement with the respective taxing authorities. As of December 31, 2020, we have approximately $79.6 million of liabilities associated with uncertain tax positions. As of December 31, 2020 and 2019, we have accrued income tax liabilities of $697.0 million, respectively, under the Transition Toll Tax. Of the amounts accrued as of December 31, 2020, $62.0 million is expected to be paid within one year. The Transition Toll Tax will be paid over an eight--year period, which started in 2018, and does not accrue interest. For additional information on the Transition Toll Tax, please read Note 16, Income Taxes, to these consolidated financial statements. Solothurn, Switzerland Manufacturing Facility In order to support our future growth and drug development pipeline, we are building a large-scale biologics manufacturing facility in Solothurn, Switzerland. We expect this facility to be partially operational during the first half of 2021. As of December 31, 2020, we had contractual commitments of approximately $9.3 million related to the construction of this facility.

Guarantees	12 Months Ended
Guarantees	Dec. 31, 2020
Guarantees [Abstract]
Guarantees	Guarantees As of December 31, 2020 and 2019, we did not have significant liabilities recorded for guarantees. We enter into indemnification provisions under our agreements with other companies in the ordinary course of business, typically with business partners, contractors, clinical sites and customers. Under these provisions, we generally indemnify and hold harmless the indemnified party for losses suffered or incurred by the indemnified party as a result of our activities. These indemnification provisions generally survive termination of the underlying agreement. The maximum potential amount of future payments we could be required to make under these indemnification provisions is unlimited. However, to date we have not incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. As a result, the estimated fair value of these agreements is minimal. Accordingly, we have no liabilities recorded for these agreements as of December 31, 2020 and 2019.

Employee Benefit Plans	12 Months Ended
Employee Benefit Plans	Dec. 31, 2020
Retirement Benefits [Abstract]
Employee Benefit Plans	Employee Benefit Plans We sponsor various retirement and pension plans. Our estimates of liabilities and expenses for these plans incorporate a number of assumptions, including expected rates of return on plan assets and interest rates used to discount future benefits. 401(k) Savings Plan We maintain a 401(k) Savings Plan, which is available to substantially all regular employees in the U.S. over the age of 21. Participants may make voluntary contributions. We make matching contributions according to the 401(k) Savings Plan’s matching formula. All matching contributions and participant contributions vest immediately. The 401(k) Savings Plan also holds certain transition contributions on behalf of participants who previously participated in the Biogen, Inc. Retirement Plan. The expense related to our 401(k) Savings Plan primarily consists of our matching contributions. Expense related to our 401(k) Savings Plan totaled $44.3 million, $44.8 million and $42.2 million for the years ended December 31, 2020, 2019 and 2018, respectively. Deferred Compensation Plan We maintain a non-qualified deferred compensation plan, known as the Supplemental Savings Plan (SSP), which allows a select group of management employees in the U.S. to defer a portion of their compensation. The SSP also provides certain credits to highly compensated U.S. employees that are paid by the company. These credits are known as the Restoration Match. The deferred compensation amounts are accrued when earned. Such deferred compensation is distributable in cash in accordance with the rules of the SSP. Deferred compensation amounts under such plan as of December 31, 2020 and 2019, totaled approximately $120.0 million and $114.6 million, respectively, and are included in other long-term liabilities in our consolidated balance sheets. The SSP also holds certain transition contributions on behalf of participants who previously participated in the Biogen, Inc. Retirement Plan. The Restoration Match and participant contributions vest immediately. Distributions to participants can be either in one lump sum payment or annual installments as elected by the participants. Pension Plans Our retiree benefit plans include defined benefit plans for employees in our affiliates in Switzerland and Germany as well as other insignificant defined benefit plans in certain other countries where we maintain an operating presence. Our Swiss plan is a government-mandated retirement fund that provides employees with a minimum investment return. The minimum investment return is determined annually by the Swiss government and was 1.00% in 2020, 2019 and 2018. Under the Swiss plan, both we and certain of our employees with annual earnings in excess of government determined amounts are required to make contributions into a fund managed by an independent investment fiduciary. Employer contributions must be in an amount at least equal to the employee’s contribution. Minimum employee contributions are based on the respective employee’s age, salary and gender. As of December 31, 2020 and 2019, the Swiss plan had an unfunded net pension obligation of $75.7 million and $42.9 million, respectively, and plan assets that totaled $170.0 million and $127.1 million, respectively. In 2020, 2019 and 2018 we recognized expense totaling $15.5 million, $14.7 million and $14.8 million, respectively, related to our Swiss plan, of which $2.6 million, $1.2 million and $1.3 million, respectively, was included in other income (expense), net. The obligations under the German plans are unfunded and totaled $75.5 million and $59.6 million as of December 31, 2020 and 2019, respectively. Net periodic pension cost related to the German plans totaled $6.2 million, $5.1 million and $5.3 million for the years ended December 31, 2020, 2019 and 2018, respectively, of which $2.0 million, $1.4 million and $1.5 million, respectively, was included in other income (expense), net.

The entire disclosure for retirement benefits.

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Segment Information

12 Months Ended

Dec. 31, 2020

Segment Reporting [Abstract]

Segment Information

We operate as one operating segment, focused on discovering, developing and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases as well as related therapeutic adjacencies. Our Chief Executive Officer (CEO), as the chief operating decision-maker, manages and allocates resources to the operations of our company on a total company basis. Our research and development organization is responsible for the research and discovery of new product candidates and supports development and registration efforts for potential future products. Our pharmaceutical, operations and technology organization manages the development of the manufacturing processes, clinical trial supply, commercial product supply, distribution, buildings and facilities. Our commercial organization is responsible for U.S. and international development of our commercial products. The company is also supported by corporate staff functions. Managing and allocating resources on a total company basis enables our CEO to assess the overall level of resources available and how to best deploy these resources across functions, therapeutic areas and research and development projects that are in line with our long-term company-wide strategic goals. Consistent with this decision-making process, our CEO uses consolidated, single-segment financial information for purposes of evaluating performance, forecasting future period financial results, allocating resources and setting incentive targets.

Enterprise-wide disclosures about product revenues, other revenues and long-lived assets by geographic area are presented below. Revenues are primarily attributed to individual countries based on location of the customer or licensee.

Geographic Information

The following tables contain certain financial information by geographic area:


	December 31, 2020
(In millions)	U.S.		Europe		Asia		Other		Total
Product revenues from external customers	$	5,900.1	$	3,656.4	$	596.7	$	539.0	$	10,692.2
Revenues from anti-CD20 therapeutic programs	1,897.4		0.1		—		80.3		1,977.8
Other revenues from external customers	733.6		8.1		32.9		—		774.6
Long-lived assets	1,496.3		2,321.4		16.2		10.9		3,844.8

	December 31, 2019
(In millions)	U.S.		Europe		Asia		Other		Total
Product revenues from external customers	$	6,713.8	$	3,794.5	$	320.3	$	551.2	$	11,379.8
Revenues from anti-CD20 therapeutic programs	2,211.9		0.2		—		78.3		2,290.4
Other revenues from external customers	585.8		9.7		112.2		—		707.7
Long-lived assets	1,493.2		2,162.9		6.2		12.0		3,674.3

	December 31, 2018
(In millions)	U.S.		Europe		Asia		Other		Total
Product revenues from external customers	$	6,800.5	$	3,370.3	$	281.2	$	434.8	$	10,886.8
Revenues from anti-CD20 therapeutic programs	1,903.4		0.2		—		76.6		1,980.2
Other revenues from external customers	457.0		32.7		96.2		—		585.9
Long-lived assets	1,152.7		2,442.8		3.9		1.8		3,601.2

Other

As of December 31, 2020, 2019 and 2018, approximately $2,180.6 million, $2,028.8 million and $1,748.5 million, respectively, of our long-lived assets were related to the construction of our large-scale biologics manufacturing facility in Solothurn, Switzerland.

In August 2019 we completed the sale of all of the outstanding shares of our subsidiary that owned our biologics manufacturing operations in Hillerød, Denmark to FUJIFILM. As of December 31, 2018, approximately $646.5 million of our long-lived assets were related to our manufacturing facility in Hillerød, Denmark.

For additional information on our large-scale biologics manufacturing facility in Solothurn, Switzerland, please read Note 10, Property, Plant and Equipment, to these consolidated financial statements. For additional information on the divestiture of our Hillerød, Denmark manufacturing operations, please read Note 3, Divestitures, to these consolidated financial statements.

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

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Quarterly Financial Information Disclosure [Abstract]

Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2020

Quarterly Financial Data (Unaudited)


(In millions, except per share amounts)	First Quarter		Second Quarter		Third Quarter		Fourth Quarter		Total Year
2020
Product revenues, net	$	2,904.6	$	2,795.7	$	2,690.3	$	2,301.6	$	10,692.2
Revenues from anti-CD20 therapeutic programs	$	520.4	$	478.3	$	560.1	$	419.0	$	1,977.8
Other revenues	$	109.3	$	407.6	$	125.7	$	132.0	$	774.6
Total revenues	$	3,534.3	$	3,681.6	$	3,376.1	$	2,852.6	$	13,444.6
Gross profit (1)	$	3,080.0	$	3,270.5	$	2,927.0	$	2,361.9	$	11,639.4
Net income	$	1,392.6	$	1,606.5	$	703.9	$	357.6	$	4,060.5
Net income attributable to Biogen Inc.	$	1,399.1	$	1,542.1	$	701.5	$	357.9	$	4,000.6
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$	8.10	$	9.60	$	4.47	$	2.33	$	24.86
Diluted earnings per share attributable to Biogen Inc.	$	8.08	$	9.59	$	4.46	$	2.32	$	24.80
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	172.8		160.6		156.9		153.7		160.9
Diluted earnings per share attributable to Biogen Inc.	173.1		160.9		157.2		154.0		161.3


(In millions, except per share amounts)	First Quarter		Second Quarter		Third Quarter		Fourth Quarter		Total Year
2019
Product revenues, net	$	2,680.0	$	2,880.3	$	2,894.7	$	2,924.8	$	11,379.8
Revenues from anti-CD20 therapeutic programs	$	517.4	$	576.4	$	595.8	$	600.8	$	2,290.4
Other revenues	$	292.4	$	160.0	$	109.6	$	145.7	$	707.7
Total revenues	$	3,489.8	$	3,616.7	$	3,600.1	$	3,671.3	$	14,377.9
Gross profit (1)	$	2,887.8	$	3,140.4	$	3,170.1	$	3,224.2	$	12,422.5
Net income	$	1,408.8	$	1,494.1	$	1,545.9	$	1,439.7	$	5,888.5
Net income attributable to Biogen Inc.	$	1,408.8	$	1,494.1	$	1,545.9	$	1,439.7	$	5,888.5
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$	7.17	$	7.85	$	8.40	$	8.10	$	31.47
Diluted earnings per share attributable to Biogen Inc.	$	7.15	$	7.85	$	8.39	$	8.08	$	31.42
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	196.6		190.3		184.0		177.8		187.1
Diluted earnings per share attributable to Biogen Inc.	197.0		190.4		184.2		178.2		187.4

(1) Gross profit is calculated as total revenues less cost of sales, excluding amortization and impairment of acquired intangible assets.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Product Revenues

Reserves for Discounts and Allowances

Product revenue reserves, which are classified as a reduction in product revenues, are generally characterized in the following categories: discounts, contractual adjustments and returns.

estimate of the pharmacy’s buying or dispensing patterns and the resulting applicable contractual rebate rate(s) to be earned over the contractual period.

Revenues from Anti-CD20 Therapeutic Programs

Revenues from anti-CD20 therapeutic programs consist of:

(i) our share of pre-tax profits and losses in the U.S. for RITUXAN and GAZYVA; and

(ii) other revenues from anti-CD20 therapeutic programs, which primarily consist of our share of pre-tax co-promotion profits on RITUXAN in Canada and royalty revenues on sales of OCREVUS.

For additional information on our relationship with Genentech, please read Note 18, Collaborative and Other Relationships, to these consolidated financial statements.

Other Revenues

Royalty Revenues

We recognize royalty revenues related to sales by our licensees of products covered under patents that we own.

Collaborative and Other Relationships

Other Corporate Revenues

Fair Value Measurements

•Level 1 — Fair values are determined utilizing quoted prices (unadjusted) in active markets for identical assets or liabilities that we have the ability to access;

•Level 3 — Prices or valuations that require inputs that are both significant to the fair value measurement and unobservable.

Other Assets and Liabilities

Cash and Cash Equivalents

Accounts Receivable

We do not adjust our receivables for the effects of a significant financing component at contract inception if we expect to collect the receivables in one year or less from the time of sale.

Concentration of Credit Risk

Financial instruments that potentially subject us to concentrations of credit risk include cash and cash equivalents, investments, derivatives and accounts receivable. We attempt to minimize the risks related to cash and cash equivalents and investments by investing in a broad and diverse range of financial instruments as previously defined by us. We have established guidelines related to credit ratings and maturities intended to safeguard principal balances and maintain liquidity. Our investment portfolio is maintained in accordance with our investment policy, which defines allowable investments, specifies credit quality standards and limits the credit exposure of any single issuer. We minimize credit risk resulting from derivative instruments by choosing only highly rated financial institutions as counterparties.Concentrations of credit risk with respect to receivables, which are typically unsecured, are somewhat mitigated due to the wide variety of customers and markets using our products, as well as their dispersion across many different geographic areas. We monitor the financial performance and creditworthiness of our customers so that we can properly assess and respond to changes in their credit profile. We continue to monitor these conditions and assess their possible impact on our business.

Marketable Securities and Other Investments

Marketable Debt Securities

Marketable Equity Securities and Venture Capital Funds

Non-Marketable Equity Securities

Evaluating Marketable Debt Securities for Other-than-Temporary Impairments

Equity Method of Accounting

Inventory

Capitalization of Inventory Costs

inventory upon a change in such judgment, due to, among other potential factors, a denial or significant delay of approval by necessary regulatory bodies.

Obsolescence and Unmarketable Inventory

Property, Plant and Equipment


Asset Category			Useful Lives
Land			Not depreciated
Buildings			15 to 40 years
Leasehold Improvements			Lesser of the useful life or the term of the respective lease
Furniture and Fixtures			5 to 7 years
Machinery and Equipment			5 to 20 years
Computer Software and Hardware			3 to 5 years

Leases

For additional information on the adoption of the new leasing standards, please read Note 11, Leases, to these consolidated financial statements.

Intangible Assets

Acquired In-process Research and Development (IPR&D)

Goodwill

Impairment of Long-Lived Assets

Contingent Consideration

The consideration for our acquisitions often includes future payments that are contingent upon the occurrence of a particular event or events. We record an obligation for such contingent payments at fair value on the acquisition date. We estimate the fair value of contingent consideration obligations through valuation models that incorporate probability-adjusted assumptions related to the achievement of the milestones and thus likelihood of making related payments. We revalue our contingent consideration obligations each reporting period. Changes in the fair value of our contingent consideration obligations are recognized in our consolidated statements of income. Changes in the fair value of the contingent consideration obligations can result from changes to one or multiple inputs, including adjustments to the discount rates, changes in the amount or timing of expected expenditures associated with product development, changes in the amount or timing of cash flows and reserves associated with products upon commercialization, changes in the assumed achievement or timing of any cumulative sales-based and development milestones, changes in the probability of certain clinical events and changes in the assumed probability associated with regulatory approval. Discount rates in our valuation models represent a measure of the credit risk associated with settling the liability. The period over which we discount our contingent obligations is based on the current development stage of the product candidates, our specific development plan for that product candidate adjusted for the probability of completing the development step and when the contingent payments would be triggered. In estimating the probability of success, we utilize data regarding similar milestone events from several sources, including industry studies and our own experience. These fair value measurements are based on significant inputs not observable in the market. Significant judgment is employed in determining the appropriateness of these assumptions as of the acquisition date and for each subsequent period.

Derivative Instruments and Hedging Activities

Cash Flow and Fair Value Derivative Instruments

Net Investment Derivative Instruments

For additional information on our derivative instruments and hedging activities, please read Note 9, Derivative Instruments, to these consolidated financial statements.

Translation of Foreign Currencies

Royalty Cost of Sales

Accounting for Share-Based Compensation

for our MSUs. The probability of actual shares expected to be earned is considered in the grant date valuation, therefore the expense is not adjusted to reflect the actual units earned.

The fair values of our RSUs are based on the market value of our stock on the date of grant. Compensation expense for RSUs is recognized straight-line over the applicable service period.

Research and Development Expenses

Selling, General and Administrative Expenses

Advertising costs are expensed as incurred. For the years ended December 31, 2020, 2019 and 2018, advertising costs totaled $111.8 million, $79.2 million and $90.2 million, respectively.

Income Taxes

Contingencies

Earnings per Share

New Accounting Pronouncements

Leases

Credit Losses

Debt Securities

Fair Value Measurements

Derivative Instruments and Hedging Activities

Collaborative Arrangements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Property, plant and equipment estimated useful lives


Asset Category			Useful Lives
Land			Not depreciated
Buildings			15 to 40 years
Leasehold Improvements			Lesser of the useful life or the term of the respective lease
Furniture and Fixtures			5 to 7 years
Machinery and Equipment			5 to 20 years
Computer Software and Hardware			3 to 5 years

Revenues (Tables)

12 Months Ended

Dec. 31, 2020

Revenue from Contract with Customer [Abstract]

Revenue by product

Revenues by product are summarized as follows:


	For the Years Ended December 31,
	2020						2019						2018
(In millions)	United States		Rest of World		Total		United States		Rest of World		Total		United States		Rest of World		Total
Multiple Sclerosis (MS):
Fumarate*	$	2,742.0	$	1,163.4	$	3,905.4	$	3,312.0	$	1,126.2	$	4,438.2	$	3,253.2	$	1,020.9	$	4,274.1
Interferon**	1,273.5		604.0		1,877.5		1,426.6		675.2		2,101.8		1,668.3		694.7		2,363.0
TYSABRI	1,096.8		849.3		1,946.1		1,041.8		850.4		1,892.2		1,025.0		839.0		1,864.0
FAMPYRA	—		103.1		103.1		—		97.1		97.1		—		92.7		92.7
ZINBRYTA	—		—		—		—		—		—		—		1.4		1.4
Subtotal: MS	5,112.3		2,719.8		7,832.1		5,780.4		2,748.9		8,529.3		5,946.5		2,648.7		8,595.2

Spinal Muscular Atrophy:
SPINRAZA	787.8		1,264.3		2,052.1		933.4		1,163.6		2,097.0		854.0		870.2		1,724.2

Biosimilars:
BENEPALI	—		481.6		481.6		—		486.2		486.2		—		485.2		485.2
IMRALDI	—		216.3		216.3		—		184.0		184.0		—		16.7		16.7
FLIXABI	—		97.9		97.9		—		68.1		68.1		—		43.2		43.2
Subtotal: Biosimilars	—		795.8		795.8		—		738.3		738.3		—		545.1		545.1

Other:
FUMADERM	—		12.2		12.2		—		15.2		15.2		—		22.3		22.3

Total product revenues	$	5,900.1	$	4,792.1	$	10,692.2	$	6,713.8	$	4,666.0	$	11,379.8	$	6,800.5	$	4,086.3	$	10,886.8

*Fumarate includes TECFIDERA and VUMERITY. VUMERITY became commercially available in the U.S. in November 2019.

**Interferon includes AVONEX and PLEGRIDY.

Analysis of change in reserves

An analysis of the change in reserves for discounts and allowances is summarized as follows:


	December 31, 2020
(In millions)	Discounts		Contractual Adjustments		Returns		Total
Beginning balance	$	131.1	$	1,027.3	$	40.5	$	1,198.9
Current provisions relating to sales in current year	774.7		3,308.8		19.0		4,102.5
Adjustments relating to prior years	(1.0)		(54.0)		1.3		(53.7)
Payments/returns relating to sales in current year	(635.1)		(2,426.1)		—		(3,061.2)
Payments/returns relating to sales in prior years	(128.3)		(763.0)		(19.2)		(910.5)
Ending balance	$	141.4	$	1,093.0	$	41.6	$	1,276.0


	December 31, 2019
(In millions)	Discounts		Contractual Adjustments		Returns		Total
Beginning balance	$	127.8	$	888.8	$	34.7	$	1,051.3
Current provisions relating to sales in current year	666.2		3,011.5		20.9		3,698.6
Adjustments relating to prior years	0.3		(54.1)		5.5		(48.3)
Payments/returns relating to sales in current year	(535.5)		(2,242.9)		(0.2)		(2,778.6)
Payments/returns relating to sales in prior years	(127.7)		(576.0)		(20.4)		(724.1)
Ending balance	$	131.1	$	1,027.3	$	40.5	$	1,198.9


	December 31, 2018
(In millions)	Discounts		Contractual Adjustments		Returns		Total
Beginning balance	$	109.6	$	606.0	$	46.0	$	761.6
Current provisions relating to sales in current year	679.3		2,686.7		23.1		3,389.1
Adjustments relating to prior years	(0.3)		(10.0)		(1.8)		(12.1)
Payments/returns relating to sales in current year	(551.7)		(1,887.6)		(1.1)		(2,440.4)
Payments/returns relating to sales in prior years	(109.1)		(506.3)		(31.5)		(646.9)
Ending balance	$	127.8	$	888.8	$	34.7	$	1,051.3

Total reserves included in consolidated balance sheet

The total reserves above, which are included in our consolidated balance sheets, are summarized as follows:


	As of December 31,
(In millions)	2020		2019
Reduction of accounts receivable	$	195.4	$	197.8
Component of accrued expenses and other	1,080.6		1,001.1
Total revenue-related reserves	$	1,276.0	$	1,198.9

Revenues from anti-CD20 therapeutic programs

Revenues from anti-CD20 therapeutic programs are summarized in the table below. For purposes of this footnote, we refer to RITUXAN and RITUXAN HYCELA collectively as RITUXAN.


