0000875045-21-000028 | 10-Q | XBRL Viewer

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Cover Page - shares	3 Months Ended
Cover Page - shares	Mar. 31, 2021	Apr. 21, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
Document Transition Report	false
Entity File Number	0-19311
Entity Registrant Name	BIOGEN INC.
Entity Central Index Key	0000875045
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	33-0112644
Entity Address, Address Line One	225 Binney Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02142
City Area Code	617
Local Phone Number	679-2000
Title of 12(b) Security	Common Stock, $0.0005 par value
Trading Symbol	BIIB
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		150,554,750

Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Total revenue	$ 2,694.0	$ 3,534.3
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	478.1	454.3
Research and development	514.2	476.3
Selling, general and administrative	595.0	570.1
Amortization and impairment of acquired intangible assets	98.1	71.5
Collaboration profit (loss) sharing	68.5	71.8
(Gain) loss on fair value remeasurement of contingent consideration	(33.8)	(4.6)
Acquired in-process research and development	0.0	75.0
Total cost and expense	1,720.1	1,714.4
Income from operations	973.9	1,819.9
Other income (expense), net	(506.9)	(120.5)
Income before income tax expense and equity in loss of investee, net of tax	467.0	1,699.4
Income tax expense	44.2	292.0
Equity in (income) loss of investee, net of tax	18.2	14.8
Net income	404.6	1,392.6
Net income (loss) attributable to noncontrolling interests, net of tax	(5.6)	(6.5)
Net income attributable to Biogen Inc.	$ 410.2	$ 1,399.1
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 2.70	$ 8.10
Diluted earnings per share attributable to Biogen Inc.	$ 2.69	$ 8.08
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	151.9	172.8
Diluted earnings per share attributable to Biogen Inc.	152.3	173.1
Product, net
Total revenue	$ 2,211.7	$ 2,904.6
Revenue from anti-CD20 therapeutic programs
Total revenue	389.0	520.4
Other
Total revenue	$ 93.3	$ 109.3

Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 410.2	$ 1,399.1
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	(0.8)	(7.8)
Unrealized gains (losses) on cash flow hedges, net of tax	149.6	33.8
Gains (losses) on net investment hedges	22.4	23.0
Unrealized gains (losses) on pension benefit obligation, net of tax	2.0	0.8
Currency translation adjustment	(48.5)	(63.9)
Total other comprehensive income (loss), net of tax	124.7	(14.1)
Comprehensive income attributable to Biogen Inc.	534.9	1,385.0
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	(4.9)	(5.9)
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	$ 530.0	$ 1,379.1

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 1,217.5	$ 1,331.2
Marketable securities	1,320.0	1,278.9
Accounts receivable, net	1,854.3	1,913.8
Due from anti-CD20 therapeutic programs	369.8	413.5
Inventory	1,171.8	1,068.6
Other current assets	786.1	881.1
Total current assets	6,719.5	6,887.1
Marketable securities	821.9	772.1
Property, plant and equipment, net	3,438.3	3,411.5
Operating lease assets	414.5	433.3
Intangible assets, net	2,988.1	3,084.3
Goodwill	5,763.1	5,762.1
Deferred tax asset	1,286.3	1,369.5
Investments and other assets	2,423.0	2,899.0
Total assets	23,854.7	24,618.9
Current liabilities:
Taxes payable	143.1	142.0
Accounts payable	430.4	454.9
Accrued expense and other	2,592.3	3,145.3
Total current liabilities	3,165.8	3,742.2
Notes payable	7,267.2	7,426.2
Deferred tax liability	966.7	1,032.8
Long-term operating lease liabilities	380.0	402.0
Other long-term liabilities	1,411.8	1,329.6
Total liabilities	13,191.5	13,932.8
Commitments and contingencies
Biogen Idec Inc. shareholders' equity
Preferred stock, par value $0.001 per share	0.0	0.0
Common stock, par value $0.0005 per share	0.1	0.1
Additional paid-in capital	0.0	0.0
Accumulated other comprehensive loss	(174.3)	(299.0)
Retained earnings	13,833.5	13,976.3
Treasury stock, at cost	(2,977.1)	(2,977.1)
Total Biogen Inc. shareholders’ equity	10,682.2	10,700.3
Noncontrolling interests	(19.0)	(14.2)
Total equity	10,663.2	10,686.1
Total liabilities and equity	$ 23,854.7	$ 24,618.9

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Common stock, par value	$ 0.0005	$ 0.0005

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flow from operating activities:
Net income	$ 404.6	$ 1,392.6
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation, amortization and impairments	146.9	119.9
Acquired in-process research and development	0.0	75.0
Share-based compensation	70.0	67.7
Contingent consideration	(33.8)	(4.6)
Deferred income taxes	(15.0)	36.9
(Gain) loss on strategic investments	437.6	62.0
Our share of Samsung Bioepis gains (losses)	18.2	16.8
Other	71.7	28.9
Changes in operating assets and liabilities, net:
Accounts receivable	37.2	(238.4)
Due from anti-CD20 therapeutic programs	43.8	78.6
Inventory	(112.5)	(62.2)
Accrued expense and other current liabilities	(283.6)	(347.4)
Income tax assets and liabilities	64.8	223.0
Other changes in operating assets and liabilities, net	(80.9)	18.5
Net cash flow provided by operating activities	769.0	1,467.3
Cash flows from investing activities:
Proceeds from sales and maturities of marketable securities	819.2	2,389.3
Purchases of marketable securities	(913.3)	(1,684.7)
Proceeds from divestiture of Hillerød, Denmark manufacturing operations	28.1	0.0
Purchases of property, plant and equipment	(92.6)	(149.7)
Acquired in-process research and development	0.0	(75.0)
Acquisitions of intangible assets	0.0	(37.0)
Proceeds from sales of strategic investments	91.2	0.5
Other	2.7	(0.5)
Net cash flow (used in) provided by investing activities	(64.7)	442.9
Cash flows from financing activities:
Purchases of treasury stock	(600.0)	(2,220.2)
Payments related to issuance of stock for share-based compensation arrangements, net	(27.1)	(29.6)
Repayments of borrowings and premiums paid on debt exchange	(169.3)	0.0
Other	11.4	4.5
Net cash flow used in financing activities	(785.0)	(2,245.3)
Net increase (decrease) in cash and cash equivalents	(80.7)	(335.1)
Effect of exchange rate changes on cash and cash equivalents	33.0	(12.7)
Cash and cash equivalents, beginning of the period	1,331.2	2,913.7
Cash and cash equivalents, end of the period	$ 1,217.5	$ 2,591.3

Condensed Consolidated Statement of Equity Statement - USD ($) shares in Millions, $ in Millions	Total	Preferred stock	Common stock	Additional paid-in capital	Accumulated Other Comprehensive Income	Retained earnings	Treasury stock	Parent	Noncontrolling interest	2018 Share Repurchase Program	2018 Share Repurchase Program Common stock	2018 Share Repurchase Program Additional paid-in capital	2018 Share Repurchase Program Retained earnings	2018 Share Repurchase Program Treasury stock	2018 Share Repurchase Program Parent	2019 Share Repurchase Program	2019 Share Repurchase Program Common stock	2019 Share Repurchase Program Additional paid-in capital	2019 Share Repurchase Program Retained earnings	2019 Share Repurchase Program Treasury stock	2019 Share Repurchase Program Parent	December 2019 Share Repurchase Program	December 2019 Share Repurchase Program Common stock	December 2019 Share Repurchase Program Additional paid-in capital	December 2019 Share Repurchase Program Retained earnings	December 2019 Share Repurchase Program Treasury stock	December 2019 Share Repurchase Program Parent
Beginning balance, shares at Dec. 31, 2019		0.0	(198.0)				(23.8)
Beginning balance at Dec. 31, 2019	$ 13,339.1	$ 0.0	$ 0.1	$ 0.0	$ (135.2)	$ 16,455.4	$ (2,977.1)	$ 13,343.2	$ (4.1)
Net income	1,392.6					1,399.1		1,399.1	(6.5)
Other comprehensive income (loss), net of tax	(13.5)				(14.1)			(14.1)	0.6
Repurchase of common stock, at cost, shares														(3.2)		(4.1)				(4.1)		(3.2)
Repurchase of common stock, at cost										$ (941.1)				$ (941.1)	$ (941.1)	$ (1,279.1)				$ (1,279.1)	$ (1,279.1)
Retirement of common stock pursuant to Share Repurchase Programs, at cost, shares											(3.2)			(3.2)			(4.1)			(4.1)
Retirement of common stock pursuant to Share Repurchase Progams, at cost										$ 0.0	$ 0.0	$ (15.6)	$ (925.5)	$ 941.1	$ 0.0			$ (71.0)	$ (1,208.1)	$ 1,279.1
Issuance of common stock under stock option and stock purchase plans, shares			0.1
Issuance of common stock under stock option and stock purchase plans	18.1		$ 0.0	18.1				18.1
Issuance of common stock under award plan, shares			0.3
Issuance of common stock under stock award plan	(47.8)		$ 0.0	0.0		(47.8)		(47.8)
Compensation related to share-based payments	69.2			69.2		0.0		69.2
Stockholders' Equity, Other	(0.6)			(0.6)				(0.6)
Ending balance at Mar. 31, 2020	12,536.9	$ 0.0	$ 0.1	0.1	(149.3)	15,673.1	$ (2,977.1)	12,546.9	(10.0)
Ending balance, shares at Mar. 31, 2020		0.0	(191.1)				(23.8)
Beginning balance, shares at Dec. 31, 2020		0.0	(176.2)				(23.8)
Beginning balance at Dec. 31, 2020	10,686.1	$ 0.0	$ 0.1	0.0	(299.0)	13,976.3	$ (2,977.1)	10,700.3	(14.2)
Net income	404.6					410.2		410.2	(5.6)
Other comprehensive income (loss), net of tax	125.4				124.7			124.7	0.7
Noncontrolling interest, increase (decrease) other	0.1							0.0	0.1
Repurchase of common stock, at cost, shares			0.0																							(2.2)
Repurchase of common stock, at cost																						$ (600.0)				$ (600.0)	$ (600.0)
Retirement of common stock pursuant to Share Repurchase Programs, at cost, shares																							(2.2)			(2.2)
Retirement of common stock pursuant to Share Repurchase Progams, at cost																						$ 0.0	$ 0.0	$ (93.8)	$ (506.2)	$ 600.0	$ 0.0
Issuance of common stock under stock option and stock purchase plans, shares			0.1
Issuance of common stock under stock option and stock purchase plans	19.7		$ 0.0	19.7				19.7
Issuance of common stock under award plan, shares			0.3
Issuance of common stock under stock award plan	(46.8)		$ 0.0	0.0		(46.8)		(46.8)
Compensation related to share-based payments	72.6			72.6				72.6
Stockholders' Equity, Other	1.5			1.5		0.0		1.5
Ending balance at Mar. 31, 2021	$ 10,663.2	$ 0.0	$ 0.1	$ 0.0	$ (174.3)	$ 13,833.5	$ (2,977.1)	$ 10,682.2	$ (19.0)
Ending balance, shares at Mar. 31, 2021		0.0	(174.4)				(23.8)