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	$	1,080.2	$	1,542.4	$	1,431.9
Other revenues from anti-CD20 therapeutic programs	897.6		748.0		548.3
Total revenues from anti-CD20 therapeutic programs	$	1,977.8	$	2,290.4	$	1,980.2

Revenues from anti-CD20 therapeutic programs are summarized as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA, including the reimbursement of selling and development expenses	$	1,080.2	$	1,542.4	$	1,431.9
Other revenues from anti-CD20 therapeutic programs	897.6		748.0		548.3
Total revenues from anti-CD20 therapeutic programs	$	1,977.8	$	2,290.4	$	1,980.2

Other revenues

Other revenues are summarized as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Revenues from collaborative and other relationships:
Revenues earned under our technical development agreement, manufacturing service agreements and royalty revenues on biosimilar products with Samsung Bioepis	$	20.9	$	106.2	$	96.4
Other revenues from collaborative and other relationships	0.7		—		(8.6)
Other royalty and corporate revenues:
Royalty	33.9		17.0		38.7
Other corporate	719.1		584.5		459.4
Total other revenues	$	774.6	$	707.7	$	585.9

Inventory (Tables)

12 Months Ended

Dec. 31, 2020

Inventory Disclosure [Abstract]

Components of inventory

The components of inventory are summarized as follows:


	As of December 31,
(In millions)	2020		2019
Raw materials	$	314.9	$	169.7
Work in process	544.5		460.0
Finished goods	209.2		174.5
Total inventory	$	1,068.6	$	804.2

Intangible Assets and Goodwill (Tables)

12 Months Ended

Dec. 31, 2020

Goodwill and Intangible Assets Disclosure [Abstract]

Intangible assets

Intangible assets, net of accumulated amortization, impairment charges and adjustments are summarized as follows:


		As of December 31, 2020						As of December 31, 2019
(In millions)	Estimated Life	Cost		Accumulated Amortization		Net		Cost		Accumulated Amortization		Net
Completed technology	4-28 years	$	7,394.3	$	(5,136.5)	$	2,257.8	$	7,379.3	$	(4,881.4)	$	2,497.9
In-process research and development	Indefinite until commercialization	762.5		—		762.5		965.5		—		965.5
Trademarks and trade names	Indefinite	64.0		—		64.0		64.0		—		64.0
Total intangible assets		$	8,220.8	$	(5,136.5)	$	3,084.3	$	8,408.8	$	(4,881.4)	$	3,527.4

Estimated future amortization of intangible assets

The estimated future amortization of finite-lived intangible assets for the next five years is expected to be as follows:


(In millions)	As of December 31, 2020
2021	$	205.0
2022	215.0
2023	215.0
2024	225.0
2025	220.0

Summary of roll forward of the changes in goodwill

The following table provides a roll forward of the changes in our goodwill balance:


	As of December 31,
(In millions)	2020		2019
Goodwill, beginning of year	$	5,757.8	$	5,706.4
Increase to goodwill	—		117.5
Elimination of goodwill allocated to Hillerød, Denmark manufacturing operations	—		(69.5)
Other	4.3		3.4
Goodwill, end of year	$	5,762.1	$	5,757.8

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2020

Fair Value Disclosures [Abstract]

Summary of assets and liabilities recorded at fair value


	As of December 31, 2020
(In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	626.9	$	—	$	626.9	$	—
Marketable debt securities:
Corporate debt securities	1,301.5		—		1,301.5		—
Government securities	627.1		—		627.1		—
Mortgage and other asset backed securities	122.4		—		122.4		—
Marketable equity securities	1,974.3		271.1		1,703.2		—
Derivative contracts	20.5		—		20.5		—
Plan assets for deferred compensation	28.2		—		28.2		—
Total	$	4,700.9	$	271.1	$	4,429.8	$	—
Liabilities:
Derivative contracts	$	217.2	$	—	$	217.2	$	—
Contingent consideration obligations	259.8		—		—		259.8
Total	$	477.0	$	—	$	217.2	$	259.8


	As of December 31, 2019
(In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	2,541.1	$	—	$	2,541.1	$	—
Marketable debt securities:
Corporate debt securities	1,695.1		—		1,695.1		—
Government securities	1,013.9		—		1,013.9		—
Mortgage and other asset backed securities	261.3		—		261.3		—
Marketable equity securities	337.5		7.9		329.6		—
Derivative contracts	43.8		—		43.8		—
Plan assets for deferred compensation	27.7		—		27.7		—
Total	$	5,920.4	$	7.9	$	5,912.5	$	—
Liabilities:
Derivative contracts	$	8.3	$	—	$	8.3	$	—
Contingent consideration obligations	346.1		—		—		346.1
Total	$	354.4	$	—	$	8.3	$	346.1

Summary of significant unobservable inputs

The following table summarizes the significant unobservable inputs in the fair value measurement of our contingent consideration obligations as of December 31, 2020:

As of December 31, 2020

(In millions)

Fair Value

Valuation Technique

Unobservable Input

Range

Weighted Average

Liabilities:

Contingent consideration obligation

$259.8

Discounted cash flow

Discount rate

0.60%

Expected timing of achievement of development milestones

2021 to 2025

—

Summary of gains or losses on assets measured at fair value, nonrecurring basis

The gains or losses on assets measured at fair value on a nonrecurring basis, are summarized as follows:


	As of December 31, 2020
(In millions)	Beginning Book Value		Impairment		Ending Book Value
BIIB111 intangible asset	$	480.0	$	(115.0)	$	365.0

Summary of fair and carrying value of debt instruments

The fair values of our debt instruments, which are Level 2 liabilities, are summarized as follows:


	As of December 31,
(In millions)	2020		2019
2.900% Senior Notes due September 15, 2020(1)	$	—	$	1,509.6
3.625% Senior Notes due September 15, 2022	1,054.1		1,038.9
4.050% Senior Notes due September 15, 2025	2,003.1		1,897.2
2.250% Senior Notes due May 1, 2030	1,557.2		—
5.200% Senior Notes due September 15, 2045	2,365.1		2,107.9
3.150% Senior Notes due May 1, 2050	1,536.4		—
Total	$	8,515.9	$	6,553.6

Fair value of contingent consideration obligations

The following table provides a roll forward of the fair values of our contingent consideration obligations, which includes Level 3 measurements:


	As of December 31,
(In millions)	2020		2019
Fair value, beginning of year	$	346.1	$	409.8
Changes in fair value	(86.3)		(63.7)
Payments and other	—		—
Fair value, end of year	$	259.8	$	346.1

Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2020

Investments, All Other Investments [Abstract]

Summary of financial assets with maturities of less than 90 days included within cash and cash equivalents

The following table summarizes our financial assets with maturities of less than 90 days from the date of purchase included in cash and cash equivalents in our consolidated balance sheets:


	As of December 31,
(In millions)	2020		2019
Commercial paper	$	61.1	$	384.4
Overnight reverse repurchase agreements	37.4		368.8
Money market funds	505.1		1,628.5
Short-term debt securities	23.3		159.4
Total	$	626.9	$	2,541.1

Debt and equity securities, available-for-sale

The following tables summarize our marketable debt and equity securities, classified as available for sale:


	As of December 31, 2020
(In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Corporate debt securities
Current	$	897.8	$	0.4	$	(0.2)	$	898.0
Non-current	402.5		1.1		(0.1)		403.5
Government securities
Current	380.6		0.1		—		380.7
Non-current	245.9		0.5		—		246.4
Mortgage and other asset backed securities
Current	0.2		—		—		0.2
Non-current	122.1		0.2		(0.1)		122.2
Total marketable debt securities	$	2,049.1	$	2.3	$	(0.4)	$	2,051.0
Marketable equity securities, current	$	70.6	$	15.9	$	—	$	86.5
Marketable equity securities, non-current	$	1,168.9	$	733.8	$	(14.9)	$	1,887.8


	As of December 31, 2019
(In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Corporate debt securities
Current	$	1,057.2	$	1.0	$	—	$	1,058.2
Non-current	633.9		3.0		—		636.9
Government securities
Current	502.9		0.4		—		503.3
Non-current	510.1		0.8		(0.3)		510.6
Mortgage and other asset backed securities
Current	0.7		—		—		0.7
Non-current	260.2		0.8		(0.4)		260.6
Total marketable debt securities	$	2,965.0	$	6.0	$	(0.7)	$	2,970.3
Marketable equity securities, non-current	$	218.4	$	132.1	$	(13.0)	$	337.5

Summary of contractual maturities: available-for-sale securities

The estimated fair value and amortized cost of our marketable debt securities available-for-sale by contractual maturity are summarized as follows:


	As of December 31, 2020				As of December 31, 2019
(In millions)	Estimated Fair Value		Amortized Cost		Estimated Fair Value		Amortized Cost
Due in one year or less	$	1,278.9	$	1,278.6	$	1,562.2	$	1,560.8
Due after one year through five years	722.6		721.3		1,234.5		1,230.4
Due after five years	49.5		49.2		173.6		173.8
Total marketable debt securities	$	2,051.0	$	2,049.1	$	2,970.3	$	2,965.0

Proceeds from marketable securities, excluding strategic investments

The proceeds from maturities and sales of marketable debt securities and resulting realized gains and losses are summarized as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Proceeds from maturities and sales	$	7,299.4	$	6,007.0	$	9,173.7
Realized gains	17.7		6.0		3.2
Realized losses	26.0		1.5		11.7

Derivative Instruments (Tables)

12 Months Ended

Dec. 31, 2020

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Foreign currency forward contracts that were entered into to hedge forecasted revenue

The notional value of foreign currency forward contracts that were entered into to hedge forecasted revenues and operating expenses is summarized as follows:


	Notional Amount As of December 31,
(In millions)	2020		2019
Euro	$	2,979.1	$	1,892.4
British pound	250.6		—



Total foreign currency forward contracts	$	3,229.7	$	1,892.4

Summary of the effect of derivatives designated as cash flow hedging instruments on our consolidated statements of income

The following table summarizes the effect of foreign currency forward contracts designated as hedging instruments in our consolidated statements of income:


For the Years Ended December 31,
Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)							Net Gains/(Losses) Recognized in Operating Income (in millions)
Location	2020		2019		2018		Location	2020		2019		2018
Revenues	$	18.3	$	118.6	$	(42.5)	Revenues	$	(9.9)	$	2.9	$	10.8
Operating expenses	3.3		(3.3)		0.2		Operating expenses	—		0.2		(0.1)

Summary of the effect of derivatives designated as net investment hedging instruments on our consolidated statement of income

The following table summarizes the effect of our net investment hedges in our consolidated financial statements:


For the Years Ended December 31,
Net Gains/(Losses) Recognized in Other Comprehensive Income (Effective Portion) (in millions)							Net Gains/(Losses) Recognized in Other Comprehensive Income (Amounts Excluded from Effectiveness Testing) (in millions)							Net Gains/(Losses) Recognized in Net Income (Amounts Excluded from Effectiveness Testing) (in millions)
Location	2020		2019		2018		Location	2020		2019		2018		Location	2020		2019		2018
Gains (losses) on net investment hedge	$	(35.1)	$	25.3	$	(3.8)	Gains (losses) on net investment hedge	$	4.5	$	3.3	$	—	Other income (expense)	$	2.9	$	7.0	$	1.5

Summary of the fair value for our outstanding derivatives

The following table summarizes the fair value and presentation in our consolidated balance sheets of our outstanding derivative instruments, including those designated as hedging instruments:


		As of December 31,
(In millions)	Balance Sheet Location	2020		2019
Cash Flow Hedging Instruments:
Asset derivative instruments	Other current assets	$	—	$	33.8
Liability derivative instruments	Accrued expenses and other	157.1		2.0
	Other long-term liabilities	35.7		1.7

Net Investment Hedging Instruments:
Asset derivative instruments	Other current assets	—		2.0
Liability derivative instruments	Accrued expenses and other	19.7		—

Fair Value Hedging Instruments
Liability derivative instruments	Accrued expenses and other	—		2.3


Other Derivative Instruments:
Asset derivative instruments	Other current assets	20.5		8.0
Liability derivative instruments	Accrued expenses and other	4.7		2.4

Property, Plant and Equipment (Tables)

12 Months Ended

Dec. 31, 2020

Property, Plant and Equipment [Abstract]

Components of property, plant and equipment, net

Components of property, plant and equipment, net are summarized as follows:


	As of December 31,
(In millions)	2020		2019
Land	$	119.8	$	118.1
Buildings	1,025.3		835.0
Leasehold improvements	104.6		99.5
Machinery and equipment	1,027.8		844.5
Computer software and hardware	903.0		798.4
Furniture and fixtures	62.5		58.3
Construction in progress	1,950.8		2,084.4
Total cost	5,193.8		4,838.2
Less: accumulated depreciation	(1,782.3)		(1,590.9)
Total property, plant and equipment, net	$	3,411.5	$	3,247.3

Leases (Tables)

12 Months Ended

Dec. 31, 2020

Leases [Abstract]

Schedule of operating leases

All of our leases qualify as operating leases. The following table summarizes the presentation in our consolidated balance sheets of our operating leases:


		As of December 31,
(In millions)	Balance sheet location	2020		2019
Assets:
Operating lease assets	Operating lease assets	$	433.3	$	427.0

Liabilities
Current operating lease liabilities	Accrued expenses and other	$	83.2	$	73.6
Non-current operating lease liabilities	Long-term operating lease liabilities	402.0		412.7
Total operating lease liabilities		$	485.2	$	486.3

Operating lease costs

The following table summarizes the effect of lease costs in our consolidated statements of income:


		For the Years Ended December 31,
(In millions)	Income Statement Location	2020		2019
Operating lease cost	Research and development	$	5.2	$	6.7
	Selling, general and administrative	93.1		84.6
Variable lease cost	Research and development	1.1		1.2
	Selling, general and administrative	21.1		23.7
Sublease income	Selling, general and administrative	(24.2)		(25.6)
	Other (income) expense, net	(3.9)		(3.9)
Net lease cost		$	92.4	$	86.7

Operating lease liability maturity

The minimum lease payments for the next five years and thereafter is expected to be as follows:


(In millions)	As of December 31, 2020
2021	$	95.8
2022	93.5
2023	82.0
2024	75.0
2025	56.9
Thereafter	129.8
Total lease payments	$	533.0
Less: interest	47.8
Present value of operating lease liabilities	$	485.2

Operating lease weighted average remaining term and discount rate

The weighted average remaining lease term and weighted average discount rate of our operating leases are as follows:


	As of December 31,
	2020		2019
Weighted average remaining lease term in years	6.30		7.07
Weighted average discount rate	2.9	%	3.2	%

Operating lease supplemental cash flow disclosure

Supplemental disclosure of cash flow information related to our operating leases included in cash flows provided by operating activities in our consolidated statements of cash flows is as follows:


	As of December 31,
(In millions)	2020		2019
Cash paid for amounts included in the measurement of lease liabilities	$	100.2	$	93.8
Operating lease assets obtained in exchange for lease obligations	59.0		35.9

Indebtedness (Tables)

12 Months Ended

Dec. 31, 2020

Debt Disclosure [Abstract]

Summary of indebtedness

Our indebtedness is summarized as follows:


	As of December 31,
(In millions)	2020		2019
Current portion:
2.900% Senior Notes due September 15, 2020(1)	$	—	$	1,495.8
Current portion of notes payable	$	—	$	1,495.8
Non-current portion:
3.625% Senior Notes due September 15, 2022	$	997.9	$	996.6
4.050% Senior Notes due September 15, 2025	1,741.2		1,739.5
2.250% Senior Notes due May 1, 2030	1,491.1		—
5.200% Senior Notes due September 15, 2045	1,723.4		1,722.9
3.150% Senior Notes due May 1, 2050	1,472.6		—
Non-current portion of notes payable	$	7,426.2	$	4,459.0

Total debt maturities

The total gross payments due under our debt arrangements are as follows:


(In millions)	As of December 31, 2020
2021	$	—
2022	1,000.0
2023	—
2024	—
2025	1,750.0
2026 and thereafter	4,750.0
Total	$	7,500.0

Equity (Tables)

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Summary of common stock

The following table describes the number of shares authorized, issued and outstanding of our common stock as of December 31, 2020, 2019 and 2018:

As of December 31, 2020

As of December 31, 2019

As of December 31, 2018

(In millions)

Authorized

Issued

Outstanding

Authorized

Issued

Outstanding

Authorized

Issued

Outstanding

Common stock

1,000.0

176.2

152.4

1,000.0

198.0

174.2

1,000.0

221.0

197.2

Schedule of Accumulated Other Comprehensive Income (Loss)

The following tables summarize the changes in accumulated other comprehensive income (loss), net of tax by component:


	December 31, 2020
(In millions)	Unrealized Gains (Losses) on Securities Available for Sale, net of tax		Unrealized Gains (Losses) on Cash Flow Hedges, net of tax		Gains (Losses) on Net Investment Hedge, Net of Tax		Unfunded Status of Postretirement Benefit Plans, net of tax		Currency Translation Adjustments		Total
Balance, December 31, 2019	$	4.2	$	7.8	$	25.1	$	(32.8)	$	(139.5)	$	(135.2)
Other comprehensive income (loss) before reclassifications	(9.3)		(165.0)		(30.7)		(33.5)		92.9		(145.6)
Amounts reclassified from accumulated other comprehensive income (loss)	6.5		(21.8)		(2.9)		—		—		(18.2)
Net current period other comprehensive income (loss)	(2.8)		(186.8)		(33.6)		(33.5)		92.9		(163.8)
Balance, December 31, 2020	$	1.4	$	(179.0)	$	(8.5)	$	(66.3)	$	(46.6)	$	(299.0)


	December 31, 2019
(In millions)	Unrealized Gains (Losses) on Securities Available for Sale, net of tax		Unrealized Gains (Losses) on Cash Flow Hedges, net of tax		Gains (Losses) on Net Investment Hedge, Net of Tax		Unfunded Status of Postretirement Benefit Plans, net of tax		Currency Translation Adjustments		Total
Balance, December 31, 2018	$	(4.0)	$	34.7	$	3.5	$	(31.3)	$	(243.3)	$	(240.4)
Other comprehensive income (loss) before reclassifications	11.8		88.1		28.6		(1.5)		103.8		230.8
Amounts reclassified from accumulated other comprehensive income (loss)	(3.6)		(115.0)		(7.0)		—		—		(125.6)
Net current period other comprehensive income (loss)	8.2		(26.9)		21.6		(1.5)		103.8		105.2
Balance, December 31, 2019	$	4.2	$	7.8	$	25.1	$	(32.8)	$	(139.5)	$	(135.2)


	December 31, 2018
(In millions)	Unrealized Gains (Losses) on Securities Available for Sale, net of tax		Unrealized Gains (Losses) on Cash Flow Hedges, net of tax		Gains (Losses) on Net Investment Hedge, Net of Tax		Unfunded Status of Postretirement Benefit Plans, net of tax		Currency Translation Adjustments		Total
Balance, December 31, 2017	$	(1.6)	$	(104.5)	$	—	$	(36.8)	$	(175.5)	$	(318.4)
Amount reclassified, net of tax, upon adoption of ASU 2016-01	1.5		—		—		—		—		1.5
Balance, January 1, 2018	(0.1)		(104.5)		—		(36.8)		(175.5)		(316.9)
Other comprehensive income (loss) before reclassifications	(10.6)		97.4		5.0		5.5		(67.8)		29.5
Amounts reclassified from accumulated other comprehensive income (loss)	6.7		41.8		(1.5)		—		—		47.0
Net current period other comprehensive income (loss)	(3.9)		139.2		3.5		5.5		(67.8)		76.5
Balance, December 31, 2018	$	(4.0)	$	34.7	$	3.5	$	(31.3)	$	(243.3)	$	(240.4)

Reclassification out of Accumulated Other Comprehensive Income

The following table summarizes the amounts reclassified from accumulated other comprehensive income:


(In millions)	Income Statement Location	Amounts Reclassified from Accumulated Other Comprehensive Income
		For the Years Ended December 31,
		2020		2019		2018
Gains (losses) on securities available for sale	Other income (expense)	$	(8.2)	$	4.5	$	(8.5)
	Income tax benefit (expense)	1.7		(0.9)		1.8

Gains (losses) on cash flow hedges	Revenues	18.3		118.6		(42.5)
	Operating expenses	3.3		(3.3)		0.2
	Other income (expense)	0.3		0.3		0.3
	Income tax benefit (expense)	(0.1)		(0.6)		0.2

Gains (losses) on net investment hedge	Other Income (expense)	2.9		7.0		1.5

Total reclassifications, net of tax		$	18.2	$	125.6	$	(47.0)

Earnings per Share (Tables)

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

Basic and diluted earnings per share

Basic and diluted earnings per share are calculated as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Numerator:
Net income attributable to Biogen Inc.	$	4,000.6	$	5,888.5	$	4,430.7
Denominator:
Weighted average number of common shares outstanding	160.9		187.1		204.9
Effect of dilutive securities:

Time-vested restricted stock units	0.2		0.2		0.3
Market stock units	0.1		0.1		0.1
Performance stock units settled in stock	0.1		—		—
Dilutive potential common shares	0.4		0.3		0.4
Shares used in calculating diluted earnings per share	161.3		187.4		205.3

Share-Based Payments (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Share-based compensation expense included in consolidated statements of income

The following table summarizes share-based compensation expense included in our consolidated statements of income:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Research and development	$	80.0	$	77.1	$	75.8
Selling, general and administrative	131.3		148.3		105.8
Subtotal	211.3		225.4		181.6
Capitalized share-based compensation costs	(6.2)		(8.9)		(11.5)
Share-based compensation expense included in total cost and expenses	205.1		216.5		170.1
Income tax effect	(33.5)		(35.7)		(27.5)
Share-based compensation expense included in net income attributable to Biogen Inc.	$	171.6	$	180.8	$	142.6

Summary of share-based compensation expense associated with each of our share-based compensating programs

The following table summarizes share-based compensation expense associated with each of our share-based compensation programs:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Market stock units	$	40.5	$	30.4	$	27.2
Time-vested restricted stock units	142.6		134.0		126.6
Cash settled performance units	(1.7)		0.7		7.8
Performance units	(0.1)		1.6		3.1
Performance stock units settled in stock	7.9		15.5		4.7
Performance stock units settled in cash	8.6		5.5		1.7
Employee stock purchase plan	13.5		11.5		10.5
NST stock options	—		26.2		—
Subtotal	211.3		225.4		181.6
Capitalized share-based compensation costs	(6.2)		(8.9)		(11.5)
Share-based compensation expense included in total cost and expenses	$	205.1	$	216.5	$	170.1

Stock option activity

The following table summarizes our stock option activity:


	Shares	Weighted Average Exercise Price
Outstanding at December 31, 2019	12,000	$	58.46
Granted	—	—
Exercised	(12,000)	58.46
Cancelled	—	—
Outstanding at December 31, 2020	—	$	—

Tax benefit and cash received from stock option exercises

The following table summarizes the amount of tax benefit realized for stock options and cash received from the exercise of stock options:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Tax benefit realized for stock options	$	2.9	$	2.5	$	2.2
Cash received from the exercise of stock options	0.7		0.4		0.8

Market stock units activity

The following table summarizes our MSU activity:


	December 31, 2020
	Shares	Weighted Average Grant Date Fair Value
Unvested at December 31, 2019	183,000	$	378.09
Granted (1)	133,000	398.61
Vested	(82,000)	375.42
Forfeited	(33,000)	402.62
Unvested at December 31, 2020	201,000	$	388.98

Assumptions used in valuation of market based stock units

The assumptions used in our valuation are summarized as follows:


	For the Years Ended December 31,
	2020	2019	2018
Expected dividend yield	—%	—%	—%
Range of expected stock price volatility	37.8% - 44.1%	31.2% - 33.6%	27.5% - 32.4%
Range of risk-free interest rates	1.41% - 1.48%	2.46% - 2.53%	1.9% - 2.3%
30 calendar day average stock price on grant date	$257.83 - $325.40	$228.59 - $331.18	$279.47 - $346.76
Weighted-average per share grant date fair value	$398.61	$378.08	$378.85

Cash settled performance shares activity

The following table summarizes our CSPU activity:


			Shares
Unvested at December 31, 2019			13,000
Granted			—
Vested			(13,000)
Forfeited			—
Unvested at December 31, 2020			—

Performance units activity

The following table summarizes our PU activity:


			Shares
Unvested at December 31, 2019			11,000
Granted			—
Vested			(11,000)
Forfeited			—
Unvested at December 31, 2020			—

Performance stock units settled in stock activity

The following table summarizes our PSUs that settle in stock activity:


	Shares	Weighted Average Grant Date Fair Value
Unvested at December 31, 2019	111,000	$	316.39
Granted (1)	73,000	293.35
Vested	—	—
Forfeited	(29,000)	323.42
Unvested at December 31, 2020	155,000	$	304.19

Performance stock units settled in cash activity

The following table summarizes our PSUs that settle in cash activity:


			Shares
Unvested at December 31, 2019			82,000
Granted (1)			63,000
Vested			(2,000)
Forfeited			(23,000)
Unvested at December 31, 2020			120,000

Time-vested restricted stock units activity

The following table summarizes our RSU activity:


	Shares	Weighted Average Grant Date Fair Value
Unvested at December 31, 2019	938,000	$	306.55
Granted (1)	620,000	318.87
Vested	(440,000)	304.45
Forfeited	(100,000)	314.46
Unvested at December 31, 2020	1,018,000	$	314.46

Shares issued under employee stock purchase plan

The following table summarizes our ESPP activity:


	For the Years Ended December 31,
(In millions, except share amounts)	2020		2019		2018
Shares issued under the 2015 ESPP	212,000		204,000		170,000
Cash received under the 2015 ESPP	$	48.6	$	40.4	$	40.5

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Income before income tax provision and the income tax expense

Income before income tax expense and the income tax expense consist of the following:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Income before income taxes (benefit):
Domestic	$	3,290.0	$	4,725.3	$	3,877.0
Foreign	1,757.5		2,400.6		2,022.6
Total	$	5,047.5	$	7,125.9	$	5,899.6
Income tax expense (benefit):
Current:
Federal	$	647.0	$	947.4	$	1,131.8
State	41.2		59.1		45.5
Foreign	155.1		84.4		140.0
Total	843.3		1,090.9		1,317.3
Deferred:
Federal	$	(1,749.9)	$	1,143.9	$	(62.0)
State	(6.8)		(2.3)		(7.4)
Foreign	1,905.7		(1,074.5)		177.7
Total	149.0		67.1		108.3
Total income tax expense	$	992.3	$	1,158.0	$	1,425.6

Components of deferred tax assets and liabilities

Significant components of our deferred tax assets and liabilities are summarized as follows:


	As of December 31,
(In millions)	2020		2019
Deferred tax assets:
Tax credits	$	113.4	$	106.6
Inventory, other reserves and accruals	165.9		162.0
Intangibles, net	1,546.0		3,380.0
Net operating loss	2,080.3		130.4
Share-based compensation	23.3		23.8
Other	103.1		103.7
Valuation allowance	(1,753.9)		(1.1)
Total deferred tax assets	$	2,278.1	$	3,905.4
Deferred tax liabilities:
Purchased intangible assets	$	(396.2)	$	(350.3)
GILTI	(1,143.7)		(1,381.6)
Tax credits	(174.6)		(1,617.2)
Depreciation, amortization and other	(227.0)		(135.0)
Total deferred tax liabilities	$	(1,941.5)	$	(3,484.1)

Reconciliation between the U.S. federal statutory tax rate and effective tax rate

A reconciliation between the U.S. federal statutory tax rate and our effective tax rate is summarized as follows:


	For the Years Ended December 31,
	2020		2019		2018
Statutory rate	21.0	%	21.0	%	21.0	%
State taxes	0.7		0.8		0.6
Taxes on foreign earnings	(3.3)		(4.5)		(1.9)
Credits and net operating loss utilization	(1.2)		(1.1)		(0.9)
Purchased intangible assets	0.7		0.4		1.2
Divestiture of Denmark manufacturing operations	(0.4)		1.0		—
Internal reorganization of certain intellectual property rights	—		(2.1)		—
TECFIDERA impairment	1.8		—		—
GILTI	1.3		1.5		1.6

U.S. tax reform	—		—		2.1
Swiss tax reform	—		(0.8)		—


Other	(0.9)		0.1		0.5
Effective tax rate	19.7	%	16.3	%	24.2	%

Reconciliation of beginning and ending amount of unrecognized tax benefits

A reconciliation of the beginning and ending amount of our unrecognized tax benefits is summarized as follows:



(In millions)	2020		2019		2018
Beginning balance at January 1,	$	129.9	$	114.2	$	66.8
Additions based on tax positions related to the current period	1.5		5.3		0.5
Additions for tax positions of prior periods	51.7		17.2		58.7
Reductions for tax positions of prior periods	(63.6)		(10.3)		(13.6)
Statute expirations	(7.9)		(0.1)		(2.9)
Settlement refund (payment)	(35.9)		3.6		4.7
Ending balance ar December 31,	$	75.7	$	129.9	$	114.2

Other Consolidated Financial Statement Detail (Tables)

12 Months Ended

Dec. 31, 2020

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Supplemental cash flow information

Supplemental disclosure of cash flow information for the years ended December 31, 2020, 2019 and 2018, is as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Cash paid during the year for:
Interest	$	272.7	$	244.2	$	243.2
Income taxes	906.7		1,064.5		1,007.1

Other income (expense), net

Components of other income (expense), net, are summarized as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Interest income	$	42.0	$	120.0	$	112.5
Interest expense	(222.5)		(187.4)		(200.6)
Gain (loss) on investments, net	685.7		204.7		119.5
Foreign exchange gains (losses), net	(10.7)		(7.0)		(9.9)
Other, net	2.9		(47.0)		(10.5)
Total other income (expense), net	$	497.4	$	83.3	$	11.0

Gain (loss) on investments

The following table summarizes our gain (loss) on investments, net that relates to our equity securities held as of December 31, 2020, 2019 and 2018:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Net gains (losses) recognized during the period on equity securities	$	693.9	$	200.1	$	127.9
Less: Net gains (losses) recognized on equity securities sold during the period and on capital distributions	12.1		50.0		(0.6)
Unrealized gains (losses) recognized during the period on equity securities	$	681.8	$	150.1	$	128.5

Accrued expenses and other

Accrued expenses and other consists of the following:


	As of December 31,
(In millions)	2020		2019
Revenue-related reserves for discounts and allowances	$	1,080.6	$	1,001.1
Collaboration expenses	389.9		281.6
Employee compensation and benefits	333.8		309.1
Royalties and licensing fees	218.5		220.9
Derivative liabilities	181.5		6.7
Current portion of contingent consideration obligations	149.6		148.4

Other	791.4		798.0
Total accrued expenses and other	$	3,145.3	$	2,765.8

Collaborative and Other Relationships (Tables)

12 Months Ended

Dec. 31, 2020

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Co-promotion profit sharing formula

Our share of annual co-promotion profits in excess of $50.0 million varies, as summarized in the table below, upon the following events:


Until GAZYVA First Non-CLL FDA Approval	40.0	%
After GAZYVA First Non-CLL FDA Approval until First GAZYVA Threshold Date	39.0	%
After First GAZYVA Threshold Date until Second GAZYVA Threshold Date	37.5	%
After Second GAZYVA Threshold Date	35.0	%

Pretax profit sharing formula

Our share of annual profits in excess of $50.0 million varies, as summarized in the table below, upon the following events:


Until First GAZYVA Threshold Date	39.0	%
After First GAZYVA Threshold Date until Second GAZYVA Threshold Date	37.5	%
After Second GAZYVA Threshold Date	35.0	%

Revenues from anti-CD20 therapeutic programs

Revenues from anti-CD20 therapeutic programs are summarized in the table below. For purposes of this footnote, we refer to RITUXAN and RITUXAN HYCELA collectively as RITUXAN.


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	$	1,080.2	$	1,542.4	$	1,431.9
Other revenues from anti-CD20 therapeutic programs	897.6		748.0		548.3
Total revenues from anti-CD20 therapeutic programs	$	1,977.8	$	2,290.4	$	1,980.2

Revenues from anti-CD20 therapeutic programs are summarized as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA, including the reimbursement of selling and development expenses	$	1,080.2	$	1,542.4	$	1,431.9
Other revenues from anti-CD20 therapeutic programs	897.6		748.0		548.3
Total revenues from anti-CD20 therapeutic programs	$	1,977.8	$	2,290.4	$	1,980.2

Summary of activity related to BAN2401 and Elenbecestat collaboration

A summary of development and sales and marketing expenses related to the BAN2401 Collaboration is as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Total development expense incurred by the collaboration related to the advancement of BAN2401 and elenbecestat	$	219.3	$	348.7	$	232.0
Biogen's share of BAN2401 and elenbecestat development expense reflected in research and development expense in our consolidated statements of income	109.6		174.3		116.0
Total sales and marketing expense incurred by the collaboration	9.8		32.4		10.7
Biogen's share of BAN2401 and elenbecestat sales and marketing expense reflected in selling, general and administrative expense in our consolidated statements of income	4.9		16.2		5.4

Summary of activity related to Aducanumab collaboration

A summary of development, sales and marketing and milestone expense related to the Aducanumab Collaboration Agreement is as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Total aducanumab development expense	$	152.0	$	179.4	$	264.8
Biogen's share of aducanumab development expense reflected in research and development expense in our consolidated statements of income	83.6		98.7		234.6
Total aducanumab sales and marketing expense incurred by the collaboration	353.0		27.4		50.6
Biogen's share of aducanumab sales and marketing expense reflected in selling, general and administrative expense in our consolidated statements of income	193.7		15.1		27.3
Total aducanumab collaboration third party milestone expense	75.0		—		—
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income	33.8		—		—

Summary of activity related to the UCB collaboration

A summary of development expense related to the UCB collaboration agreement is as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Total UCB development expense	$	58.3	$	31.9	$	29.7
Biogen's share of UCB development expense reflected in research and development expense in our consolidated statements of income	29.2		16.0		14.9

Segment Information (Tables)

12 Months Ended

Dec. 31, 2020

Segment Reporting [Abstract]

Geographic information

The following tables contain certain financial information by geographic area:


	December 31, 2020
(In millions)	U.S.		Europe		Asia		Other		Total
Product revenues from external customers	$	5,900.1	$	3,656.4	$	596.7	$	539.0	$	10,692.2
Revenues from anti-CD20 therapeutic programs	1,897.4		0.1		—		80.3		1,977.8
Other revenues from external customers	733.6		8.1		32.9		—		774.6
Long-lived assets	1,496.3		2,321.4		16.2		10.9		3,844.8

	December 31, 2019
(In millions)	U.S.		Europe		Asia		Other		Total
Product revenues from external customers	$	6,713.8	$	3,794.5	$	320.3	$	551.2	$	11,379.8
Revenues from anti-CD20 therapeutic programs	2,211.9		0.2		—		78.3		2,290.4
Other revenues from external customers	585.8		9.7		112.2		—		707.7
Long-lived assets	1,493.2		2,162.9		6.2		12.0		3,674.3

	December 31, 2018
(In millions)	U.S.		Europe		Asia		Other		Total
Product revenues from external customers	$	6,800.5	$	3,370.3	$	281.2	$	434.8	$	10,886.8
Revenues from anti-CD20 therapeutic programs	1,903.4		0.2		—		76.6		1,980.2
Other revenues from external customers	457.0		32.7		96.2		—		585.9
Long-lived assets	1,152.7		2,442.8		3.9		1.8		3,601.2

Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2020

Quarterly Financial Information Disclosure [Abstract]

Quarterly Financial Data (Unaudited)


(In millions, except per share amounts)	First Quarter		Second Quarter		Third Quarter		Fourth Quarter		Total Year
2020
Product revenues, net	$	2,904.6	$	2,795.7	$	2,690.3	$	2,301.6	$	10,692.2
Revenues from anti-CD20 therapeutic programs	$	520.4	$	478.3	$	560.1	$	419.0	$	1,977.8
Other revenues	$	109.3	$	407.6	$	125.7	$	132.0	$	774.6
Total revenues	$	3,534.3	$	3,681.6	$	3,376.1	$	2,852.6	$	13,444.6
Gross profit (1)	$	3,080.0	$	3,270.5	$	2,927.0	$	2,361.9	$	11,639.4
Net income	$	1,392.6	$	1,606.5	$	703.9	$	357.6	$	4,060.5
Net income attributable to Biogen Inc.	$	1,399.1	$	1,542.1	$	701.5	$	357.9	$	4,000.6
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$	8.10	$	9.60	$	4.47	$	2.33	$	24.86
Diluted earnings per share attributable to Biogen Inc.	$	8.08	$	9.59	$	4.46	$	2.32	$	24.80
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	172.8		160.6		156.9		153.7		160.9
Diluted earnings per share attributable to Biogen Inc.	173.1		160.9		157.2		154.0		161.3


(In millions, except per share amounts)	First Quarter		Second Quarter		Third Quarter		Fourth Quarter		Total Year
2019
Product revenues, net	$	2,680.0	$	2,880.3	$	2,894.7	$	2,924.8	$	11,379.8
Revenues from anti-CD20 therapeutic programs	$	517.4	$	576.4	$	595.8	$	600.8	$	2,290.4
Other revenues	$	292.4	$	160.0	$	109.6	$	145.7	$	707.7
Total revenues	$	3,489.8	$	3,616.7	$	3,600.1	$	3,671.3	$	14,377.9
Gross profit (1)	$	2,887.8	$	3,140.4	$	3,170.1	$	3,224.2	$	12,422.5
Net income	$	1,408.8	$	1,494.1	$	1,545.9	$	1,439.7	$	5,888.5
Net income attributable to Biogen Inc.	$	1,408.8	$	1,494.1	$	1,545.9	$	1,439.7	$	5,888.5
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$	7.17	$	7.85	$	8.40	$	8.10	$	31.47
Diluted earnings per share attributable to Biogen Inc.	$	7.15	$	7.85	$	8.39	$	8.08	$	31.42
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	196.6		190.3		184.0		177.8		187.1
Diluted earnings per share attributable to Biogen Inc.	197.0		190.4		184.2		178.2		187.4

(1) Gross profit is calculated as total revenues less cost of sales, excluding amortization and impairment of acquired intangible assets.