Summary of Significant Accounting Policies	3 Months Ended
Summary of Significant Accounting Policies	Mar. 31, 2021
Accounting Policies [Abstract]
Summary of significant accounting policies	References in these notes to "Biogen," the "company," "we," "us" and "our" refer to Biogen Inc. and its consolidated subsidiaries. Business Overview Biogen is a global biopharmaceutical company 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 core growth areas include multiple sclerosis (MS) and neuroimmunology; Alzheimer’s disease and dementia; neuromuscular disorders, including spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS); movement disorders, including Parkinson's disease; ophthalmology; and neuropsychiatry. We are also focused on discovering, developing and delivering worldwide innovative therapies in our emerging growth areas of immunology; acute neurology; and neuropathic pain. In addition, we commercialize biosimilars of advanced biologics. We support our drug discovery and development efforts through the commitment of significant resources to discovery, research and development programs and business development opportunities. Our marketed products include TECFIDERA, VUMERITY, AVONEX, PLEGRIDY, TYSABRI and FAMPYRA for the treatment of MS; SPINRAZA for the treatment of SMA; and FUMADERM for the treatment of severe plaque psoriasis. We have certain business and financial rights with respect to RITUXAN for the treatment of non-Hodgkin's lymphoma, chronic lymphocytic leukemia (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 primary progressive MS and relapsing MS; and other potential anti-CD20 therapies pursuant to our collaboration arrangements with Genentech, Inc. (Genentech), a wholly-owned member of the Roche Group. For additional information on our collaboration arrangements with Genentech, please read Note 18, Collaborative and Other Relationships, to our consolidated financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2020 (2020 Form 10-K). Our innovative drug development and commercialization activities are complemented by our biosimilar business that expands access to medicines and reduces the cost burden for healthcare systems. Through our agreements with Samsung Bioepis Co., Ltd. (Samsung Bioepis), our joint venture with Samsung BioLogics Co., Ltd. (Samsung BioLogics), we market and sell BENEPALI, an etanercept biosimilar referencing ENBREL, IMRALDI, an adalimumab biosimilar referencing HUMIRA, and FLIXABI, an infliximab biosimilar referencing REMICADE, in certain countries in Europe and have an option to acquire exclusive rights to commercialize these products in China. Additionally, we have 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 United States (U.S.), Canada, Europe, Japan and Australia. For additional information on our collaboration arrangements with Samsung Bioepis, please read Note 16, Collaborative and Other Relationships, to these unaudited condensed consolidated financial statements (condensed consolidated financial statements). Basis of Presentation In the opinion of management, our condensed consolidated financial statements include all adjustments, consisting of normal recurring accruals, necessary for a fair statement of our financial statements for interim periods in accordance with accounting principles generally accepted in the United States (U.S. GAAP). The information included in this quarterly report on Form 10-Q should be read in conjunction with our audited consolidated financial statements and the accompanying notes included in our 2020 Form 10-K. Our accounting policies are described in the Notes to Consolidated Financial Statements in our 2020 Form 10-K and updated, as necessary, in this report. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from our audited financial statements, but does not include all disclosures required by U.S. GAAP. The results of operations for the three months ended March 31, 2021, are not necessarily indicative of the operating results for the full year or for any other subsequent interim period. 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. Consolidation Our condensed consolidated financial statements reflect our financial statements, those of our wholly-owned subsidiaries and those of certain variable interest entities where we are the primary beneficiary. For consolidated entities where we own or are exposed to less than 100.0% of the economics, we record net income (loss) attributable to noncontrolling interests in our condensed consolidated statements of income equal to the percentage of the economic or ownership interest retained in such entities by the respective noncontrolling parties. Intercompany balances and transactions are eliminated in consolidation. In determining whether we are the primary beneficiary of a variable interest entity, we apply a qualitative approach that determines whether we have both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. These considerations impact the way we account for our existing collaborative relationships and other arrangements. We continuously assess whether we are the primary beneficiary of a variable interest entity as changes to existing relationships or future transactions may result in us consolidating or deconsolidating one or more of our collaborators or partners. Use of Estimates The preparation of our condensed consolidated financial statements requires us to make estimates, judgments and assumptions that may affect the reported amounts of assets, liabilities, equity, revenue and expense and related disclosure of contingent assets and liabilities. On an ongoing basis we evaluate our estimates, judgments and methodologies. We base our estimates on historical experience and on various other assumptions that we believe are reasonable, the results of which form the basis for making judgments about the carrying values of assets, liabilities and equity and the amount of revenue and expense. Actual results may differ from these estimates. The length of time and full extent to which the COVID-19 pandemic directly or indirectly impacts our business, results of operations and financial condition, including sales, expense, reserves and allowances, manufacturing, clinical trials, research and development costs and employee-related amounts, depends on future developments that are highly uncertain, subject to change and are difficult to predict, including as a result of new information that may emerge concerning COVID-19 and the actions taken to contain or treat COVID-19 as well as the economic impact on local, regional, national and international customers and markets. We have made estimates of the impact of the COVID-19 pandemic within our condensed consolidated financial statements and there may be changes to those estimates in future periods. New Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (FASB) or other standard setting bodies that we adopt as of the specified effective date. Unless otherwise discussed below, we do not believe that the adoption of recently issued standards have or may have a material impact on our condensed consolidated financial statements or disclosures. Income Taxes In December 2019 the FASB issued Accounting Standards Update No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. This standard removes certain exceptions to the general principles in Topic 740 and simplifies certain other aspects of the accounting for income taxes. This standard became effective for us on January 1, 2021, and did not have a material impact on our condensed consolidated financial statements and related disclosures.

Acquisitions	3 Months Ended
Acquisitions	Mar. 31, 2021
Business Combinations [Abstract]
Acquisitions	BIIB118 Acquisition In March 2020 we acquired BIIB118 (CK1 inhibitor) for the potential treatment of patients with behavioral and neurological symptoms across various psychiatric and neurological diseases from Pfizer Inc. (Pfizer). We are developing BIIB118 for the potential treatment of irregular sleep wake rhythm disorder in Parkinson's disease and plan to develop BIIB118 for the potential treatment of sundowning in Alzheimer's disease. In connection with this acquisition, we made an upfront payment of $75.0 million to Pfizer, which was accounted for as an asset acquisition and recorded as acquired in-process research and development (IPR&D) in our condensed consolidated statements of income as BIIB118 has not yet reached technological feasibility. We may also pay Pfizer up to $635.0 million in potential additional development and commercialization milestone payments as well as tiered royalties in the high single digits to sub-teens.

Revenues

3 Months Ended

Mar. 31, 2021

Revenue from Contract with Customer [Abstract]

Revenues

Product Revenue

Revenue by product is summarized as follows:


	For the Three Months Ended March 31,
	2021						2020
(In millions)	United States		Rest of World		Total		United States		Rest of World		Total
Multiple Sclerosis (MS):
Fumarate*	$	236.0	$	316.9	$	552.9	$	777.5	$	323.3	$	1,100.8
Interferon**	241.8		158.7		400.5		292.6		173.4		466.0
TYSABRI	273.3		230.0		503.3		277.7		244.7		522.4
FAMPYRA	—		26.6		26.6		—		28.3		28.3
Subtotal: MS product revenue	751.1		732.2		1,483.3		1,347.8		769.7		2,117.5

Spinal Muscular Atrophy:
SPINRAZA	148.7		371.8		520.5		235.4		329.6		565.0

Biosimilars:
BENEPALI	—		121.7		121.7		—		133.5		133.5
IMRALDI	—		57.9		57.9		—		61.6		61.6
FLIXABI	—		25.5		25.5		—		23.7		23.7
Subtotal: Biosimilar product revenue	—		205.1		205.1		—		218.8		218.8

Other:
FUMADERM	—		2.8		2.8		—		3.3		3.3
Total product revenue	$	899.8	$	1,311.9	$	2,211.7	$	1,583.2	$	1,321.4	$	2,904.6

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

**Interferon includes AVONEX and PLEGRIDY.

We recognized revenue from two wholesalers accounting for 30.0% and 9.3% of gross product revenue for the three months ended March 31, 2021, and 30.0% and 14.6% for the three months ended March 31, 2020.

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


(In millions)	Discounts		Contractual Adjustments		Returns		Total
Balance, December 31, 2020	$	141.4	$	1,093.0	$	41.6	$	1,276.0
Current provisions relating to sales in current year	185.1		775.7		3.8		964.6
Adjustments relating to prior years	0.9		(32.1)		1.0		(30.2)
Payments/credits relating to sales in current year	(111.8)		(284.8)		—		(396.6)
Payments/credits relating to sales in prior years	(82.8)		(562.9)		(1.7)		(647.4)
Balance, March 31, 2021	$	132.8	$	988.9	$	44.7	$	1,166.4

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


(In millions)	As of March 31, 2021		As of December 31, 2020
Reduction of accounts receivable	$	173.4	$	195.4
Component of accrued expense and other	993.0		1,080.6
Total revenue-related reserves	$	1,166.4	$	1,276.0

Revenue from Anti-CD20 Therapeutic Programs

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


	For the Three Months Ended March 31,
(In millions)	2021		2020
Biogen’s share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	$	174.1	$	341.3
Other revenue from anti-CD20 therapeutic programs	214.9		179.1
Total revenue from anti-CD20 therapeutic programs	$	389.0	$	520.4

For additional information on our collaboration arrangements with Genentech, please read Note 18, Collaborative and Other Relationships, to our consolidated financial statements included in our 2020 Form 10-K.

Other Revenue

Other revenue is summarized as follows:


	For the Three Months Ended March 31,
(In millions)	2021		2020
Revenue from collaborative and other relationships:
Revenue earned under our technical development agreement, manufacturing services agreements and royalty revenue on biosimilar products with Samsung Bioepis	$	3.9	$	3.7
Other revenue from collaborative and other relationships	—		0.2
Other royalty and corporate revenue:
Royalty	6.2		11.4
Other corporate	83.2		94.0
Total other revenue	$	93.3	$	109.3

- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-URI http://asc.fasb.org/topic&trid=49130388

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130551-203045

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130556-203045

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 12
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130554-203045

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130550-203045

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 12
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130554-203045

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 12
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130554-203045

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 12
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130554-203045

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130558-203045

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 12
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130554-203045

+ Details

Inventory

3 Months Ended

Mar. 31, 2021

Inventory Disclosure [Abstract]