Summary of Significant Accounting Policies - Narrative (Details) $ in Millions	9 Months Ended	12 Months Ended			24 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2020 USD ($) product segment	Dec. 31, 2019 USD ($) product	Dec. 31, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2017 USD ($)
Property, Plant and Equipment [Line Items]
Number of ophthalmolgy biosimilar products \| product		2	2
Interest in subsidiary (less than given percentage)		100.00%
Operating lease assets		$ 433.3	$ 427.0		$ 433.3
Present value of operating lease liabilities		$ 485.2	486.3		485.2
Number of operating segments \| segment		1
Advertising costs		$ 111.8	79.2	$ 90.2
Stockholders equity	$ 13,031.6	10,686.1	$ 13,339.1	13,031.6	10,686.1	$ 12,598.1
Eisai
Property, Plant and Equipment [Line Items]
Capitalized inventory		$ 93.8			$ 93.8
Percentage of future development costs related to Eisai	15.00%	45.00%			45.00%
Cumulative Effect, Period of Adoption, Adjustment
Property, Plant and Equipment [Line Items]
Operating lease assets	$ 463.0			463.0
Present value of operating lease liabilities	526.0			526.0
Stockholders equity						106.1
Minimum
Property, Plant and Equipment [Line Items]
Payment terms of accounts receivable arising from product sales		30 days
Minimum \| Computer Software and Hardware
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life		3 years
Maximum
Property, Plant and Equipment [Line Items]
Payment terms of accounts receivable arising from product sales		90 days
Maximum \| Computer Software and Hardware
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life		5 years
Deferred Income Tax Charge \| Cumulative Effect, Period of Adoption, Adjustment
Property, Plant and Equipment [Line Items]
Stockholders equity						500.0
Retained Earnings \| Cumulative Effect, Period of Adoption, Adjustment
Property, Plant and Equipment [Line Items]
Stockholders equity						$ 500.0
Deferred tax liability \| Cumulative Effect, Period of Adoption, Adjustment
Property, Plant and Equipment [Line Items]
Stockholders equity	$ 400.0			$ 400.0

Summary of Significant Accounting Policies - Summary of Property, Plant and Equipment Estimated Useful Lives (Details)	12 Months Ended
	Dec. 31, 2020
Minimum \| Buildings
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	15 years
Minimum \| Furniture and Fixtures
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Minimum \| Machinery and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Minimum \| Computer Software and Hardware
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Maximum \| Buildings
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	40 years
Maximum \| Furniture and Fixtures
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	7 years
Maximum \| Machinery and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	20 years
Maximum \| Computer Software and Hardware
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years

Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions	1 Months Ended					12 Months Ended
	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($) product	Jul. 30, 2018 USD ($)	Apr. 30, 2018 USD ($)	Jan. 31, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 07, 2019 USD ($) $ / shares
Business Acquisition [Line Items]
Acquired in-process research and development						$ 75.0	$ 0.0	$ 112.5
Clinical assets acquired \| product		2
BIIB118
Business Acquisition [Line Items]
Acquired in-process research and development	$ 75.0
Estimated additional payments upon achievement of development and commercial milestones	$ 635.0
Nightstar
Business Acquisition [Line Items]
Price per share \| $ / shares									$ 25.50
Total transaction value		$ 847.6
Payments for pre-combination equity compensation		4.6
Fair value, in-process research and development, discount rate									12.50%
Intangible assets									$ 585.0
Deferred tax liabilities, goodwill									60.9
Nightstar \| BIIB111
Business Acquisition [Line Items]
Intangible assets									480.0
Nightstar \| BIIB112
Business Acquisition [Line Items]
Intangible assets									$ 220.0
Nightstar \| Post-acquisition equity compensation
Business Acquisition [Line Items]
Fair-value post-combination equity compensation		26.2
Nightstar \| Post-acquisition equity compensation \| Selling, general and administrative
Business Acquisition [Line Items]
Fair-value post-combination equity compensation		18.4
Nightstar \| Post-acquisition equity compensation \| Research and development
Business Acquisition [Line Items]
Fair-value post-combination equity compensation		$ 7.8
Karyopharm
Business Acquisition [Line Items]
Acquired in-process research and development					$ 10.0
Estimated additional payments upon achievement of development and commercial milestones						207.0
Pfizer
Business Acquisition [Line Items]
Acquired in-process research and development				$ 75.0
Estimated additional payments upon achievement of development and commercial milestones						515.0
AliveGen
Business Acquisition [Line Items]
Acquired in-process research and development			$ 27.5
Estimated additional payments upon achievement of development and commercial milestones						$ 535.0

Divestitures (Details) - USD ($) $ in Millions	12 Months Ended
Divestitures (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Aug. 31, 2019
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
(Gain) loss on divestiture of Hillerød, Denmark manufacturing operations	$ 92.5	$ (55.3)	$ 0.0
Inventory raw materials sold to FUJIFILM				$ 41.8
Denmark Manufacturing Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration received for sale of Denmark manufacturing operations				$ 881.9
Loss on divestiture of Hillerød, Denmark manufacturing operations, net of tax		124.2
(Gain) loss on divestiture of Hillerød, Denmark manufacturing operations	30.5	(55.3)
Expected costs to sell disposal group		11.2
Future minimum batch production for Denmark manufacturing operations		74.0
Tax expense on disposal group		$ 68.9
Purchase commitment, reduction	$ 62.0

Revenues - Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 2,852.6	$ 3,376.1	$ 3,681.6	$ 3,534.3	$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	$ 13,444.6	$ 14,377.9	$ 13,452.9
Fumarate
Disaggregation of Revenue [Line Items]
Revenues									3,905.4	4,438.2	4,274.1
Interferon
Disaggregation of Revenue [Line Items]
Revenues									1,877.5	2,101.8	2,363.0
TYSABRI
Disaggregation of Revenue [Line Items]
Revenues									1,946.1	1,892.2	1,864.0
FAMPYRA
Disaggregation of Revenue [Line Items]
Revenues									103.1	97.1	92.7
ZINBRYTA
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	1.4
Subtotal: MS
Disaggregation of Revenue [Line Items]
Revenues									7,832.1	8,529.3	8,595.2
SPINRAZA
Disaggregation of Revenue [Line Items]
Revenues									2,052.1	2,097.0	1,724.2
BENEPALI
Disaggregation of Revenue [Line Items]
Revenues									481.6	486.2	485.2
IMRALDI
Disaggregation of Revenue [Line Items]
Revenues									216.3	184.0	16.7
FLIXABI
Disaggregation of Revenue [Line Items]
Revenues									97.9	68.1	43.2
Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Revenues									795.8	738.3	545.1
FUMADERM
Disaggregation of Revenue [Line Items]
Revenues									12.2	15.2	22.3
Total product revenues
Disaggregation of Revenue [Line Items]
Revenues	$ 2,301.6	$ 2,690.3	$ 2,795.7	$ 2,904.6	$ 2,924.8	$ 2,894.7	$ 2,880.3	$ 2,680.0	10,692.2	11,379.8	10,886.8
United States \| Fumarate
Disaggregation of Revenue [Line Items]
Revenues									2,742.0	3,312.0	3,253.2
United States \| Interferon
Disaggregation of Revenue [Line Items]
Revenues									1,273.5	1,426.6	1,668.3
United States \| TYSABRI
Disaggregation of Revenue [Line Items]
Revenues									1,096.8	1,041.8	1,025.0
United States \| FAMPYRA
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0
United States \| ZINBRYTA
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0
United States \| Subtotal: MS
Disaggregation of Revenue [Line Items]
Revenues									5,112.3	5,780.4	5,946.5
United States \| SPINRAZA
Disaggregation of Revenue [Line Items]
Revenues									787.8	933.4	854.0
United States \| BENEPALI
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0
United States \| IMRALDI
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0
United States \| FLIXABI
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0
United States \| Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0
United States \| FUMADERM
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0
United States \| Total product revenues
Disaggregation of Revenue [Line Items]
Revenues									5,900.1	6,713.8	6,800.5
Rest of World \| Fumarate
Disaggregation of Revenue [Line Items]
Revenues									1,163.4	1,126.2	1,020.9
Rest of World \| Interferon
Disaggregation of Revenue [Line Items]
Revenues									604.0	675.2	694.7
Rest of World \| TYSABRI
Disaggregation of Revenue [Line Items]
Revenues									849.3	850.4	839.0
Rest of World \| FAMPYRA
Disaggregation of Revenue [Line Items]
Revenues									103.1	97.1	92.7
Rest of World \| ZINBRYTA
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	1.4
Rest of World \| Subtotal: MS
Disaggregation of Revenue [Line Items]
Revenues									2,719.8	2,748.9	2,648.7
Rest of World \| SPINRAZA
Disaggregation of Revenue [Line Items]
Revenues									1,264.3	1,163.6	870.2
Rest of World \| BENEPALI
Disaggregation of Revenue [Line Items]
Revenues									481.6	486.2	485.2
Rest of World \| IMRALDI
Disaggregation of Revenue [Line Items]
Revenues									216.3	184.0	16.7
Rest of World \| FLIXABI
Disaggregation of Revenue [Line Items]
Revenues									97.9	68.1	43.2
Rest of World \| Subtotal: Biosimilars
Disaggregation of Revenue [Line Items]
Revenues									795.8	738.3	545.1
Rest of World \| FUMADERM
Disaggregation of Revenue [Line Items]
Revenues									12.2	15.2	22.3
Rest of World \| Total product revenues
Disaggregation of Revenue [Line Items]
Revenues									$ 4,792.1	$ 4,666.0	$ 4,086.3

Revenues - Narrative (Details) $ in Millions	3 Months Ended								12 Months Ended
Revenues - Narrative (Details) $ in Millions	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) wholesaler	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Number of wholesalers \| wholesaler									2
Percentage of total revenues									14.70%	15.90%	14.70%
Revenues	$ 2,852.6	$ 3,376.1	$ 3,681.6	$ 3,534.3	$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	$ 13,444.6	$ 14,377.9	$ 13,452.9
Other
Disaggregation of Revenue [Line Items]
Revenues	132.0	$ 125.7	407.6	$ 109.3	$ 145.7	$ 109.6	$ 160.0	$ 292.4	774.6	707.7	585.9
Other corporate
Disaggregation of Revenue [Line Items]
Revenues									719.1	584.5	459.4
Remaining performance obligation, amount	$ 153.8		$ 500.0						153.8
Bioverativ \| Other
Disaggregation of Revenue [Line Items]
Revenues									48.6	$ 383.2	$ 206.7
Contract Manufacturing Customer \| Other corporate
Disaggregation of Revenue [Line Items]
Revenues									$ 346.2
Distributor One
Disaggregation of Revenue [Line Items]
Percentage of revenue from major distributors									30.50%	30.00%	32.00%
Percentage receivables of wholesale distributor accounted in consolidated receivables	21.10%				24.10%				21.10%	24.10%
Distributor Two
Disaggregation of Revenue [Line Items]
Percentage of revenue from major distributors									15.30%	17.20%	18.40%
Percentage receivables of wholesale distributor accounted in consolidated receivables	8.50%				13.90%				8.50%	13.90%

Revenues - Reserves for Discounts and Allowances (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balance	$ 1,198.9	$ 1,051.3	$ 761.6
Current provisions relating to sales in current year	4,102.5	3,698.6	3,389.1
Adjustments relating to prior years	(53.7)	(48.3)	(12.1)
Payments/returns relating to sales in current year	(3,061.2)	(2,778.6)	(2,440.4)
Payments/returns relating to sales in prior years	(910.5)	(724.1)	(646.9)
Ending balance	1,276.0	1,198.9	1,051.3
Discounts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balance	131.1	127.8	109.6
Current provisions relating to sales in current year	774.7	666.2	679.3
Adjustments relating to prior years	(1.0)	0.3	(0.3)
Payments/returns relating to sales in current year	(635.1)	(535.5)	(551.7)
Payments/returns relating to sales in prior years	(128.3)	(127.7)	(109.1)
Ending balance	141.4	131.1	127.8
Contractual Adjustments
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balance	1,027.3	888.8	606.0
Current provisions relating to sales in current year	3,308.8	3,011.5	2,686.7
Adjustments relating to prior years	(54.0)	(54.1)	(10.0)
Payments/returns relating to sales in current year	(2,426.1)	(2,242.9)	(1,887.6)
Payments/returns relating to sales in prior years	(763.0)	(576.0)	(506.3)
Ending balance	1,093.0	1,027.3	888.8
Returns
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balance	40.5	34.7	46.0
Current provisions relating to sales in current year	19.0	20.9	23.1
Adjustments relating to prior years	1.3	5.5	(1.8)
Payments/returns relating to sales in current year	0.0	(0.2)	(1.1)
Payments/returns relating to sales in prior years	(19.2)	(20.4)	(31.5)
Ending balance	$ 41.6	$ 40.5	$ 34.7

Revenues - Total Reserves for Discounts and Allowances (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Revenue-related reserves for discounts and allowances	$ 1,276.0	$ 1,198.9	$ 1,051.3	$ 761.6
Reduction of accounts receivable
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Revenue-related reserves for discounts and allowances	195.4	197.8
Component of accrued expenses and other
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Revenue-related reserves for discounts and allowances	$ 1,080.6	$ 1,001.1

Revenues - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 2,852.6	$ 3,376.1	$ 3,681.6	$ 3,534.3	$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	$ 13,444.6	$ 14,377.9	$ 13,452.9
Revenues from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 419.0	$ 560.1	$ 478.3	$ 520.4	$ 600.8	$ 595.8	$ 576.4	$ 517.4	1,977.8	2,290.4	1,980.2
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA									1,080.2	1,542.4	1,431.9
Other revenues from anti-CD20 therapeutic programs									$ 897.6	$ 748.0	$ 548.3

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Revenues - Other Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
Revenues - Other Revenues (Details) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 2,852.6	$ 3,376.1	$ 3,681.6	$ 3,534.3	$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	$ 13,444.6	$ 14,377.9	$ 13,452.9
Royalty
Disaggregation of Revenue [Line Items]
Total revenues									33.9	17.0	38.7
Other corporate
Disaggregation of Revenue [Line Items]
Total revenues									719.1	584.5	459.4
Total other revenues
Disaggregation of Revenue [Line Items]
Total revenues	$ 132.0	$ 125.7	$ 407.6	$ 109.3	$ 145.7	$ 109.6	$ 160.0	$ 292.4	774.6	707.7	585.9
Collaborative Arrangement \| Samsung Bioepis
Disaggregation of Revenue [Line Items]
Total revenues									20.9	106.2	96.4
Collaborative Arrangement \| Other Relationships
Disaggregation of Revenue [Line Items]
Total revenues									$ 0.7	$ 0.0	$ (8.6)

Share-based Compensation Arrangement by Share-based Payment Award

Inventory - Components of Inventory (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Inventory Disclosure [Abstract]
Raw materials	$ 314.9	$ 169.7
Work in process	544.5	460.0
Finished goods	209.2	174.5
Total inventory	$ 1,068.6	$ 804.2

Inventory - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Inventory - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Aug. 31, 2019
Inventory [Line Items]
Write-downs on excess, obsolete, unmarketable or other inventory	$ 26.6	$ 52.2	$ 41.9
Inventory raw materials sold to FUJIFILM				$ 41.8
Work-in-process
Inventory [Line Items]
Inventory work-in-process sold to FUJIFILM				$ 14.0

Intangible Assets and Goodwill - Summary of Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Cost	$ 7,394.3	$ 7,379.3
Total intangible assets	8,220.8	8,408.8
Accumulated Amortization	(5,136.5)	(4,881.4)
Net	2,257.8	2,497.9
Total intangible assets	$ 3,084.3	3,527.4
Minimum
Finite-Lived Intangible Assets [Line Items]
Estimated life (in years)	4 years
Maximum
Finite-Lived Intangible Assets [Line Items]
Estimated life (in years)	28 years
In-process research and development
Finite-Lived Intangible Assets [Line Items]
Cost and net	$ 762.5	965.5
Trademarks and trade names
Finite-Lived Intangible Assets [Line Items]
Cost and net	$ 64.0	$ 64.0

Intangible Assets and Goodwill - Narrative (Details) - USD ($)	3 Months Ended			12 Months Ended
	Dec. 31, 2020	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Jun. 07, 2019
Goodwill [Line Items]
Amortization and impairment of acquired intangible assets				$ 464,800,000	$ 489,900,000	$ 747,300,000
Amortization of acquired intangible assets				255,100,000	274,000,000.0	381,200,000
Accumulated impairment losses related to goodwill	$ 0			0
Intangible Assets, Net (Excluding Goodwill)	3,084,300,000			3,084,300,000	3,527,400,000
TECFIDERA
Goodwill [Line Items]
Impairment						176,800,000
In-process research and development
Goodwill [Line Items]
Amortization and impairment of acquired intangible assets				19,300,000
In-process research and development
Goodwill [Line Items]
Impairment		$ 215,900,000	$ 189,300,000		215,900,000	$ 189,300,000
Intangible assets (excluding goodwill)	762,500,000			762,500,000	$ 965,500,000
BIIB111 \| In-process research and development
Goodwill [Line Items]
Intangible Assets, Net (Excluding Goodwill)	365,000,000.0			365,000,000.0			$ 480,000,000.0
TGN \| In-process research and development
Goodwill [Line Items]
Intangible assets (excluding goodwill)	177,500,000			177,500,000
Nightstar
Goodwill [Line Items]
Intangible assets								$ 585,000,000.0
Nightstar \| BIIB111
Goodwill [Line Items]
Impairment				115,000,000.0
Intangible assets								$ 480,000,000.0
Nightstar \| BIIB111 \| In-process research and development
Goodwill [Line Items]
Impairment	115,000,000.0
Nightstar \| BIIB054
Goodwill [Line Items]
Impairment	$ 75,400,000			$ 75,400,000

Intangible Assets and Goodwill - Estimated Future Amortization of Intangible Assets (Details) $ in Millions	Dec. 31, 2020 USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2021	$ 205.0
2022	215.0
2023	215.0
2024	225.0
2025	$ 220.0

Intangible Assets and Goodwill - Changes in Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
Goodwill, beginning of year	$ 5,757.8	$ 5,706.4
Increase to goodwill	0.0	117.5
Elimination of goodwill allocated to Hillerød, Denmark manufacturing operations	0.0	(69.5)
Other	4.3	3.4
Goodwill, end of year	$ 5,762.1	$ 5,757.8

Fair Value Measurements - Summary of Assets and Liabilities Recorded at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Assets:
Fair Value	$ 2,051.0	$ 2,970.3
Fair value, measurements, recurring
Assets:
Cash equivalents	626.9	2,541.1
Marketable equity securities	1,974.3	337.5
Derivative contracts	20.5	43.8
Plan assets for deferred compensation	28.2	27.7
Total	4,700.9	5,920.4
Liabilities:
Derivative contracts	217.2	8.3
Contingent consideration obligations	259.8	346.1	$ 409.8
Total	477.0	354.4
Fair value, measurements, recurring \| Corporate debt securities
Assets:
Fair Value	1,301.5	1,695.1
Fair value, measurements, recurring \| Government securities
Assets:
Fair Value	627.1	1,013.9
Fair value, measurements, recurring \| Mortgage and other asset backed securities
Assets:
Fair Value	122.4	261.3
Quoted Prices in Active Markets (Level 1) \| Fair value, measurements, recurring
Assets:
Cash equivalents	0.0	0.0
Marketable equity securities	271.1	7.9
Derivative contracts	0.0	0.0
Plan assets for deferred compensation	0.0	0.0
Total	271.1	7.9
Liabilities:
Derivative contracts	0.0	0.0
Contingent consideration obligations	0.0	0.0
Total	0.0	0.0
Quoted Prices in Active Markets (Level 1) \| Fair value, measurements, recurring \| Corporate debt securities
Assets:
Fair Value	0.0	0.0
Quoted Prices in Active Markets (Level 1) \| Fair value, measurements, recurring \| Government securities
Assets:
Fair Value	0.0	0.0
Quoted Prices in Active Markets (Level 1) \| Fair value, measurements, recurring \| Mortgage and other asset backed securities
Assets:
Fair Value	0.0	0.0
Significant Other Observable Inputs (Level 2) \| Fair value, measurements, recurring
Assets:
Cash equivalents	626.9	2,541.1
Marketable equity securities	1,703.2	329.6
Derivative contracts	20.5	43.8
Plan assets for deferred compensation	28.2	27.7
Total	4,429.8	5,912.5
Liabilities:
Derivative contracts	217.2	8.3
Contingent consideration obligations	0.0	0.0
Total	217.2	8.3
Significant Other Observable Inputs (Level 2) \| Fair value, measurements, recurring \| Corporate debt securities
Assets:
Fair Value	1,301.5	1,695.1
Significant Other Observable Inputs (Level 2) \| Fair value, measurements, recurring \| Government securities
Assets:
Fair Value	627.1	1,013.9
Significant Other Observable Inputs (Level 2) \| Fair value, measurements, recurring \| Mortgage and other asset backed securities
Assets:
Fair Value	122.4	261.3
Significant Unobservable Inputs (Level 3) \| Fair value, measurements, recurring
Assets:
Cash equivalents	0.0	0.0
Marketable equity securities	0.0	0.0
Derivative contracts	0.0	0.0
Plan assets for deferred compensation	0.0	0.0
Total	0.0	0.0
Liabilities:
Derivative contracts	0.0	0.0
Contingent consideration obligations	259.8	346.1
Total	259.8	346.1
Significant Unobservable Inputs (Level 3) \| Fair value, measurements, recurring \| Corporate debt securities
Assets:
Fair Value	0.0	0.0
Significant Unobservable Inputs (Level 3) \| Fair value, measurements, recurring \| Government securities
Assets:
Fair Value	0.0	0.0
Significant Unobservable Inputs (Level 3) \| Fair value, measurements, recurring \| Mortgage and other asset backed securities
Assets:
Fair Value	$ 0.0	$ 0.0

Fair Value Measurements - Summary of Significant Unobservable Inputs (Details) $ in Millions	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Discount rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.0060
Weighted Average \| Discount rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Input	0.0060
Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 259.8	$ 346.1	$ 409.8

Fair Value Measurements - Narrative (Details) - USD ($)	1 Months Ended		3 Months Ended			12 Months Ended
Fair Value Measurements - Narrative (Details) - USD ($)	Feb. 28, 2015	Dec. 31, 2010	Dec. 31, 2020	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jun. 07, 2019	Mar. 31, 2012
Business Acquisition, Contingent Consideration [Line Items]
Technological and regulatory success, probability						39.90%
Reduction in contingent consideration						$ 51,000,000.0	$ 61,200,000
Maximum contingent consideration in the form of development and approval milestones			$ 400,000,000.0			400,000,000.0
In-process research and development
Business Acquisition, Contingent Consideration [Line Items]
Impairment				$ 215,900,000	$ 189,300,000		215,900,000	$ 189,300,000
Intangible asset, fair value			0	$ 0		0
Convergence Pharmaceuticals
Business Acquisition, Contingent Consideration [Line Items]
Additions	$ 274,500,000
Fair Value			259,800,000			259,800,000	244,600,000
Maximum contingent consideration in the form of development and approval milestones			400,000,000.0			400,000,000.0
Intangible assets	$ 424,600,000
Biogen Idec International Neuroscience GmbH
Business Acquisition, Contingent Consideration [Line Items]
Additions		$ 81,200,000				0
Fair Value			101,500,000			101,500,000
Biogen Idec International Neuroscience GmbH \| In-process research and development
Business Acquisition, Contingent Consideration [Line Items]
Acquired intangible asset						110,900,000
Stromedix, Inc.
Business Acquisition, Contingent Consideration [Line Items]
Intangible assets										$ 219,200,000
Nightstar
Business Acquisition, Contingent Consideration [Line Items]
Intangible assets									$ 585,000,000.0
Nightstar \| BIIB054
Business Acquisition, Contingent Consideration [Line Items]
Impairment			$ 75,400,000			$ 75,400,000
Discount rate \| Convergence Pharmaceuticals
Business Acquisition, Contingent Consideration [Line Items]
Fair value measurement and measurement inputs, recurring and nonrecurring			0.60%			0.60%
Other long-term liabilities
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration obligations			$ 110,300,000			$ 110,300,000	$ 197,700,000