Inventory

The components of inventory are summarized as follows:


(In millions)	As of March 31, 2021		As of December 31, 2020
Raw materials	$	331.3	$	314.9
Work in process	634.8		544.5
Finished goods	205.7		209.2
Total inventory	$	1,171.8	$	1,068.6

Intangible Assets and Goodwill

3 Months Ended

Mar. 31, 2021

Goodwill and Intangible Assets Disclosure [Abstract]

Intangible Assets and Goodwill

Intangible Assets

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


		As of March 31, 2021						As of December 31, 2020
(In millions)	Estimated Life	Cost		Accumulated Amortization		Net		Cost		Accumulated Amortization		Net
Completed technology	4-28 years	$	7,394.3	$	(5,190.3)	$	2,204.0	$	7,394.3	$	(5,136.5)	$	2,257.8
In-process research and development	Indefinite until commercialization	720.1		—		720.1		762.5		—		762.5
Trademarks and trade names	Indefinite	64.0		—		64.0		64.0		—		64.0
Total intangible assets		$	8,178.4	$	(5,190.3)	$	2,988.1	$	8,220.8	$	(5,136.5)	$	3,084.3

For the three months ended March 31, 2021, amortization and impairment of acquired intangible assets totaled $98.1 million, compared to $71.5 million in the prior year comparative period. For the three months ended March 31, 2021, amortization and impairment of acquired intangible assets reflects a $44.3 million impairment charge related to vixotrigine (BIIB074) for the potential treatment of trigeminal neuralgia (TGN). For the three months ended March 31, 2020, we had no impairment charges.

Completed Technology

Completed technology primarily relates to our acquisition of all remaining rights to TYSABRI 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 March 31, 2021, relates to the various IPR&D programs we acquired in connection with our acquisitions of Nightstar Therapeutics plc (NST) and Convergence Pharmaceuticals Holdings Ltd. (Convergence). The majority of the balance relates to our acquisition of NST in June 2019. For additional information on our acquisition of NST, please read Note 2, Acquisitions, to our consolidated financial statements included in our 2020 Form 10-K.

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 and for the potential treatment of diabetic painful neuropathy (DPN), another form of neuropathic pain. We have engaged with the U.S. Food and Drug Administration (FDA) regarding the design of the Phase 3 studies of vixotrigine for TGN and DPN and now plan to perform an additional clinical trial of vixotrigine before initiating a Phase 3 study of DPN.

The performance of this additional clinical trial has delayed the initiation of the Phase 3 studies of vixotrigine for the potential treatment of TGN, and, as a result, we recognized an impairment charge of $44.3 million related to vixotrigine for the potential treatment of TGN during the first quarter of 2021. As of March 31, 2021, the carrying value associated with our remaining vixotrigine IPR&D assets was $135.1 million, all of which is related to DPN.

BIIB111 and BIIB112

During the fourth quarter of 2020 we recognized an impairment charge of $115.0 million related to BIIB111 (timrepigene emparvovec) for the potential treatment of choroideremia, a rare, degenerative, X-linked inherited retinal disorder that leads to blindness and currently has no approved treatments, as a result of third-party manufacturing delays that may impact our timeline for a potential filing of a Biologics License Application (BLA) for regulatory approval by up to one year, and increase the costs associated with advancing BIIB111 through Phase 3 development.

We are continuing to monitor the manufacturing issues encountered with BIIB111 and their potential impact on that program's timing. As of March 31, 2021, we have no indication that BIIB111 or BIIB112 (cotoretigene toliparvovec) for the potential treatment of X-linked retinitis pigmentosa, which is a rare inherited retinal disease with no currently approved treatments, were impaired; however, we will continue to monitor the capabilities of our manufacturing partners and how any additional new manufacturing issues might affect the planned timing of such gene therapy programs.

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 March 31, 2021
2021 (remaining nine months)	$	165.0
2022	215.0
2023	215.0
2024	225.0
2025	220.0
2026	200.0

Goodwill

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


(In millions)	As of March 31, 2021
Goodwill, beginning of period	$	5,762.1

Other	1.0
Goodwill, end of period	$	5,763.1

As of March 31, 2021, we had no accumulated impairment losses related to goodwill. Other includes adjustments related to foreign currency exchange rate fluctuations.

Fair Value Measurements

3 Months Ended

Mar. 31, 2021

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 March 31, 2021
(In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	745.8	$	—	$	745.8	$	—
Marketable debt securities:
Corporate debt securities	1,336.2		—		1,336.2		—
Government securities	677.4		—		677.4		—
Mortgage and other asset backed securities	128.3		—		128.3		—
Marketable equity securities	1,442.8		148.2		1,294.6		—
Derivative contracts	51.2		—		51.2		—
Plan assets for deferred compensation	31.1		—		31.1		—
Total	$	4,412.8	$	148.2	$	4,264.6	$	—
Liabilities:
Derivative contracts	$	88.6	$	—	$	88.6	$	—
Contingent consideration obligations	226.0		—		—		226.0
Total	$	314.6	$	—	$	88.6	$	226.0


	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

There have been no material impairments of our assets measured and carried at fair value during the three months ended March 31, 2021. In addition, there have been no changes in valuation techniques during the three months ended March 31, 2021. 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 the common stock of Sangamo Therapeutics, Inc. (Sangamo), Denali Therapeutics Inc. (Denali) and Sage Therapeutics, Inc. (Sage) 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 7, Financial Instruments, to these condensed 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 our consolidated financial statements included in our 2020 Form 10-K.

The following tables summarize the significant unobservable inputs in the fair value measurement of our contingent consideration obligations as of March 31, 2021 and December 31, 2020:

As of March 31, 2021

(In millions)

Fair Value

Valuation Technique

Unobservable Input(s)

Range

Weighted Average

Liabilities:

Contingent consideration obligation

226.0

Discounted cash flow

Discount rate

0.77%

Expected timing of achievement of development milestones

2022 to 2027

—

As of December 31, 2020

(In millions)

Fair Value

Valuation Technique

Unobservable Input(s)

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 21.7% to certain probability, to the valuation models to estimate the fair values of our contingent consideration obligations.

Debt Instruments

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


	As of March 31, 2021				As of December 31, 2020
(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value

3.625% Senior Notes due September 15, 2022	$	1,044.2	$	998.2	$	1,054.1	$	997.9
4.050% Senior Notes due September 15, 2025	1,940.8		1,741.5		2,003.1		1,741.2
2.250% Senior Notes due May 1, 2030	1,455.3		1,491.4		1,557.2		1,491.1
5.200% Senior Notes due September 15, 2045 (1)	1,394.8		1,099.7		2,365.1		1,723.4
3.150% Senior Notes due May 1, 2050	1,369.8		1,472.7		1,536.4		1,472.6
3.250% Senior Notes due February 15, 2051 (1)	654.3		463.7		—		—
Total	$	7,859.2	$	7,267.2	$	8,515.9	$	7,426.2

(1) In February 2021 we completed a private offer to exchange (Exchange Offer) our 5.200% Senior Notes due September 15, 2045 (2045 Senior Notes), whereby approximately $624.6 million of our 2045 Senior Notes were exchanged for approximately $700.7 million of a new series of 3.250% Senior Notes due February 15, 2051 (2051 Senior Notes). For additional information on our Exchange Offer, please read Note 10, Indebtedness, to these condensed 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 issued on April 30, 2020 and September 15, 2015, please read Note 12, Indebtedness, to our consolidated financial statements included in our 2020 Form 10-K.

Contingent Consideration Obligations

In connection with our acquisitions of Convergence and Biogen International Neuroscience GmbH, 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:


	For the Three Months Ended March 31,
(In millions)	2021		2020
Fair value, beginning of period	$	259.8	$	346.1
Changes in fair value	(33.8)		(4.6)

Fair value, end of period	$	226.0	$	341.5

As of March 31, 2021 and December 31, 2020, approximately $226.0 million and $110.3 million, respectively, of the fair value of our total contingent consideration obligations was reflected as a component of other long-term liabilities in our condensed consolidated balance sheets with the remaining balance reflected as a component of accrued expense and other.

For the three months ended March 31, 2021, changes in the fair value of our contingent consideration obligations were primarily due to delays in the expected timing of the achievement of certain remaining developmental milestones related to our vixotrigine programs.

- Definition

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

3 Months Ended

Mar. 31, 2021

Investments, Debt and Equity Securities [Abstract]

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 condensed consolidated balance sheets:


(In millions)	As of March 31, 2021		As of December 31, 2020
Commercial paper	$	50.7	$	61.1
Overnight reverse repurchase agreements	93.7		37.4
Money market funds	541.5		505.1
Short-term debt securities	59.9		23.3
Total	$	745.8	$	626.9

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 condensed consolidated statements of income. The following tables summarize our marketable debt and equity securities, classified as available for sale:


	As of March 31, 2021
(In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Marketable debt securities
Corporate debt securities:
Current	$	979.7	$	0.2	$	(0.4)	$	979.5
Non-current	356.0		0.8		(0.1)		356.7
Government securities:
Current	340.5		—		—		340.5
Non-current	336.5		0.4		—		336.9
Mortgage and other asset backed securities:
Current	—		—		—		—
Non-current	128.2		0.2		(0.1)		128.3
Total marketable debt securities	$	2,140.9	$	1.6	$	(0.6)	$	2,141.9

Marketable equity securities

Marketable equity securities, non-current	$	1,168.9	$	351.0	$	(77.1)	$	1,442.8
Total marketable equity securities	$	1,168.9	$	351.0	$	(77.1)	$	1,442.8


	As of December 31, 2020
(In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Marketable debt securities
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
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
Total marketable equity securities	$	1,239.5	$	749.7	$	(14.9)	$	1,974.3

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 March 31, 2021				As of December 31, 2020
(In millions)	Estimated Fair Value		Amortized Cost		Estimated Fair Value		Amortized Cost
Due in one year or less	$	1,320.0	$	1,320.2	$	1,278.9	$	1,278.6
Due after one year through five years	781.2		780.4		722.6		721.3
Due after five years	40.7		40.3		49.5		49.2
Total marketable debt securities	$	2,141.9	$	2,140.9	$	2,051.0	$	2,049.1

The average maturity of our marketable debt securities available-for-sale as of March 31, 2021 and December 31, 2020, was approximately 12 months and 11 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 Three Months Ended March 31,
(In millions)	2021		2020
Proceeds from maturities and sales	$	819.2	$	2,389.3
Realized gains	0.2		5.7
Realized losses	(0.7)		(19.1)

Strategic Investments

As of March 31, 2021 and December 31, 2020, our strategic investment portfolio was comprised of investments totaling $1,493.1 million and $2,024.6 million, respectively, which are included in investments and other assets in our condensed 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 6, Fair Value Measurements, to these condensed consolidated financial statements, venture capital funds where the underlying investments are in equity securities of certain biotechnology companies and non-marketable equity securities.