Fair Value Measurements - Gains and Losses on Assets Measured at Fair Value (Details) $ in Millions	7 Months Ended	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 07, 2019 USD ($)
Finite-lived Intangible Assets [Roll Forward]
Beginning Book Value	$ 8,220.8	$ 8,408.8
Ending Book Value	3,084.3	$ 3,527.4
BIIB111 \| Fair Value, Nonrecurring
Finite-lived Intangible Assets [Roll Forward]
Beginning Book Value			$ 480.0
Impairment	(115.0)
Ending Book Value	$ 365.0

Fair Value Measurements - Summary of Fair and Carrying Value of Debt Instruments (Details) - USD ($)	Dec. 31, 2020	Apr. 30, 2020	Dec. 31, 2019	Sep. 15, 2015
Debt Instrument
Notes payable, fair value	$ 8,515,900,000		$ 6,553,600,000
Senior Notes
Debt Instrument
Aggregate principal amount		$ 3,000,000,000.0
2.900% Senior Notes due September 15, 2020
Debt Instrument
Notes payable, fair value	$ 0		1,509,600,000
Interest rate on senior notes	2.90%
2.900% Senior Notes due September 15, 2020 \| Senior Notes
Debt Instrument
Interest rate on senior notes	2.90%
Aggregate principal amount				$ 1,500,000,000
3.625% Senior Notes due September 15, 2022
Debt Instrument
Notes payable, fair value	$ 1,054,100,000		1,038,900,000
Interest rate on senior notes	3.625%
Aggregate principal amount	$ 1,000,000,000.0
3.625% Senior Notes due September 15, 2022 \| Senior Notes
Debt Instrument
Interest rate on senior notes	3.625%
4.050% Senior Notes due September 15, 2025
Debt Instrument
Notes payable, fair value	$ 2,003,100,000		1,897,200,000
Interest rate on senior notes	4.05%
Aggregate principal amount	$ 1,750,000,000
4.050% Senior Notes due September 15, 2025 \| Senior Notes
Debt Instrument
Interest rate on senior notes	4.05%
2.250% Senior Notes due May 1, 2030
Debt Instrument
Notes payable, fair value	$ 1,557,200,000		0
2.250% Senior Notes due May 1, 2030 \| Senior Notes
Debt Instrument
Interest rate on senior notes	2.25%	2.25%
Aggregate principal amount		$ 1,500,000,000
5.200% Senior Notes due September 15, 2045
Debt Instrument
Notes payable, fair value	$ 2,365,100,000		2,107,900,000
Interest rate on senior notes	5.20%
Aggregate principal amount	$ 1,750,000,000
5.200% Senior Notes due September 15, 2045 \| Senior Notes
Debt Instrument
Interest rate on senior notes	5.20%
3.150% Senior Notes due May 1, 2050
Debt Instrument
Notes payable, fair value	$ 1,536,400,000		$ 0
3.150% Senior Notes due May 1, 2050 \| Senior Notes
Debt Instrument
Interest rate on senior notes	3.15%	3.15%
Aggregate principal amount		$ 1,500,000,000

Fair Value Measurements - Fair Value of Contingent Consideration Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Contingent consideration	$ (86.3)	$ (63.7)	$ (12.3)
Fair value, measurements, recurring
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of year	346.1	409.8
Contingent consideration	(86.3)	(63.7)
Payments and other	0.0	0.0
Fair value, end of year	$ 259.8	$ 346.1	$ 409.8

Financial Instruments - Summary of Financial Assets with Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Line Items]
Cash equivalents	$ 626.9	$ 2,541.1
Commercial paper
Cash and Cash Equivalents [Line Items]
Cash equivalents	61.1	384.4
Overnight reverse repurchase agreements
Cash and Cash Equivalents [Line Items]
Cash equivalents	37.4	368.8
Money market funds
Cash and Cash Equivalents [Line Items]
Cash equivalents	505.1	1,628.5
Short-term debt securities
Cash and Cash Equivalents [Line Items]
Cash equivalents	$ 23.3	$ 159.4

Financial Instruments - Summary of Marketable Securities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,049.1	$ 2,965.0
Gross Unrealized Gains	2.3	6.0
Gross Unrealized Losses	(0.4)	(0.7)
Fair Value	2,051.0	2,970.3
Corporate debt securities Current
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	897.8	1,057.2
Gross Unrealized Gains	0.4	1.0
Gross Unrealized Losses	(0.2)	0.0
Fair Value	898.0	1,058.2
Corporate debt securities Non-current
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	402.5	633.9
Gross Unrealized Gains	1.1	3.0
Gross Unrealized Losses	(0.1)	0.0
Fair Value	403.5	636.9
Government securities Current
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	380.6	502.9
Gross Unrealized Gains	0.1	0.4
Gross Unrealized Losses	0.0	0.0
Fair Value	380.7	503.3
Government securities Non-current
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	245.9	510.1
Gross Unrealized Gains	0.5	0.8
Gross Unrealized Losses	0.0	(0.3)
Fair Value	246.4	510.6
Mortgage and other asset backed securities Current
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	0.2	0.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	0.2	0.7
Mortgage and other asset backed securities Non-current
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	122.1	260.2
Gross Unrealized Gains	0.2	0.8
Gross Unrealized Losses	(0.1)	(0.4)
Fair Value	122.2	260.6
Marketable equity securities, current
Debt Securities, Available-for-sale [Line Items]
Marketable equity securities, amortized cost	70.6
Marketable equity securities, gross unrealized gains	15.9
Marketable equity securities, gross unrealized losses	0.0
Marketable equity securities, fair value	86.5
Marketable equity securities, non-current
Debt Securities, Available-for-sale [Line Items]
Marketable equity securities, amortized cost	1,168.9	218.4
Marketable equity securities, gross unrealized gains	733.8	132.1
Marketable equity securities, gross unrealized losses	(14.9)	(13.0)
Marketable equity securities, fair value	$ 1,887.8	$ 337.5

Financial Instruments - Summary of Contractual Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Estimated Fair Value
Due in one year or less	$ 1,278.9	$ 1,562.2
Due after one year through five years	722.6	1,234.5
Due after five years	49.5	173.6
Total marketable debt securities	2,051.0	2,970.3
Amortized Cost
Due in one year or less	1,278.6	1,560.8
Due after one year through five years	721.3	1,230.4
Due after five years	49.2	173.8
Amortized Cost	$ 2,049.1	$ 2,965.0

Financial Instruments - Proceeds from Marketable Debt Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments, All Other Investments [Abstract]
Proceeds from maturities and sales	$ 7,299.4	$ 6,007.0	$ 9,173.7
Realized gains	17.7	6.0	3.2
Realized losses	$ 26.0	$ 1.5	$ 11.7

Financial Instruments - Narrative (Details) shares in Millions, $ in Millions, ₩ in Billions	1 Months Ended		12 Months Ended
	Nov. 30, 2018 USD ($)	Nov. 30, 2018 KRW (₩)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2020 KRW (₩)	Nov. 30, 2020 shares	Sep. 30, 2020 shares	Apr. 30, 2020 shares	Dec. 31, 2019 KRW (₩)	Jun. 30, 2018	Feb. 29, 2012 USD ($)	Feb. 29, 2012 KRW (₩)
Business Acquisition [Line Items]
Average maturity of marketable securities, months			11 months	14 months
Payments to increase investment in Samsung Bioepis \| $			$ 0.0	$ 0.0	$ 676.6
Sage Therapeutics
Business Acquisition [Line Items]
Dividend yield percentage			0.00%
Sage Therapeutics \| Significant Other Observable Inputs (Level 2)
Business Acquisition [Line Items]
Investment in common stock, shares purchased (in shares)							6.2
Denali Therapeutics
Business Acquisition [Line Items]
Dividend yield percentage			0.00%
Denali Therapeutics \| Significant Other Observable Inputs (Level 2)
Business Acquisition [Line Items]
Investment in common stock, shares purchased (in shares)								13.0
Sangamo Therapeutics
Business Acquisition [Line Items]
Investment in common stock, shares purchased (in shares)									24.0
Dividend yield percentage			0.00%
Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio \| $			$ 2,024.6	393.9
Samsung Bioepis
Business Acquisition [Line Items]
Samsung Bioepis, ownership percentage before additional purchase transaction											5.00%
Samsung Bioepis, ownership percentage			49.90%			49.90%						15.00%	15.00%
Payments to increase investment in Samsung Bioepis	$ 676.6	₩ 759.5
Investment in Samsung Bioepis			$ 620.2	$ 580.2		₩ 673.8				₩ 670.8		$ 45.0	₩ 49.5

Derivative Instruments - Narrative (Details) ₩ in Billions	1 Months Ended		12 Months Ended
Derivative Instruments - Narrative (Details) ₩ in Billions	Nov. 30, 2018 USD ($)	Nov. 30, 2018 KRW (₩)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018	Feb. 29, 2012
Derivatives, Fair Value [Line Items]
Range of durations of foreign currency forward contracts			24 months
Payments to increase investment in Samsung Bioepis			$ 0	$ 0	$ 676,600,000
Net gains (losses) of other income (expense) related to foreign currency forward contracts			$ (30,100,000)	5,900,000	2,000,000.0
Samsung Bioepis
Derivatives, Fair Value [Line Items]
Samsung Bioepis, ownership percentage before additional purchase transaction						5.00%
Samsung Bioepis, ownership percentage			49.90%				15.00%
Payments to increase investment in Samsung Bioepis	$ 676,600,000	₩ 759.5
Net investment hedge
Derivatives, Fair Value [Line Items]
Forward contracts duration			10 months
Derivative qualifying as hedge, excluded component, after tax			$ 200,000	2,900,000
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, notional amount			3,229,700,000	1,892,400,000
Not designated as hedging instrument
Derivatives, Fair Value [Line Items]
Derivative, notional amount			$ 1,158,000,000.0	793,800,000
2.900% Senior Notes due September 15, 2020
Derivatives, Fair Value [Line Items]
Interest rate on senior notes			2.90%
Foreign Exchange Forward
Derivatives, Fair Value [Line Items]
Unrealized gain (loss) on derivative instruments			$ 212,500,000	500,000	$ 27,300,000
Foreign Exchange Forward \| Net investment hedge
Derivatives, Fair Value [Line Items]
Unrealized gain (loss) on derivative instruments			21,200,000	$ 1,500,000
Interest rate swap \| Interest Expense
Derivatives, Fair Value [Line Items]
Gain (loss) on derivative instruments, net, pretax			3,300,000
Interest rate swap \| Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, notional amount			$ 675,000,000.0
Short-term derivative
Derivatives, Fair Value [Line Items]
Range of durations of foreign currency forward contracts			12 months
Unrealized gain (loss) on derivative instruments			$ 175,200,000
Minimum
Derivatives, Fair Value [Line Items]
Range of durations of foreign currency forward contracts			1 month	1 month
Maximum
Derivatives, Fair Value [Line Items]
Range of durations of foreign currency forward contracts			24 months	15 months

Derivative Instruments - Foreign Currency Forward Contracts Entered into Hedge Forecasted Revenues (Details) - Designated as Hedging Instrument - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Derivative, notional amount	$ 3,229.7	$ 1,892.4
Euro
Derivatives, Fair Value [Line Items]
Derivative, notional amount	2,979.1	1,892.4
British pound
Derivatives, Fair Value [Line Items]
Derivative, notional amount	$ 250.6	$ 0.0

Derivative Instruments - Summary of the Effect of Derivatives Designated as Cash Flow Hedging Instruments (Details) - Foreign exchange contract - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues
Derivative Instruments, Gain (Loss) [Line Items]
Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)	$ 18.3	$ 118.6	$ (42.5)
Operating expenses
Derivative Instruments, Gain (Loss) [Line Items]
Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)	3.3	(3.3)	0.2
Cash flows, revenue \| Revenues
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivative instruments, net, pretax	(9.9)	2.9	10.8
Cash flows, operating expenses \| Operating expenses
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivative instruments, net, pretax	$ 0.0	$ 0.2	$ (0.1)

Derivative Instruments - Summary of the Effect of Derivatives Designated as Net Investment Hedging Instruments (Details) - Net investment hedge - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Net Gains/(Losses) Recognized in Other Comprehensive Income (Effective Portion) (in millions)	$ (35.1)	$ 25.3	$ (3.8)
Net Gains/(Losses) Recognized in Other Comprehensive Income (Amounts Excluded from Effectiveness Testing) (in millions)	4.5	3.3	0.0
Other (income) expense, net
Derivative Instruments, Gain (Loss) [Line Items]
Net Gains/(Losses) Recognized in Net Income (Amounts Excluded from Effectiveness Testing) (in millions)	$ 2.9	$ 7.0	$ 1.5

Derivative Instruments - Summary of the Fair Value for our Outstanding Derivatives (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Other current assets \| Foreign exchange contract \| Not designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Asset derivative instruments	$ 20.5	$ 8.0
Other current assets \| Foreign exchange contract \| Cash flow hedge
Derivative Instruments, Gain (Loss) [Line Items]
Asset derivative instruments	0.0	33.8
Other current assets \| Foreign exchange contract \| Net investment hedge
Derivative Instruments, Gain (Loss) [Line Items]
Asset derivative instruments	0.0	2.0
Accrued expenses and other \| Foreign exchange contract \| Not designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Liability derivative instruments	4.7	2.4
Accrued expenses and other \| Foreign exchange contract \| Cash flow hedge
Derivative Instruments, Gain (Loss) [Line Items]
Liability derivative instruments	157.1	2.0
Accrued expenses and other \| Interest rate swap \| Net investment hedge
Derivative Instruments, Gain (Loss) [Line Items]
Liability derivative instruments	19.7	0.0
Accrued expenses and other \| Interest rate swap \| Fair value hedge
Derivative Instruments, Gain (Loss) [Line Items]
Liability derivative instruments	0.0	2.3
Other long-term liabilities \| Foreign exchange contract \| Cash flow hedge
Derivative Instruments, Gain (Loss) [Line Items]
Liability derivative instruments	$ 35.7	$ 1.7

Property, Plant and Equipment (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Land	$ 119.8	$ 118.1
Buildings	1,025.3	835.0
Leasehold improvements	104.6	99.5
Machinery and equipment	1,027.8	844.5
Computer software and hardware	903.0	798.4
Furniture and fixtures	62.5	58.3
Construction in progress	1,950.8	2,084.4
Total cost	5,193.8	4,838.2
Less: accumulated depreciation	(1,782.3)	(1,590.9)
Total property, plant and equipment, net	$ 3,411.5	$ 3,247.3

Property, Plant and Equipment - Narrative (Details) ft² in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) ft²	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Aug. 31, 2019 USD ($)
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 201.9	$ 190.6	$ 269.4
Interest costs capitalized	65.2	68.8	$ 54.0
Construction in progress	1,950.8	2,084.4
Property, plant and equipment, gross	5,193.8	4,838.2
Denmark Manufacturing Operations
Property, Plant and Equipment [Line Items]
Property, plant and equipment sold to Fujifilm				$ 631.5
Denmark Manufacturing Operations \| Buildings
Property, Plant and Equipment [Line Items]
Property, plant and equipment sold to Fujifilm				312.5
Denmark Manufacturing Operations \| Machinery and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment sold to Fujifilm				$ 287.3
Solothurn
Property, Plant and Equipment [Line Items]
Construction in progress	1,800.0	$ 1,900.0
Property, plant and equipment, gross	$ 256.8
Biologics Manufacturing \| Solothurn
Property, Plant and Equipment [Line Items]
Number of square feet \| ft²	393
Warehouse, Utilities and Support Space \| Solothurn
Property, Plant and Equipment [Line Items]
Number of square feet \| ft²	290
Administrative Space \| Solothurn
Property, Plant and Equipment [Line Items]
Number of square feet \| ft²	51

Leases - Narrative (Details)	Dec. 31, 2020
Minimum
Operating Leased Assets [Line Items]
Operating lease, contract term	1 year
Operating lease, renewal term	1 year
Minimum \| Sublease
Operating Leased Assets [Line Items]
Operating lease, contract term	4 years
Maximum
Operating Leased Assets [Line Items]
Operating lease, contract term	10 years
Operating lease, renewal term	6 years
Maximum \| Sublease
Operating Leased Assets [Line Items]
Operating lease, contract term	8 years

Leases - Schedule of Operating Leases (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Operating lease assets	$ 433.3	$ 427.0
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent
Long-term operating lease liabilities	$ 402.0	412.7
Present value of operating lease liabilities	485.2	486.3
Operating lease assets
Operating lease assets	433.3	427.0
Accrued expenses and other
Operating lease liability, current	83.2	73.6
Long-term operating lease liabilities
Long-term operating lease liabilities	$ 402.0	$ 412.7

Leases - Operating Lease Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Net lease cost	$ 92.4	$ 86.7
Research and development
Operating lease cost	5.2	6.7
Variable lease cost	1.1	1.2
Selling, general and administrative
Operating lease cost	93.1	84.6
Variable lease cost	21.1	23.7
Sublease income	(24.2)	(25.6)
Other (income) expense, net
Sublease income	$ (3.9)	$ (3.9)

Leases - Operating Lease Liability Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
2021	$ 95.8
2022	93.5
2023	82.0
2024	75.0
2025	56.9
Thereafter	129.8
Total lease payments	533.0
Less: interest	47.8
Present value of operating lease liabilities	$ 485.2	$ 486.3

Leases - Operating Lease Weighted Average Remaining Term and Discount Rate (Details)	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Weighted average remaining lease term in years	6 years 3 months 18 days	7 years 25 days
Weighted average discount rate	2.90%	3.20%

Leases - Operating Lease Supplemental Cash Flow Disclosure (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Cash paid for amounts included in the measurement of lease liabilities	$ 100.2	$ 93.8
Operating lease assets obtained in exchange for lease obligations	$ 59.0	$ 35.9

Indebtedness - Summary of Indebtedness (Details) - USD ($)	Dec. 31, 2020	Apr. 30, 2020	Dec. 31, 2019	Sep. 15, 2015
Current portion:
Current portion of notes payable	$ 0		$ 1,495,800,000
Non-current portion:
Non-current notes payable	7,500,000,000.0
Non-current portion of notes payable	7,426,200,000		4,459,000,000.0
Senior Notes
Current portion:
Current portion of notes payable	0		1,495,800,000
Non-current portion:
Non-current portion of notes payable	$ 7,426,200,000		4,459,000,000.0
Aggregate principal amount		$ 3,000,000,000.0
2.900% Senior Notes due September 15, 2020
Non-current portion:
Interest rate on senior notes	2.90%
2.900% Senior Notes due September 15, 2020 \| Senior Notes
Current portion:
2.900% Senior Notes due September 15, 2020	$ 0		1,495,800,000
Non-current portion:
Interest rate on senior notes	2.90%
Aggregate principal amount				$ 1,500,000,000
3.625% Senior Notes due September 15, 2022
Non-current portion:
Interest rate on senior notes	3.625%
Aggregate principal amount	$ 1,000,000,000.0
3.625% Senior Notes due September 15, 2022 \| Senior Notes
Non-current portion:
Non-current notes payable	$ 997,900,000		996,600,000
Interest rate on senior notes	3.625%
4.050% Senior Notes due September 15, 2025
Non-current portion:
Interest rate on senior notes	4.05%
Aggregate principal amount	$ 1,750,000,000
4.050% Senior Notes due September 15, 2025 \| Senior Notes
Non-current portion:
Non-current notes payable	$ 1,741,200,000		1,739,500,000
Interest rate on senior notes	4.05%
2.250% Senior Notes due May 1, 2030 \| Senior Notes
Non-current portion:
Non-current notes payable	$ 1,491,100,000		0
Interest rate on senior notes	2.25%	2.25%
Aggregate principal amount		$ 1,500,000,000
5.200% Senior Notes due September 15, 2045
Non-current portion:
Interest rate on senior notes	5.20%
Aggregate principal amount	$ 1,750,000,000
5.200% Senior Notes due September 15, 2045 \| Senior Notes
Non-current portion:
Non-current notes payable	$ 1,723,400,000		1,722,900,000
Interest rate on senior notes	5.20%
3.150% Senior Notes due May 1, 2050 \| Senior Notes
Non-current portion:
Non-current notes payable	$ 1,472,600,000		$ 0
Interest rate on senior notes	3.15%	3.15%
Aggregate principal amount		$ 1,500,000,000