The decrease in our strategic investment portfolio for the three months ended March 31, 2021, was primarily due to decreases in the fair values of our investments in Ionis Pharmaceuticals Inc. (Ionis), Sage, Denali and Sangamo common stock.

Sage Therapeutics, Inc.

In November 2020 we entered into a global collaboration and license agreement with Sage. In connection with 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. 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 arrangement with Sage, please read Note 16, Collaborative and Other Relationships, to these condensed 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 $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. 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's 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 arrangement with Denali, please read Note 16, Collaborative and Other Relationships, to these condensed 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 $225.0 million of Sangamo common stock, or approximately 24 million shares at approximately $9.21 per share, which are subject to transfer restrictions. 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’s 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 arrangement with Sangamo, please read Note 16, Collaborative and Other Relationships, to these condensed consolidated financial statements.

Derivative Instruments

3 Months Ended

Mar. 31, 2021

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 revenue and operating expense are recorded in currencies other than the U.S. dollar. The value of revenue and operating expense 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 revenue and operating expense.

Foreign currency forward contracts in effect as of March 31, 2021 and December 31, 2020, had durations of 1 to 21 months and 1 to 24 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 tables below). Realized gains and losses of such contracts are recognized in revenue when the sale of product in the currency being hedged is recognized and in operating expense 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 condensed 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 revenue and operating expense is summarized as follows:


	Notional Amount
(In millions)	As of March 31, 2021		As of December 31, 2020
Euro	$	2,657.6	$	2,979.1
British pound	190.4		250.6
Swiss franc	159.4		—
Japanese yen	98.5		—
Canadian dollar	80.5		—
Total foreign currency forward contracts	$	3,186.4	$	3,229.7

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 as of March 31, 2021, reflected aggregate net unrealized losses of $41.5 million, composed of gross unrealized losses of approximately $73.5 million and gross unrealized gains of approximately $32.0 million, compared to aggregate net unrealized losses of $212.5 million as of December 31, 2020. We expect the net unrealized losses of $41.5 million to be settled over the next 21 months, of which $54.3 million of unrealized 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 revenue or operating expense. 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 March 31, 2021 and December 31, 2020, 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 condensed consolidated statements of income:


For the Three Months Ended March 31,
Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)					Net Gains/(Losses) Recognized in Operating Income (in millions)
Location	2021		2020		Location	2021		2020
Revenue	$	(23.1)	$	27.0	Revenue	$	(3.0)	$	9.3
Operating expense	(0.4)		(0.1)		Operating expense	(0.1)		(0.9)

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 originally 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, 2020, 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, which was recorded as a component of interest expense in our condensed consolidated statements of income during the second quarter of 2020.

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 the 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 March 31, 2021, had a remaining duration of seven 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 gains of $2.7 million and net losses of $21.2 million as of March 31, 2021 and December 31, 2020, 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 condensed 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 $0.1 million and $0.2 million as of March 31, 2021 and December 31, 2020, respectively.

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


For the Three Months Ended March 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	2021		2020		Location	2021		2020		Location	2021		2020
Gains (losses) on net investment hedge	$	23.8	$	24.2	Gains (losses) on net investment hedge	$	(1.4)	$	(0.3)	Other income (expense)	$	0.1	$	0.9

For additional information on our collaboration arrangements with Samsung Bioepis, please read Note 16, Collaborative and Other Relationships, to these condensed 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 forward contracts was $1,358.8 million and $1,158.0 million as of March 31, 2021 and December 31, 2020, respectively. Net losses of $17.4 million and $2.4 million related to these contracts were recorded as a component of other income (expense), net for the three months ended March 31, 2021 and 2020, 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 condensed 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 condensed consolidated balance sheets of our outstanding derivative instruments, including those designated as hedging instruments:


(In millions)	Balance Sheet Location	As of March 31, 2021		As of December 31, 2020
Cash Flow Hedging Instruments:
Asset derivative instruments	Other current assets	$	20.6	$	—
	Investments and other assets	15.2		—
Liability derivative instruments	Accrued expense and other	60.8		157.1
	Other long-term liabilities	—		35.7

Net Investment Hedging Instruments:
Asset derivative instruments	Other current assets	2.7		—
Liability derivative instruments	Accrued expense and other	—		19.7





Other Derivative Instruments:
Asset derivative instruments	Other current assets	12.7		20.5
Liability derivative instruments	Accrued expense and other	27.8		4.7

Property, Plant and Equipment Property, Plant and Equipment	3 Months Ended
	Mar. 31, 2021
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, plant and equipment are recorded at historical cost, net of accumulated depreciation. Accumulated depreciation on property, plant and equipment was $1,827.6 million and $1,782.3 million as of March 31, 2021 and December 31, 2020, respectively. For the three months ended March 31, 2021, depreciation expense totaled $48.8 million compared to $48.4 million in the prior year comparative period. 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 March 31, 2021 and December 31, 2020, we had approximately $1.9 billion and $1.8 billion, respectively, capitalized as construction in progress related to this facility.