Indebtedness - Narrative (Details) - USD ($)	1 Months Ended	12 Months Ended
Indebtedness - Narrative (Details) - USD ($)	Jan. 31, 2020	Dec. 31, 2020	Apr. 30, 2020	Sep. 15, 2015
Debt Instrument
Redemptions percentage of 2015 notes		100.00%
Redemption percentage for change in control provision		101.00%
Debt instrument, term	5 years
Interest Expense \| Interest rate swap
Debt Instrument
Gain (loss) on derivative instruments, net, pretax		$ 3,300,000
3.625% Senior Notes due September 15, 2022
Debt Instrument
Senior notes aggregate principal amount		$ 1,000,000,000.0
Interest rate on senior notes		3.625%
Redemption percentage par value of senior notes		99.92%
4.050% Senior Notes due September 15, 2025
Debt Instrument
Senior notes aggregate principal amount		$ 1,750,000,000
Interest rate on senior notes		4.05%
Redemption percentage par value of senior notes		99.764%
5.200% Senior Notes due September 15, 2045
Debt Instrument
Senior notes aggregate principal amount		$ 1,750,000,000
Interest rate on senior notes		5.20%
Redemption percentage par value of senior notes		99.294%
2.900% Senior Notes due September 15, 2020
Debt Instrument
Interest rate on senior notes		2.90%
Senior Notes
Debt Instrument
Senior notes aggregate principal amount			$ 3,000,000,000.0
Redemptions percentage of 2015 notes		100.00%
Redemption percentage for change in control provision		101.00%
Senior Notes \| 2020 Senior Notes
Debt Instrument
Senior notes aggregate principal amount			3,000,000,000.0
Costs associated with the senior Nnotes offerings		$ 24,400,000
Senior Notes \| 2.250% Senior Notes due May 1, 2030
Debt Instrument
Senior notes aggregate principal amount			$ 1,500,000,000
Interest rate on senior notes		2.25%	2.25%
Redemption percentage par value of senior notes			99.973%
Senior Notes \| 3.150% Senior Notes due May 1, 2050
Debt Instrument
Senior notes aggregate principal amount			$ 1,500,000,000
Interest rate on senior notes		3.15%	3.15%
Redemption percentage par value of senior notes			99.174%
Senior Notes \| 2015 Senior Notes
Debt Instrument
Costs associated with the senior Nnotes offerings		$ 47,500,000
Senior Notes \| 3.625% Senior Notes due September 15, 2022
Debt Instrument
Interest rate on senior notes		3.625%
Senior Notes \| 4.050% Senior Notes due September 15, 2025
Debt Instrument
Interest rate on senior notes		4.05%
Senior Notes \| 5.200% Senior Notes due September 15, 2045
Debt Instrument
Interest rate on senior notes		5.20%
Senior Notes \| 2.900% Senior Notes due September 15, 2020
Debt Instrument
Senior notes aggregate principal amount				$ 1,500,000,000
Interest rate on senior notes		2.90%
Redemption percentage par value of senior notes				99.792%
Pre-tax charge		$ 12,700,000
Senior Notes \| 2.900% Senior Notes due September 15, 2020 \| Interest Expense
Debt Instrument
Pre-tax charge		9,400,000
Line of Credit \| Revolving Credit Facility
Debt Instrument
Senior unsecured revolving credit facility maximum borrowing capacity	$ 1,000,000,000.0
Outstanding borrowings		$ 0

Indebtedness - Total Debt Maturities (Details) $ in Millions	Dec. 31, 2020 USD ($)
Debt Disclosure [Abstract]
2021	$ 0.0
2022	1,000.0
2023	0.0
2024	0.0
2025	1,750.0
2026 and thereafter	4,750.0
Total	$ 7,500.0

Equity - Narrative (Details) - USD ($)	12 Months Ended
Equity - Narrative (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Oct. 31, 2020	Mar. 31, 2019	Aug. 31, 2018	Jul. 31, 2016
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)	8,000,000.0
Preferred stock, shares issued (in shares)	0	0	0
Preferred stock, shares outstanding (in shares)	0	0	0
Payments for repurchase of common stock	$ 6,679,100,000	$ 5,868,300,000	$ 4,352,600,000
Series A
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)	1,750,000
Series X junior participating
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)	1,000,000.0
Undesignated
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)	5,250,000
2020 Share Repurchase Program
Class of Stock [Line Items]
Authorized amount of share repurchases				$ 5,000,000,000.0
Repurchase of common stock, at cost (in shares)	1,600,000
Payments for repurchase of common stock	$ 400,000,000.0
December 2019 Share Repurchase Program
Class of Stock [Line Items]
Authorized amount of share repurchases		$ 5,000,000,000.0
Repurchase of common stock, at cost (in shares)	16,700,000
Payments for repurchase of common stock	$ 5,000,000,000.0
March 2019 Share Repurchase Program
Class of Stock [Line Items]
Authorized amount of share repurchases					$ 5,000,000,000.0
Repurchase of common stock, at cost (in shares)	4,100,000	14,700,000
Payments for repurchase of common stock	$ 1,300,000,000	$ 3,700,000,000
2018 Share Repurchase Program
Class of Stock [Line Items]
Authorized amount of share repurchases						$ 3,500,000,000
Repurchase of common stock, at cost (in shares)		8,900,000	4,300,000
Payments for repurchase of common stock		$ 2,100,000,000	$ 1,400,000,000
2016 Share Repurchase Program
Class of Stock [Line Items]
Authorized amount of share repurchases							$ 5,000,000,000.0
Repurchase of common stock, at cost (in shares)			10,500,000
Payments for repurchase of common stock			$ 3,000,000,000.0

Equity - Summary of Common Stock (Details) - shares	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Common stock authorized (in shares)	1,000,000,000.0	1,000,000,000.0	1,000,000,000.0
Common stock issued (in shares)	176,200,000	198,000,000.0	221,000,000.0
Common stock outstanding (in shares)	152,400,000	174,200,000	197,200,000

Equity - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance	$ (135.2)
Amount reclassified, net of tax, upon adoption of ASU 2016-01	10,686.1	$ 13,339.1	$ 13,031.6	$ 12,598.1
Other comprehensive income (loss) before reclassifications	(145.6)	230.8	29.5
Amounts reclassified from accumulated other comprehensive income (loss)	(18.2)	(125.6)	47.0
Net current period other comprehensive income (loss)	(163.8)	105.2	76.5
AOCI, ending balance	(299.0)	(135.2)
Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amount reclassified, net of tax, upon adoption of ASU 2016-01				106.1
Accumulated other comprehensive loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance	(135.2)	(240.4)	(318.4)
Amount reclassified, net of tax, upon adoption of ASU 2016-01	(299.0)	(135.2)	(240.4)	(318.4)
AOCI, ending balance	(299.0)	(135.2)	(240.4)
Accumulated other comprehensive loss \| Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amount reclassified, net of tax, upon adoption of ASU 2016-01				1.5
Accumulated other comprehensive loss \| Cumulative Effect, Period of Adoption, Adjusted Balance
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance			(316.9)
Unrealized Gains (Losses) on Securities Available for Sale, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance	4.2	(4.0)	(1.6)
Other comprehensive income (loss) before reclassifications	(9.3)	11.8	(10.6)
Amounts reclassified from accumulated other comprehensive income (loss)	6.5	(3.6)	6.7
Net current period other comprehensive income (loss)	(2.8)	8.2	(3.9)
AOCI, ending balance	1.4	4.2	(4.0)
Unrealized Gains (Losses) on Securities Available for Sale, net of tax \| Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amount reclassified, net of tax, upon adoption of ASU 2016-01				1.5
Unrealized Gains (Losses) on Securities Available for Sale, net of tax \| Cumulative Effect, Period of Adoption, Adjusted Balance
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance			(0.1)
Unrealized Gains (Losses) on Cash Flow Hedges, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance	7.8	34.7	(104.5)
Other comprehensive income (loss) before reclassifications	(165.0)	88.1	97.4
Amounts reclassified from accumulated other comprehensive income (loss)	(21.8)	(115.0)	41.8
Net current period other comprehensive income (loss)	(186.8)	(26.9)	139.2
AOCI, ending balance	(179.0)	7.8	34.7
Unrealized Gains (Losses) on Cash Flow Hedges, net of tax \| Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amount reclassified, net of tax, upon adoption of ASU 2016-01				0.0
Unrealized Gains (Losses) on Cash Flow Hedges, net of tax \| Cumulative Effect, Period of Adoption, Adjusted Balance
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance			(104.5)
Gains (Losses) on Net Investment Hedge, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance	25.1	3.5	0.0
Other comprehensive income (loss) before reclassifications	(30.7)	28.6	5.0
Amounts reclassified from accumulated other comprehensive income (loss)	(2.9)	(7.0)	(1.5)
Net current period other comprehensive income (loss)	(33.6)	21.6	3.5
AOCI, ending balance	(8.5)	25.1	3.5
Gains (Losses) on Net Investment Hedge, Net of Tax \| Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amount reclassified, net of tax, upon adoption of ASU 2016-01				0.0
Gains (Losses) on Net Investment Hedge, Net of Tax \| Cumulative Effect, Period of Adoption, Adjusted Balance
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance			0.0
Unfunded Status of Postretirement Benefit Plans, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance	(32.8)	(31.3)	(36.8)
Other comprehensive income (loss) before reclassifications	(33.5)	(1.5)	5.5
Amounts reclassified from accumulated other comprehensive income (loss)	0.0	0.0	0.0
Net current period other comprehensive income (loss)	(33.5)	(1.5)	5.5
AOCI, ending balance	(66.3)	(32.8)	(31.3)
Unfunded Status of Postretirement Benefit Plans, net of tax \| Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amount reclassified, net of tax, upon adoption of ASU 2016-01				0.0
Unfunded Status of Postretirement Benefit Plans, net of tax \| Cumulative Effect, Period of Adoption, Adjusted Balance
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance			(36.8)
Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance	(139.5)	(243.3)	(175.5)
Other comprehensive income (loss) before reclassifications	92.9	103.8	(67.8)
Amounts reclassified from accumulated other comprehensive income (loss)	0.0	0.0	0.0
Net current period other comprehensive income (loss)	92.9	103.8	(67.8)
AOCI, ending balance	$ (46.6)	$ (139.5)	(243.3)
Currency Translation Adjustments \| Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amount reclassified, net of tax, upon adoption of ASU 2016-01				$ 0.0
Currency Translation Adjustments \| Cumulative Effect, Period of Adoption, Adjusted Balance
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance			$ (175.5)

Equity - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income (expense)									$ 497.4	$ 83.3	$ 11.0
Income tax benefit (expense)									992.3	1,158.0	1,425.6
Revenues	$ 2,852.6	$ 3,376.1	$ 3,681.6	$ 3,534.3	$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	13,444.6	14,377.9	13,452.9
Total reclassifications, net of tax	$ 357.9	$ 701.5	$ 1,542.1	$ 1,399.1	$ 1,439.7	$ 1,545.9	$ 1,494.1	$ 1,408.8	4,000.6	5,888.5	4,430.7
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Total reclassifications, net of tax									18.2	125.6	(47.0)
Gains (losses) on securities available for sale \| Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income (expense)									(8.2)	4.5	(8.5)
Income tax benefit (expense)									1.7	(0.9)	1.8
Gains (losses) on cash flow hedges \| Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income (expense)									0.3	0.3	0.3
Income tax benefit (expense)									(0.1)	(0.6)	0.2
Revenues									18.3	118.6	(42.5)
Operating expenses									3.3	(3.3)	0.2
Gains (losses) on net investment hedge \| Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income (expense)									$ 2.9	$ 7.0	$ 1.5

Earnings per Share - Basic and Diluted Earnings Per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net income attributable to Biogen Inc.	$ 357.9	$ 701.5	$ 1,542.1	$ 1,399.1	$ 1,439.7	$ 1,545.9	$ 1,494.1	$ 1,408.8	$ 4,000.6	$ 5,888.5	$ 4,430.7
Denominator:
Basic earnings per share attributable to Biogen Inc. (in shares)	153.7	156.9	160.6	172.8	177.8	184.0	190.3	196.6	160.9	187.1	204.9
Effect of dilutive securities:
Dilutive potential common shares (in shares)									0.4	0.3	0.4
Diluted earnings per share attributable to Biogen Inc. (in shares)	154.0	157.2	160.9	173.1	178.2	184.2	190.4	197.0	161.3	187.4	205.3
Time-vested restricted stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan (in shares)									0.2	0.2	0.3
Market stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan (in shares)									0.1	0.1	0.1
Performance stock units settled in stock
Effect of dilutive securities:
Stock options and employee stock purchase plan (in shares)									0.1	0.0	0.0

Earnings per Share - Narrative (Details) - shares shares in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Repurchase of common stock (in shares)	22.4	23.6	14.8

Share-Based Payments - Share-based Compensation Expense Included in Consolidation Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$ (204.5)	$ (191.2)	$ (168.7)
Subtotal	211.3	225.4	181.6
Capitalized share-based compensation costs	(6.2)	(8.9)	(11.5)
Share-based compensation expense included in total cost and expenses	205.1	216.5	170.1
Income tax effect	(33.5)	(35.7)	(27.5)
Research and development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	80.0	77.1	75.8
Selling, general and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	131.3	148.3	105.8
Share-based compensation expense included in net income attributable to Biogen Inc.
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 171.6	$ 180.8	$ 142.6

Share-Based Payments - Summary of Share-based Compensation Expense Associated with Each Share-based Compensating Programs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$ (204.5)	$ (191.2)	$ (168.7)
Subtotal	211.3	225.4	181.6
Capitalized share-based compensation costs	(6.2)	(8.9)	(11.5)
Share-based compensation expense included in total cost and expenses	205.1	216.5	170.1
Market stock units
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	40.5	30.4	27.2
Time-vested restricted stock units
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	142.6	134.0	126.6
Cash settled performance units
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	(1.7)	0.7	7.8
Performance units
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	(0.1)	1.6	3.1
Performance stock units settled in stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	7.9	15.5	4.7
Performance stock units settled in cash
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	8.6	5.5	1.7
Employee stock purchase plan
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	13.5	11.5	10.5
Nightstar \| Share-based Payment Arrangement, Option
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
NST stock options	$ 0.0	$ 26.2	$ 0.0

Share-Based Payments - Narrative (Details)
$ / shares in Units, shares in Millions, $ in Millions

1 Months Ended

12 Months Ended

Jun. 30, 2015

Dec. 31, 2020

USD ($)

Plan

Increment

$ / shares

Dec. 31, 2019

USD ($)

$ / shares

Dec. 31, 2018

USD ($)

$ / shares

Dec. 31, 2013

Increment

Jun. 30, 2017

Unrecognized compensation cost related to unvested share-based compensation

$ 207.6

Weighted-average period to recognize the cost of unvested awards

1 year 10 months 24 days

Number of share-based compensation plans pursuant to which awards are currently being made | Plan

Total intrinsic value of options exercised

$ 2.9

$ 4.2

$ 4.0

Shares issued under the 2015 ESPP (in shares) | shares

6.2

Market stock units

Share-based Compensation Arrangement by Share-based Payment Award

Number of equal annual increments | Increment

Percentage of earnings of the target number of units granted based on actual stock performance

0.00%

Percentage of earnings of the target number of units granted based on actual stock performance

200.00%

150.00%

Number of days for calculation of average closing stock price

30 days

Total fair value of vested awards

$ 26.9

$ 32.5

$ 26.9

Cash settled performance units

$ 398.61

^[1]

$ 378.08

$ 378.85

Share-based Compensation Arrangement by Share-based Payment Award

Number of equal annual increments | Increment

Percentage of earnings of the target number of units granted based on actual stock performance

0.00%

Percentage of earnings of the target number of units granted based on actual stock performance

200.00%

Number of days for calculation of average closing stock price

30 days

Cash settlement of awards upon vesting

$ 3.8

$ 10.6

$ 15.1

Performance units

Share-based Compensation Arrangement by Share-based Payment Award

Number of equal annual increments | Increment

Percentage of earnings of the target number of units granted based on actual stock performance

0.00%

Percentage of earnings of the target number of units granted based on actual stock performance

200.00%

Number of days for calculation of average closing stock price

30 days

Cash settlement of awards upon vesting

$ 3.4

10.4

17.0

Performance stock units settled in stock

Share-based Compensation Arrangement by Share-based Payment Award

Percentage of earnings of the target number of units granted based on actual stock performance

3 years

0.00%

Percentage of earnings of the target number of units granted based on actual stock performance

200.00%

Performance stock units settled in cash

^[2]

$ 293.35

Share-based Compensation Arrangement by Share-based Payment Award

Percentage of earnings of the target number of units granted based on actual stock performance

3 years

0.00%

Percentage of earnings of the target number of units granted based on actual stock performance

200.00%

Number of days for calculation of average closing stock price

30 days

Time-vested restricted stock units

Share-based Compensation Arrangement by Share-based Payment Award

Total fair value of vested awards

3 years

$ 140.5

$ 131.5

$ 111.7

Time-vested restricted stock units | Share-based Payment Arrangement, Tranche One

$ 318.87

^[3]

$ 304.44

$ 316.32

Share-based Compensation Arrangement by Share-based Payment Award

Vesting percentage

33.33%

Time-vested restricted stock units | Share-based Payment Arrangement, Tranche Two

Share-based Compensation Arrangement by Share-based Payment Award

Vesting percentage

33.33%

Time-vested restricted stock units | Share-based Payment Arrangement, Tranche Three

Share-based Compensation Arrangement by Share-based Payment Award

Vesting percentage

33.33%

Share-based Payment Arrangement, Option

Share-based Compensation Arrangement by Share-based Payment Award

Share based payment arrangement, term

10 years

Share-based Payment Arrangement, Option | Minimum

Share-based Compensation Arrangement by Share-based Payment Award

Share-based Payment Arrangement, Option | Maximum

1 year

Share-based Compensation Arrangement by Share-based Payment Award

Share-based Compensation Arrangement by Share-based Payment Award

4 years

Directors Plan

Total number of shares of common stock for issuance (in shares) | shares

1.6

Ratio of total number of shares reserved under the plan

1.5

2017 Omnibus Equity Plan

Share-based Compensation Arrangement by Share-based Payment Award

Ratio of total number of shares reserved under the plan

1.5

Omnibus Plan number of shares authorized (in shares) | shares

8.0

[1]	MSUs granted during 2020 include awards granted in conjunction with our annual awards made in February 2020 and MSUs granted in conjunction with the hiring of employees. These grants reflect the target number of shares eligible to be earned at the time of grant. MSUs granted in 2020 also reflect an adjustment based upon the final performance multiplier in relation to shares granted in 2019, 2018 and 2017.
[2]	PSUs settled in stock granted in 2020 include awards granted in conjunction with our annual awards made in February 2020 and PSUs granted in conjunction with the hiring of employees. These grants reflect the target number of shares eligible to be earned at the time of grant.
[3]	RSUs granted in 2020 primarily represent RSUs granted in conjunction with our annual awards made in February 2020 and awards made in conjunction with the hiring of new employees. RSUs granted in 2020 also include approximately 10,000 RSUs granted to our Board of Directors.

Share-Based Payments - Stock Option Activity (Details) - Share-based Payment Arrangement, Option shares in Thousands	12 Months Ended
	Dec. 31, 2020 $ / shares shares
Shares
Beginning balance, outstanding shares (in shares) \| shares	12
Granted (in shares) \| shares	0
Exercised (in shares) \| shares	(12)
Cancelled (in shares) \| shares	0
Ending balance, outstanding shares (in shares) \| shares	0
Weighted Average Exercise Price
Beginning balance, weighted average exercise price (in dollars per share) \| $ / shares	$ 58.46
Granted (in dollars per share) \| $ / shares	0
Exercised (in dollars per share) \| $ / shares	58.46
Cancelled (in dollars per share) \| $ / shares	0
Ending balance, weighted average exercise price (in dollars per share) \| $ / shares	$ 0

Share-Based Payments - Tax Benefit and Cash Received from Stock Option Exercises (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Payment Arrangement [Abstract]
Tax benefit realized for stock options	$ 2.9	$ 2.5	$ 2.2
Cash received from the exercise of stock options	$ 0.7	$ 0.4	$ 0.8

Share-Based Payments - Market Stock Units Activity (Details) - Market stock units - $ / shares
shares in Thousands

12 Months Ended

Dec. 31, 2020

Dec. 31, 2019

Dec. 31, 2018

Shares

Beginning balance, unvested shares (in shares)

183

Granted (in shares)

^[1]

133

Vested (in shares)

(82)

Forfeited (in shares)

(33)

Ending balance, unvested shares (in shares)

201

183

Beginning balance, weighted average grant date fair value (in dollars per share)

$ 378.09

Granted (in dollars per share)

398.61

^[1]

$ 378.08

$ 378.85

Vested (in dollars per share)

375.42

Forfeited (in dollars per share)

402.62

Ending balance, weighted average grant date fair value (in dollars per share)

$ 388.98

$ 378.09

[1]

MSUs granted during 2020 include awards granted in conjunction with our annual awards made in February 2020 and MSUs granted in conjunction with the hiring of employees. These grants reflect the target number of shares eligible to be earned at the time of grant. MSUs granted in 2020 also reflect an adjustment based upon the final performance multiplier in relation to shares granted in 2019, 2018 and 2017.

Share-Based Payments - Assumptions Used in Valuation of Market Based Stock Units (Details) - Market stock units - USD ($)

12 Months Ended

Dec. 31, 2020

Dec. 31, 2019

Dec. 31, 2018

Share-based Compensation Arrangement by Share-based Payment Award

Expected dividend yield

0.00%

Minimum range of risk-free interest rates

1.41%

2.46%

1.90%

Maximum range of risk-free interest rates

1.48%

2.53%

2.30%

Average stock price on grant date minimum

$ 257.83

$ 228.59

$ 279.47

Average stock price on grant date maximum

$ 325.40

$ 331.18

$ 346.76

Granted (in dollars per share)

$ 398.61

^[1]

$ 378.08

$ 378.85

Minimum

Share-based Compensation Arrangement by Share-based Payment Award

Range of expected stock price volatility

37.80%

31.20%

27.50%

Maximum

Share-based Compensation Arrangement by Share-based Payment Award

Range of expected stock price volatility

44.10%

33.60%

32.40%

[1]

Share-Based Payments - Cash Settled Performance Shares Activity (Details) - Cash settled performance units shares in Thousands	12 Months Ended
	Dec. 31, 2020 shares
Shares
Beginning balance, unvested shares (in shares)	13,000
Granted (in shares)	0
Vested (in shares)	(13,000)
Forfeited (in shares)	0
Ending balance, unvested shares (in shares)	0

Share-Based Payments - Performance Units Activity (Details) - Performance units shares in Thousands	12 Months Ended
	Dec. 31, 2020 shares
Shares
Beginning balance, unvested shares (in shares)	11,000
Granted (in shares)	0
Vested (in shares)	(11,000)
Forfeited (in shares)	0
Ending balance, unvested shares (in shares)	0

Share-Based Payments - Performance Stock Units Settled in Stock Activity (Details) - Performance stock units settled in stock
shares in Thousands

12 Months Ended

Dec. 31, 2020

$ / shares

Shares

Beginning balance, unvested shares (in shares) | shares

111,000

Granted (in shares) | shares

73,000

^[1]

Vested (in shares) | shares

Forfeited (in shares) | shares

(29,000)

Ending balance, unvested shares (in shares) | shares

155,000

Beginning balance, weighted average grant date fair value (in dollars per share) | $ / shares

$ 316.39

Vested (in dollars per share) | $ / shares

293.35

^[1]

Forfeited (in dollars per share) | $ / shares

323.42

Ending balance, weighted average grant date fair value (in dollars per share) | $ / shares

$ 304.19

[1]	PSUs settled in stock granted in 2020 include awards granted in conjunction with our annual awards made in February 2020 and PSUs granted in conjunction with the hiring of employees. These grants reflect the target number of shares eligible to be earned at the time of grant.

Share-Based Payments - Performance Stock Units Settled in Cash Activity (Details) - Performance stock units settled in cash shares in Thousands	12 Months Ended
	Dec. 31, 2020 shares
Shares
Beginning balance, unvested shares (in shares)	82,000
Granted (in shares)	63,000
Vested (in shares)	(2,000)
Forfeited (in shares)	(23,000)
Ending balance, unvested shares (in shares)	120,000

Share-Based Payments - Time-Vested Restricted Stock Units Activity (Details) - Time-vested restricted stock units - $ / shares

12 Months Ended

Dec. 31, 2020

Dec. 31, 2019

Dec. 31, 2018

Shares

Beginning balance, unvested shares (in shares)

938,000

Granted (in shares)

^[1]

620,000

Vested (in shares)

(440,000)

Forfeited (in shares)

(100,000)

Ending balance, unvested shares (in shares)

1,018,000

938,000

Beginning balance, weighted average grant date fair value (in dollars per share)

$ 306.55

Granted (in dollars per share)

318.87

^[1]

$ 304.44

$ 316.32

Vested (in dollars per share)

304.45

Forfeited (in dollars per share)

314.46

Ending balance, weighted average grant date fair value (in dollars per share)

$ 314.46

$ 306.55

Number of shares granted (in shares)

^[1]

620,000

Director

Shares

Granted (in shares)

^[1]

10,000

Number of shares granted (in shares)

^[1]

10,000

[1]	RSUs granted in 2020 primarily represent RSUs granted in conjunction with our annual awards made in February 2020 and awards made in conjunction with the hiring of new employees. RSUs granted in 2020 also include approximately 10,000 RSUs granted to our Board of Directors.

Share-Based Payments - Shares Issued Under Employee Stock Purchase Plan (Details) - USD ($) shares in Thousands, $ in Millions	1 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Shares issued under the 2015 ESPP (in shares)	6,200
2015 ESPP
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Shares issued under the 2015 ESPP (in shares)		212	204	170
Cash received under the 2015 ESPP		$ 48.6	$ 40.4	$ 40.5

Income Taxes - Income Before Income Tax Provision and the Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income before income taxes (benefit):
Domestic	$ 3,290.0	$ 4,725.3	$ 3,877.0
Foreign	1,757.5	2,400.6	2,022.6
Income before income tax expense and equity in loss of investee, net of tax	5,047.5	7,125.9	5,899.6
Current:
Federal	647.0	947.4	1,131.8
State	41.2	59.1	45.5
Foreign	155.1	84.4	140.0
Total	843.3	1,090.9	1,317.3
Deferred:
Federal	(1,749.9)	1,143.9	(62.0)
State	(6.8)	(2.3)	(7.4)
Foreign	1,905.7	(1,074.5)	177.7
Total	149.0	67.1	108.3
Total income tax expense	$ 992.3	$ 1,158.0	$ 1,425.6

Income Taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Income Taxes - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Tax Credit Carryforward [Line Items]
GILTI	$ (1,143.7)	$ (1,381.6)
Income taxes paid	62.0
Deferred tax liabilities, temporary differences, purchase accounting, foreign subsidiaries	1,500.0
Total deferred charges and prepaid taxes	142.2	243.8
Valuation allowance	1,753.9	1.1
Other	103.1	103.7
Unrecognized tax benefit	68.8	122.7	$ 109.1
Net interest expense	1.0	4.7	2.2
Unrecognized tax benefits, interest on income taxes accrued	21.2	20.0
Decrease in unrecognized tax benefits is reasonably possible	25.0
2017 Tax Act
Tax Credit Carryforward [Line Items]
Reduction in deferred tax liability, undistributed foreign earnings			34.6
Income tax expense (benefit)			12.7
GILTI			135.8
Other tax expense (benefit)			$ 11.0
Transition toll tax liabilities	697.0	697.0
General Business \| Domestic Country
Tax Credit Carryforward [Line Items]
Operating loss carryforwards	0.6
Tax credit carryforward, amount	1.3
Operating loss carryforward, subject to expiration	18,000.0
General Business \| Foreign Tax Authority
Tax Credit Carryforward [Line Items]
Operating loss carryforward, subject to expiration	16,800.0
Research \| State and Local Jurisdiction
Tax Credit Carryforward [Line Items]
Tax credit carryforward, amount	142.4
Earnings in the form of cash and cash equivalents \| 2017 Tax Act
Tax Credit Carryforward [Line Items]
Transition toll tax repatriation tax rate				15.50%
Other Foreign Earnings \| 2017 Tax Act
Tax Credit Carryforward [Line Items]
Transition toll tax repatriation tax rate				8.00%
Minimum
Tax Credit Carryforward [Line Items]
Residual U.S. tax liability on reversal of deferred tax liabilities on foreign subsidiaries purchase accounting	300.0
Maximum
Tax Credit Carryforward [Line Items]
Residual U.S. tax liability on reversal of deferred tax liabilities on foreign subsidiaries purchase accounting	400.0
Denmark Manufacturing Operations
Tax Credit Carryforward [Line Items]
Tax expense on disposal group		68.9
TECFIDERA
Tax Credit Carryforward [Line Items]
Deferred tax assets, value, reduction in value	1,700.0
Deferred tax liabilities, value, reduction in value	1,600.0
Income tax expense, reduction in deferred tax assets and liabilities, net	$ 90.3
Intellectual Property
Tax Credit Carryforward [Line Items]
Other		754.1
Deferred tax liabilities, other		$ 603.3

Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Tax credits	$ 113.4	$ 106.6
Inventory, other reserves and accruals	165.9	162.0
Intangibles, net	1,546.0	3,380.0
Net operating loss	2,080.3	130.4
Share-based compensation	23.3	23.8
Other	103.1	103.7
Valuation allowance	(1,753.9)	(1.1)
Total deferred tax assets	2,278.1	3,905.4
Deferred tax liabilities:
Purchased intangible assets	(396.2)	(350.3)
GILTI	(1,143.7)	(1,381.6)
Tax credits	(174.6)	(1,617.2)
Depreciation, amortization and other	(227.0)	(135.0)
Total deferred tax liabilities	$ (1,941.5)	$ (3,484.1)

Income Taxes - Tax Rate (Details)	12 Months Ended
Income Taxes - Tax Rate (Details)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Statutory rate	21.00%	21.00%	21.00%
State taxes	0.70%	0.80%	0.60%
Taxes on foreign earnings	(3.30%)	(4.50%)	(1.90%)
Credits and net operating loss utilization	(1.20%)	(1.10%)	(0.90%)
Purchased intangible assets	0.70%	0.40%	1.20%
Divestiture of Denmark manufacturing operations	(0.40%)	1.00%	0.00%
Internal reorganization of certain intellectual property rights	0.00%	(2.10%)	0.00%
TECFIDERA impairment	1.80%	0.00%	0.00%
GILTI	1.30%	1.50%	1.60%
U.S. tax reform	0.00%	0.00%	2.10%
Swiss tax reform	0.00%	(0.80%)	0.00%
Other	(0.90%)	0.10%	0.50%
Effective tax rate	19.70%	16.30%	24.20%

Income Taxes - Accounting for Uncertainty in Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance at January 1,	$ 129.9	$ 114.2	$ 66.8
Additions based on tax positions related to the current period	1.5	5.3	0.5
Additions for tax positions of prior periods	51.7	17.2	58.7
Reductions for tax positions of prior periods	(63.6)	(10.3)	(13.6)
Statute expirations	(7.9)	(0.1)	(2.9)
Settlement refund (payment)		3.6	4.7
Settlement refund (payment)	(35.9)
Ending balance ar December 31,	$ 75.7	$ 129.9	$ 114.2

Other Consolidated Financial Statement Detail - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Interest	$ 272.7	$ 244.2	$ 243.2
Income taxes	$ 906.7	$ 1,064.5	$ 1,007.1

Other Consolidated Financial Statement Detail - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Increase to goodwill		$ 0	$ 117,500,000
Unrealized gain (loss) on investments		(681,800,000)	150,100,000
Realized investment gains (losses)		12,100,000	50,000,000.0
Unrealized gains (losses) recognized during the period on equity securities		681,800,000	150,100,000	$ 128,500,000
Other long-term liabilities		1,329,600,000	1,348,900,000
Accrued income taxes		709,900,000	803,300,000
Denali and Sangamo
Business Acquisition [Line Items]
Unrealized gains (losses) recognized during the period on equity securities		703,900,000
Solothurn
Business Acquisition [Line Items]
Construction in progress		12,400,000	$ 50,000,000.0
Fumarate \| Fumapharm AG \| Twenty billion
Business Acquisition [Line Items]
Increase to goodwill	$ 300,000,000.0
Cumulative sales level	20,000,000,000.0			20,000,000,000.0
Fumarate \| Fumapharm AG \| Each additional one billion up to twenty billion
Business Acquisition [Line Items]
Cumulative sales level	$ 20,000,000,000.0	$ 20,000,000,000.0		$ 20,000,000,000.0

Other Consolidated Financial Statement Detail - Other Income (Expense), Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Interest income	$ 42.0	$ 120.0	$ 112.5
Interest expense	(222.5)	(187.4)	(200.6)
Gain (loss) on investments, net	685.7	204.7	119.5
Foreign exchange gains (losses), net	(10.7)	(7.0)	(9.9)
Other, net	2.9	(47.0)	(10.5)
Total other income (expense), net	$ 497.4	$ 83.3	$ 11.0

Other Consolidated Financial Statement Detail - Gain (Loss) on Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Net gains (losses) recognized during the period on equity securities	$ 693.9	$ 200.1	$ 127.9
Less: Net gains (losses) recognized on equity securities sold during the period and on capital distributions	(12.1)	(50.0)	0.6
Unrealized gains (losses) recognized during the period on equity securities	$ 681.8	$ 150.1	$ 128.5

Other Consolidated Financial Statement Detail - Accrued Expenses and Other (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Revenue-related reserves for discounts and allowances	$ 1,276.0	$ 1,198.9	$ 1,051.3	$ 761.6
Total accrued expenses and other	3,145.3	2,765.8
Component of accrued expenses and other
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Revenue-related reserves for discounts and allowances	1,080.6	1,001.1
Collaboration expenses	389.9	281.6
Employee-related Liabilities, Current	333.8	309.1
Accrued Royalties, Current	218.5	220.9
Derivative liabilities	181.5	6.7
Business Combination, Contingent Consideration, Liability, Current	149.6	148.4
Other Accrued Liabilities, Current	791.4	798.0
Total accrued expenses and other	$ 3,145.3	$ 2,765.8

Collaborative and Other Relationships - OCREVUS (Details) - OCREVUS $ in Millions	1 Months Ended
	Mar. 31, 2017 USD ($)
Collaborative arrangements and non-collaborative arrangement transactions
Royalty operating profit threshold for highest royalty rate percentage	$ 900.0
Reduction in royalty rate	50.00%
Collaboration agreement term	11 years
Foreign Tax Authority
Collaborative arrangements and non-collaborative arrangement transactions
Royalty percentage to be received	3.00%
Minimum
Collaborative arrangements and non-collaborative arrangement transactions
Royalty percentage to be received	13.50%
Maximum
Collaborative arrangements and non-collaborative arrangement transactions
Royalty percentage to be received	24.00%

Collaborative and Other Relationships - Co-promotion Profit Sharing Formula (Details) - RITUXAN	Dec. 31, 2020
Co-promotion profit sharing formula
Until GAZYVA First Non-CLL FDA Approval	40.00%
After First GA101 Threshold Date
After GAZYVA First Non-CLL FDA Approval until First GAZYVA Threshold Date	39.00%
After First GAZYVA Threshold Date until Second GAZYVA Threshold Date	37.50%
After Second GAZYVA Threshold Date	35.00%

Collaborative and Other Relationships - Profit Sharing (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
RITUXAN
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of co promotion operating profits first fifty million	30.00%
Until First GAZYVA Threshold Date	39.00%
After First GAZYVA Threshold Date until Second GAZYVA Threshold Date	37.50%
GAZYVA
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of co promotion operating profits first fifty million	35.00%
Co-promotion operating profit threshold for Rituxan in U S and Canada to determine share of co promotion operating profit prior to amendment	$ 50.0
Limit of gross sale of GAZYVA to be achieved in preceding 12 months under option one	150.0
Limit of gross sale of GAZYVA to be achieved in any 12 months under option one	150.0
Sales trigger gross sales threshold	$ 500.0
Until First GAZYVA Threshold Date	39.00%
After First GAZYVA Threshold Date until Second GAZYVA Threshold Date	37.50%	37.50%	37.50%
New Anti Cd20
Collaborative arrangements and non-collaborative arrangement transactions
Future percentage of co-promotion operating profits	37.50%	37.50%	37.50%
New Anti Cd20 \| Minimum
Collaborative arrangements and non-collaborative arrangement transactions
Future percentage of co-promotion operating profits	30.00%
New Anti Cd20 \| Maximum
Collaborative arrangements and non-collaborative arrangement transactions
Future percentage of co-promotion operating profits	37.50%

Collaborative and Other Relationships - Pretax Profit Sharing Formula (Details) - GAZYVA	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Co-promotion profit sharing formula
Until First GAZYVA Threshold Date	39.00%
After First GA101 Threshold Date
After First GAZYVA Threshold Date until Second GAZYVA Threshold Date	37.50%	37.50%	37.50%
After Second GAZYVA Threshold Date	35.00%

Collaborative and Other Relationships - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative arrangements and non-collaborative arrangement transactions
Total revenues from anti-CD20 therapeutic programs	$ 2,852.6	$ 3,376.1	$ 3,681.6	$ 3,534.3	$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	$ 13,444.6	$ 14,377.9	$ 13,452.9
Revenues from anti-CD20 therapeutic programs
Collaborative arrangements and non-collaborative arrangement transactions
Total revenues from anti-CD20 therapeutic programs	$ 419.0	$ 560.1	$ 478.3	$ 520.4	$ 600.8	$ 595.8	$ 576.4	$ 517.4	1,977.8	2,290.4	1,980.2
Genentech
Collaborative arrangements and non-collaborative arrangement transactions
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA, including the reimbursement of selling and development expenses									1,080.2	1,542.4	1,431.9
Other revenues from anti-CD20 therapeutic programs									$ 897.6	$ 748.0	$ 548.3

Selected Quarterly Financial Information [Abstract]

Collaborative and Other Relationships - Ionis (Details) - USD ($) shares in Millions, $ in Millions	1 Months Ended						12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2013	Dec. 31, 2012	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2015
Collaborative arrangements and non-collaborative arrangement transactions
Cost of sales, excluding amortization and impairment of acquired intangible assets							$ 1,805.2	$ 1,955.4	$ 1,816.3
Research and development							3,990.9	2,280.6	2,597.2
Ionis Pharmaceuticals
Collaborative arrangements and non-collaborative arrangement transactions
License fee					$ 70.0	$ 70.0
Expected license fee and regulatory milestone payments					$ 130.0	130.0
Upfront payment for collaboration agreement			$ 375.0			30.0
Total aducanumab collaboration third party milestone expense	$ 155.0					$ 10.0
Research and development			$ 324.1				11.3	30.0			$ 10.0
Research and development expense asset acquired	$ 45.0
Term of collaboration agreement			10 years		6 years
Total payment to enter collaboration agreement			$ 1,000.0
Investment in common stock, shares purchased (in shares)			11.5
Purchase of common stock			$ 625.0
Prepaid research and discovery services			50.9
Investment in common stock, value			462.9
Premium on purchase of common stock			162.1
Estimated additional payments upon achievement of development and commercial milestones				$ 260.0
Upfront and milestone payments made to collaborative partner				25.0
Expected additional milestone payments when certain sales threshold is met				$ 800.0
Ionis Pharmaceuticals \| SOD1
Collaborative arrangements and non-collaborative arrangement transactions
Research and development							28.0	20.0	18.0
Estimated additional payments upon achievement of development and commercial milestones		$ 55.0
Upfront and milestone payments made to collaborative partner		$ 35.0
Ionis Pharmaceuticals \| SPINRAZA
Collaborative arrangements and non-collaborative arrangement transactions
Cost of sales, excluding amortization and impairment of acquired intangible assets							$ 286.6	$ 293.0	$ 238.0
Ionis Pharmaceuticals \| Minimum
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones			125.0
Ionis Pharmaceuticals \| Minimum \| SPINRAZA
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of royalties as per collaboration							11.00%			11.00%
Ionis Pharmaceuticals \| Maximum
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones			$ 270.0
Ionis Pharmaceuticals \| Maximum \| SPINRAZA
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of royalties as per collaboration							15.00%			15.00%

Collaborative and Other Relationships - Eisai (Details) $ in Millions	3 Months Ended			9 Months Ended	12 Months Ended			24 Months Ended
	Jun. 30, 2020 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2020 USD ($) product	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2020
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of development costs incurred			100.00%
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income					$ 3.3	$ (77.4)	$ 0.0
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Number of product candidates \| product					2
Loss on contract termination		$ 45.0
Percentage of future development costs related to Eisai				15.00%	45.00%			45.00%
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income					$ 33.8	0.0	0.0
Total aducanumab collaboration third party milestone expense	$ 75.0				$ 75.0	$ 0.0	$ 0.0
Eisai \| United States
Collaborative arrangements and non-collaborative arrangement transactions
Biogen share of co-promotion profits or losses					55.00%
Eisai \| European Union
Collaborative arrangements and non-collaborative arrangement transactions
Biogen share of co-promotion profits or losses					68.50%
Eisai \| JAPAN
Collaborative arrangements and non-collaborative arrangement transactions
Biogen share of co-promotion profits or losses					20.00%

Collaborative and Other Relationships - Summary of Activity Related to BAN2401 and Elenbecestat Collaboration (Details) - Eisai - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative arrangements and non-collaborative arrangement transactions
Total development expense incurred by the collaboration related to the advancement of BAN2401 and elenbecestat	$ 219.3	$ 348.7	$ 232.0
Biogen's share of UCB development expense reflected in research and development expense in our consolidated statements of income	109.6	174.3	116.0
Selling, general and administrative \| E2609 and BAN2401
Collaborative arrangements and non-collaborative arrangement transactions
Biogen's share of UCB development expense reflected in research and development expense in our consolidated statements of income	4.9	16.2	5.4
Total sales and marketing expense incurred by the collaboration	$ 9.8	$ 32.4	$ 10.7

Collaborative and Other Relationships - Summary of Activity Related to Aducanumab Collaboration (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative arrangements and non-collaborative arrangement transactions
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income		$ 3.3	$ (77.4)	$ 0.0
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Biogen's share of UCB development expense reflected in research and development expense in our consolidated statements of income		109.6	174.3	116.0
Total aducanumab collaboration third party milestone expense	$ 75.0	75.0	0.0	0.0
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income		33.8	0.0	0.0
Aducanumab \| Research and development \| Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Total UCB development expense		152.0	179.4	264.8
Biogen's share of UCB development expense reflected in research and development expense in our consolidated statements of income		83.6	98.7	234.6
Aducanumab \| Selling, general and administrative \| Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Total UCB development expense		353.0	27.4	50.6
Biogen's share of UCB development expense reflected in research and development expense in our consolidated statements of income		$ 193.7	$ 15.1	$ 27.3

Collaborative and Other Relationships - Summary of Activity Related to the UCB Collaboration (Details) - Research and development - UCB Pharma S.A. - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative arrangements and non-collaborative arrangement transactions
Total UCB development expense	$ 58.3	$ 31.9	$ 29.7
Biogen's share of UCB development expense reflected in research and development expense in our consolidated statements of income	$ 29.2	$ 16.0	$ 14.9

Collaborative and Other Relationships - Alkermes (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Oct. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative arrangements and non-collaborative arrangement transactions
Research and development		$ 3,990.9	$ 2,280.6	$ 2,597.2
Potential future milestone payments commitment to third party approximately		10,200.0
Alkermes
Collaborative arrangements and non-collaborative arrangement transactions
Research and development		$ 32.4	53.5	$ 68.7
Potential future milestone payments commitment to third party approximately			$ 155.0
VUMERITY \| Alkermes
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of royalties as per collaboration	15.00%	15.00%
Royalties payable period, post FDA approval		5 years

Collaborative and Other Relationships - Bristol Myers Squibb (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative arrangements and non-collaborative arrangement transactions
Research and development		$ 3,990.9	$ 2,280.6	$ 2,597.2
Bristol-Myers Squibb
Collaborative arrangements and non-collaborative arrangement transactions
Research and development	$ 300.0	62.4	$ 144.0	$ 97.0
Total aducanumab collaboration third party milestone expense		360.0
iPerian
Collaborative arrangements and non-collaborative arrangement transactions
Total aducanumab collaboration third party milestone expense		$ 370.0

Collaborative and Other Relationships - Acorda (Details) - Acorda - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative arrangements and non-collaborative arrangement transactions
Expected additional milestone payments when certain sales threshold is met	$ 15.0
Foreign sales required to trigger milestone	100.0
Biogen's share of UCB development expense reflected in research and development expense in our consolidated statements of income	$ 44.5	$ 42.0	$ 36.5

Collaborative and Other Relationships - Other Arrangements (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended			12 Months Ended
	Nov. 30, 2020	Sep. 30, 2020	Feb. 29, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative arrangements and non-collaborative arrangement transactions
Research and development				$ 3,990.9	$ 2,280.6	$ 2,597.2
Sage Therapeutics
Collaborative arrangements and non-collaborative arrangement transactions
Global licensing collaboration, shares purchased, amount	$ 650.0
Global licensing collaboration, shares purchased (in shares)	6.2
Global licensing collaboration, purchase price per share (in dollars per share)	$ 104.14
Research and development expense asset acquired	$ 209.0
Upfront and milestone payments made to collaborative partner	875.0
Global licensing collaboration, development and commercial milestone payments	$ 1,600.0
Denali Therapeutics
Collaborative arrangements and non-collaborative arrangement transactions
Global licensing collaboration, shares purchased, amount		$ 465.0
Global licensing collaboration, shares purchased (in shares)		13.0
Global licensing collaboration, purchase price per share (in dollars per share)		$ 34.94
Research and development expense asset acquired		$ 41.3
Upfront and milestone payments made to collaborative partner		560.0
Global licensing collaboration, development and commercial milestone payments		$ 1,100.0
Research and development				8.8
Sangamo Therapeutics, Inc. Agreement
Collaborative arrangements and non-collaborative arrangement transactions
Global licensing collaboration, shares purchased, amount			$ 225.0
Global licensing collaboration, shares purchased (in shares)			24.0
Global licensing collaboration, purchase price per share (in dollars per share)			$ 9.21
Research and development expense asset acquired			$ 83.0
Upfront and milestone payments made to collaborative partner			125.0
Global licensing collaboration, development and commercial milestone payments			2,400.0
Research and development				$ 6.4
Term of collaboration agreement				5 years
Global licensing collaboration, payment, selection of targets			80.0
Global licensing collaboration, payment,first milestone			1,900.0
Global licensing collaboration, payment, sales based milestone			$ 380.0
Other research and discovery
Collaborative arrangements and non-collaborative arrangement transactions
Research and development				$ 92.1	$ 77.0	$ 48.6

Collaborative and Other Relationships - Samsung Bioepis (Details) $ in Millions, ₩ in Billions	1 Months Ended					3 Months Ended								12 Months Ended
	Jan. 31, 2020 USD ($)	Nov. 30, 2018 USD ($)	Nov. 30, 2018 KRW (₩)	Dec. 31, 2013 USD ($)	Feb. 29, 2012 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) product	Dec. 31, 2019 USD ($) product	Dec. 31, 2018 USD ($)	Dec. 31, 2020 KRW (₩)	Dec. 31, 2019 KRW (₩)	Jun. 30, 2018	Feb. 29, 2012 KRW (₩)
Collaborative arrangements and non-collaborative arrangement transactions
Payments to increase investment in Samsung Bioepis														$ 0.0	$ 0.0	$ 676.6
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income														$ 3.3	$ (77.4)	0.0
Number of ophthalmolgy biosimilar products \| product														2	2
Research and Development Expense (Excluding Acquired in Process Cost)														$ 3,990.9	$ 2,280.6	2,597.2
Collaboration profit (loss) sharing														232.9	241.6	185.0
Revenues						$ 2,852.6	$ 3,376.1	$ 3,681.6	$ 3,534.3	$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	13,444.6	14,377.9	13,452.9
Samsung Bioepis
Collaborative arrangements and non-collaborative arrangement transactions
Upfront and milestone payments made to collaborative partner	$ 100.0
Research and development expense asset acquired	63.0
Prepaid research and discovery services	$ 37.0
Estimated additional payments upon achievement of development and commercial milestones														$ 195.0
Term of collaboration agreement				10 years										5 years
Option exercise fee														$ 60.0
Research and Development Expense (Excluding Acquired in Process Cost)				$ 46.0
Expected profit share percentage														50.00%
Collaboration profit (loss) sharing														$ 266.5	241.6	187.4
Revenues														20.9	106.2	$ 96.4
Due from related parties						5.1				85.0				5.1	85.0
Due to related parties						99.0				100.0				99.0	100.0
Samsung Bioepis \| Development Milestones
Collaborative arrangements and non-collaborative arrangement transactions
Total aducanumab collaboration third party milestone expense							$ 15.0
Samsung Bioepis
Collaborative arrangements and non-collaborative arrangement transactions
Contributions by third party					$ 250.0															₩ 280.5
Joint venture ownership percentage					85.00%
Investment in Samsung Bioepis					$ 45.0	$ 620.2				$ 580.2				$ 620.2	580.2		₩ 673.8	₩ 670.8		₩ 49.5
Samsung Bioepis, ownership percentage					15.00%	49.90%								49.90%			49.90%			15.00%
Samsung Bioepis, ownership percentage before additional purchase transaction																			5.00%
Payments to increase investment in Samsung Bioepis		$ 676.6	₩ 759.5
Total basis difference		$ 675.0
Net profits on investment														$ 5.3	79.4
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income														45.3	1.2
Amortization														$ 40.0	$ 78.2
Total aducanumab collaboration third party milestone expense				25.0
Net change in acquired and in-licensed rights and patents				$ 75.0
Samsung Bioepis \| Inventories
Collaborative arrangements and non-collaborative arrangement transactions
Equity method investment basis difference amortization period														1 year 6 months
Samsung Bioepis \| Completed technology
Collaborative arrangements and non-collaborative arrangement transactions
Equity method investment basis difference amortization period														15 years

Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended	9 Months Ended	12 Months Ended			24 Months Ended
	May 31, 2018	Oct. 31, 2017	Jun. 30, 2020	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
Variable Interest Entity [Line Items]
Payment to Neurimmune					$ 4.6	$ 0.0	$ 0.0
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income					3.3	(77.4)	0.0
Potential future milestone payments commitment to third party approximately					10,200.0			$ 10,200.0
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities					12.8	22.7		12.8
Twelve months
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately					$ 200.0			200.0
Neurimmune
Variable Interest Entity [Line Items]
Term of collaboration agreement					12 years
Research and development costs, percentage					100.00%
Payment to Neurimmune	$ 50.0	$ 150.0
Reduction in royalty rate payable on commercial sales		15.00%
Additional reduction in royalty rate payable on commercial sales	5.00%
Regulatory Milestones
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately					$ 1,300.0			$ 1,300.0
Eisai
Variable Interest Entity [Line Items]
Total aducanumab collaboration third party milestone expense			$ 75.0		75.0	0.0	0.0
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income					$ 33.8	$ 0.0	$ 0.0
Percentage of future development costs related to Eisai				15.00%	45.00%			45.00%
Neurimmune \| Regulatory Milestones
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately					$ 100.0			$ 100.0
Aducanumab \| Regulatory Milestones \| European Union
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately					50.0			50.0
Aducanumab \| Regulatory Milestones \| JAPAN
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately					$ 50.0			$ 50.0

Litigation - Narrative (Details) $ in Millions	2 Months Ended
Litigation - Narrative (Details) $ in Millions	Dec. 31, 2019 USD ($)
Loss Contingency, Information about Litigation Matters [Abstract]
Claims asserted	$ 200.0

Commitments and Contingencies - Narrative (Details) - USD ($)	1 Months Ended			3 Months Ended	12 Months Ended
	Dec. 31, 2019	Oct. 31, 2019	Dec. 31, 2012	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2021	Dec. 31, 2013
Business Acquisition [Line Items]
Maximum contingent consideration in the form of development and approval milestones					$ 400,000,000.0
Contingent payment				$ 300,000,000.0	0	$ 0	$ 58,200,000
Potential future milestone payments commitment, approximately					10,200,000,000
Accrued expenses					21,700,000
Cancellable future commitments					593,000,000.0
Liabilities associated with uncertain tax positions					79,600,000
Income taxes paid					62,000,000.0
Solothurn
Business Acquisition [Line Items]
Contractual obligation					9,300,000
2017 Tax Act
Business Acquisition [Line Items]
Transition toll tax liabilities	$ 697,000,000.0				697,000,000.0	697,000,000.0
Twelve months
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately					200,000,000.0
Development Milestones
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately					1,900,000,000
Regulatory Milestones
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately					1,300,000,000
Commercial Milestones
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately					$ 7,000,000,000.0
Ionis Pharmaceuticals
Business Acquisition [Line Items]
Total aducanumab collaboration third party milestone expense	155,000,000.0		$ 10,000,000.0
Ionis Pharmaceuticals \| Minimum \| SPINRAZA
Business Acquisition [Line Items]
Percentage of royalties as per collaboration					11.00%			11.00%
Ionis Pharmaceuticals \| Maximum \| SPINRAZA
Business Acquisition [Line Items]
Percentage of royalties as per collaboration					15.00%			15.00%
Alkermes
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately	$ 155,000,000.0					$ 155,000,000.0
Alkermes \| VUMERITY
Business Acquisition [Line Items]
Percentage of royalties as per collaboration		15.00%			15.00%
Royalties payable period, post FDA approval					5 years
Aducanumab \| Forecast
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately									$ 86,200,000
Aducanumab \| Regulatory Milestones \| European Union
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately					$ 50,000,000.0
Aducanumab \| Regulatory Milestones \| JAPAN
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately					50,000,000.0
Neurimmune \| Regulatory Milestones
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately					100,000,000.0
TYSABRI
Business Acquisition [Line Items]
Future contingent payment for annual worldwide net sales up to $2.0 billion										18.00%
Future contingent payment threshold										$ 2,000,000,000.0
Future contingent payment for annual worldwide net sales that exceed $2.0 billion										25.00%
Fumapharm AG \| Fumarate \| Twenty billion
Business Acquisition [Line Items]
Cumulative sales level							20,000,000,000.0
Fumapharm AG \| Fumarate \| Each additional one billion up to twenty billion
Business Acquisition [Line Items]
Cumulative sales level					$ 20,000,000,000.0		$ 20,000,000,000.0

Employee Benefit Plans - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SWITZERLAND
Defined Contribution Plan Disclosure [Line Items]
Percentage of minimum investment return	1.00%	1.00%	1.00%
Unfunded net pension	$ 75.7	$ 42.9
Employee benefit plan obligations	170.0	127.1
Pension cost (reversal)	15.5	14.7	$ 14.8
SWITZERLAND \| Other (income) expense, net
Defined Contribution Plan Disclosure [Line Items]
Pension cost (reversal)	2.6	1.2	1.3
GERMANY
Defined Contribution Plan Disclosure [Line Items]
Employee benefit plan obligations	75.5	59.6
Periodic pension cost	6.2	5.1	5.3
GERMANY \| Other (income) expense, net
Defined Contribution Plan Disclosure [Line Items]
Periodic pension cost	$ 2.0	1.4	1.5
Domestic Plan
Defined Contribution Plan Disclosure [Line Items]
Minimum qualifying age for employee benefit plan	21 years
Expenses related to savings plan	$ 44.3	44.8	$ 42.2
Deferred Compensation Plan
Defined Contribution Plan Disclosure [Line Items]
Deferred compensation liability	$ 120.0	$ 114.6

Segment Information - Narrative (Details) $ in Millions	12 Months Ended
Segment Information - Narrative (Details) $ in Millions	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Revenue from External Customer [Line Items]
Number of reportable segment \| segment	1
Long-lived assets related to operations in Denmark	$ 3,844.8	$ 3,674.3	$ 3,601.2
Solothurn
Revenue from External Customer [Line Items]
Long-lived assets related to operations in Denmark	$ 2,180.6	$ 2,028.8	1,748.5
DENMARK
Revenue from External Customer [Line Items]
Long-lived assets related to operations in Denmark			$ 646.5

Segment Information - Geographic Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue, Major Customer [Line Items]
Revenues	$ 2,852.6	$ 3,376.1	$ 3,681.6	$ 3,534.3	$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	$ 13,444.6	$ 14,377.9	$ 13,452.9
Long-lived assets	3,844.8				3,674.3				3,844.8	3,674.3	3,601.2
U.S.
Revenue, Major Customer [Line Items]
Long-lived assets	1,496.3				1,493.2				1,496.3	1,493.2	1,152.7
Europe
Revenue, Major Customer [Line Items]
Long-lived assets	2,321.4				2,162.9				2,321.4	2,162.9	2,442.8
Asia
Revenue, Major Customer [Line Items]
Long-lived assets	16.2				6.2				16.2	6.2	3.9
Other
Revenue, Major Customer [Line Items]
Long-lived assets	10.9				12.0				10.9	12.0	1.8
Product, net
Revenue, Major Customer [Line Items]
Revenues	2,301.6	2,690.3	2,795.7	2,904.6	2,924.8	2,894.7	2,880.3	2,680.0	10,692.2	11,379.8	10,886.8
Product, net \| U.S.
Revenue, Major Customer [Line Items]
Revenues									5,900.1	6,713.8	6,800.5
Product, net \| Europe
Revenue, Major Customer [Line Items]
Revenues									3,656.4	3,794.5	3,370.3
Product, net \| Asia
Revenue, Major Customer [Line Items]
Revenues									596.7	320.3	281.2
Product, net \| Other
Revenue, Major Customer [Line Items]
Revenues									539.0	551.2	434.8
Revenues from anti-CD20 therapeutic programs
Revenue, Major Customer [Line Items]
Revenues	419.0	560.1	478.3	520.4	600.8	595.8	576.4	517.4	1,977.8	2,290.4	1,980.2
Revenues from anti-CD20 therapeutic programs \| U.S.
Revenue, Major Customer [Line Items]
Revenues									1,897.4	2,211.9	1,903.4
Revenues from anti-CD20 therapeutic programs \| Europe
Revenue, Major Customer [Line Items]
Revenues									0.1	0.2	0.2
Revenues from anti-CD20 therapeutic programs \| Asia
Revenue, Major Customer [Line Items]
Revenues									0.0	0.0	0.0
Revenues from anti-CD20 therapeutic programs \| Other
Revenue, Major Customer [Line Items]
Revenues									80.3	78.3	76.6
Other
Revenue, Major Customer [Line Items]
Revenues	$ 132.0	$ 125.7	$ 407.6	$ 109.3	$ 145.7	$ 109.6	$ 160.0	$ 292.4	774.6	707.7	585.9
Other \| U.S.
Revenue, Major Customer [Line Items]
Revenues									733.6	585.8	457.0
Other \| Europe
Revenue, Major Customer [Line Items]
Revenues									8.1	9.7	32.7
Other \| Asia
Revenue, Major Customer [Line Items]
Revenues									32.9	112.2	96.2
Other \| Other
Revenue, Major Customer [Line Items]
Revenues									$ 0.0	$ 0.0	$ 0.0

Quarterly Financial Data (Unaudited) (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions

3 Months Ended

12 Months Ended

Dec. 31, 2020

Sep. 30, 2020

Jun. 30, 2020

Mar. 31, 2020

Dec. 31, 2019

Sep. 30, 2019

Jun. 30, 2019

Mar. 31, 2019

Dec. 31, 2020

Dec. 31, 2019

Dec. 31, 2018

Net income attributable to Biogen Inc.

$ 2,852.6

$ 3,376.1

$ 3,681.6

$ 3,534.3

$ 3,671.3

$ 3,600.1

$ 3,616.7

$ 3,489.8

$ 13,444.6

$ 14,377.9

$ 13,452.9

Gross profit

^[1]

2,361.9

2,927.0

3,270.5

3,080.0

3,224.2

3,170.1

3,140.4

2,887.8

11,639.4

12,422.5

Net income

357.6

703.9

1,606.5

1,392.6

1,439.7

1,545.9

1,494.1

1,408.8

4,060.5

5,888.5

4,474.0

$ 357.9

$ 701.5

$ 1,542.1

$ 1,399.1

$ 1,439.7

$ 1,545.9

$ 1,494.1

$ 1,408.8

$ 4,000.6

$ 5,888.5

$ 4,430.7

Net income per share:

Basic earnings per share attributable to Biogen Inc. (in dollars per share)

$ 2.33

$ 4.47

$ 9.60

$ 8.10

$ 8.40

$ 7.85

$ 7.17

$ 24.86

$ 31.47

$ 21.63

Diluted earnings per share attributable to Biogen Inc. (in dollars per share)

$ 2.32

$ 4.46

$ 9.59

$ 8.08

$ 8.39

$ 7.85

$ 7.15

$ 24.80

$ 31.42

$ 21.58

Weighted-average shares used in calculating:

Basic earnings per share attributable to Biogen Inc. (in shares)

153.7

156.9

160.6

172.8

177.8

184.0

190.3

196.6

160.9

187.1

204.9

Diluted earnings per share attributable to Biogen Inc. (in shares)

154.0

157.2

160.9

173.1

178.2

184.2

190.4

197.0

161.3

187.4

205.3

Product, net

Selected Quarterly Financial Information [Abstract]

Revenues from anti-CD20 therapeutic programs

$ 2,301.6

$ 2,690.3

$ 2,795.7

$ 2,904.6

$ 2,924.8

$ 2,894.7

$ 2,880.3

$ 2,680.0

$ 10,692.2

$ 11,379.8

$ 10,886.8

Selected Quarterly Financial Information [Abstract]

Selected Quarterly Financial Information [Abstract]

419.0

560.1

478.3

520.4

600.8

595.8

576.4

517.4

1,977.8

2,290.4

1,980.2

Other