Indebtedness (Notes)	3 Months Ended
Indebtedness (Notes)	Mar. 31, 2021
Debt Disclosure [Abstract]
Indebtedness	Indebtedness Exchange Offer In February 2021 we completed our Exchange Offer of our tendered 2045 Senior Notes for our 2051 Senior Notes and cash, and an offer to purchase our tendered 2045 Senior Notes for cash. An aggregate principal amount of approximately $624.6 million of our 2045 Senior Notes was exchanged for an aggregate principal amount of approximately $700.7 million of our 2051 Senior Notes and aggregate cash payments of approximately $151.8 million. Our Exchange Offer has been accounted for as a debt modification; as such, the cash component has been reflected as additional debt discount and is amortized as an adjustment to interest expense over the term of our 2051 Senior Notes. In addition, we redeemed an aggregate principal amount of approximately $8.9 million of our 2045 Senior Notes for aggregate cash payments of approximately $12.1 million, excluding accrued and unpaid interest. The redemption has been accounted for as a debt extinguishment; as such, we recognized a pre-tax charge of $3.2 million upon the extinguishment of such 2045 Senior Notes. This charge, which was recognized in interest expense in other income (expense), net in our condensed consolidated statements of income for the three months ended March 31, 2021, reflects the payment of an early call premium and the write-off of the remaining unamortized original debt issuance costs and discount balances associated with such 2045 Senior Notes. Upon settlement, we also made aggregate cash payments of approximately $13.8 million to settle all accrued and unpaid interest from the last interest payment date on our 2045 Senior Notes that were exchanged or redeemed. We incurred approximately $6.1 million of costs associated with our Exchange Offer, which was recognized in interest expense in other income (expense), net in our condensed consolidated statements of income for the three months ended March 31, 2021. 2020 Senior Notes On April 30, 2020, we issued senior unsecured notes for an aggregate principal amount of $3.0 billion (2020 Senior Notes), 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. 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 condensed consolidated balance sheet. For additional information on our 2020 Senior Notes, please read Note 12, Indebtedness, to our consolidated financial statements included in our 2020 Form 10-K. 2.90% Senior Notes due September 15, 2020 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 during the second quarter of 2020. This charge, which was recognized in interest expense in other income (expense), net in our condensed consolidated statements of income and 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 swap contracts, please read Note 8, Derivative Instruments, to these condensed consolidated financial statements.

Equity

3 Months Ended

Mar. 31, 2021

Equity [Abstract]

Equity

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 2.2 million shares of our common stock at a cost of approximately $600.0 million during the three months ended March 31, 2021. Approximately $4.0 billion remained available under our 2020 Share Repurchase Program as of March 31, 2021.

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 3.2 million shares of our common stock at a cost of approximately $941.1 million during the three months ended March 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 shares of our common stock at a cost of approximately $1.3 billion during the three months ended March 31, 2020.

Accumulated Other Comprehensive Income (Loss)

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


(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		Unfunded Status of Postretirement Benefit Plans, Net of Tax		Currency Translation Adjustments		Total
Balance, December 31, 2020	$	1.4	$	(179.0)	$	(8.5)	$	(66.3)	$	(46.6)	$	(299.0)
Other comprehensive income (loss) before reclassifications	(1.2)		128.6		22.4		2.0		(48.5)		103.3
Amounts reclassified from accumulated other comprehensive income (loss)	0.4		21.0		—		—		—		21.4
Net current period other comprehensive income (loss)	(0.8)		149.6		22.4		2.0		(48.5)		124.7
Balance, March 31, 2021	$	0.6	$	(29.4)	$	13.9	$	(64.3)	$	(95.1)	$	(174.3)


(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		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	(20.8)		60.7		23.9		0.8		(63.9)		0.7
Amounts reclassified from accumulated other comprehensive income (loss)	13.0		(26.9)		(0.9)		—		—		(14.8)
Net current period other comprehensive income (loss)	(7.8)		33.8		23.0		0.8		(63.9)		(14.1)
Balance, March 31, 2020	$	(3.6)	$	41.6	$	48.1	$	(32.0)	$	(203.4)	$	(149.3)

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


(In millions)	Income Statement Location	Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)
		For the Three Months Ended March 31,
		2021		2020
Gains (losses) on securities available for sale	Other income (expense)	$	(0.5)	$	(16.4)
	Income tax benefit (expense)	0.1		3.4

Gains (losses) on cash flow hedges	Revenue	(23.1)		27.0
	Operating expense	(0.4)		(0.1)
	Other income (expense)	0.2		0.1
	Income tax benefit (expense)	2.3		(0.1)

Gains (losses) on net investment hedge	Other income (expense)	—		0.9


Total reclassifications, net of tax		$	(21.4)	$	14.8

Earnings per Share

3 Months Ended

Mar. 31, 2021

Earnings Per Share [Abstract]

Earnings per Share

Basic and diluted earnings per share are calculated as follows:


	For the Three Months Ended March 31,
(In millions)	2021		2020
Numerator:
Net income attributable to Biogen Inc.	$	410.2	$	1,399.1
Denominator:
Weighted average number of common shares outstanding	151.9		172.8
Effect of dilutive securities:

Time-vested restricted stock units	0.2		0.1
Market stock units	0.1		0.1
Performance stock units settled in stock	0.1		0.1
Dilutive potential common shares	0.4		0.3
Shares used in calculating diluted earnings per share	152.3		173.1

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

Share-based Payments

3 Months Ended

Mar. 31, 2021

Share-based Payment Arrangement [Abstract]

Share-based Payments

Share-based Compensation Expense

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


	For the Three Months Ended March 31,
(In millions)	2021		2020
Research and development	$	33.6	$	33.3
Selling, general and administrative	44.9		43.1
Subtotal	78.5		76.4
Capitalized share-based compensation costs	(2.6)		(1.5)
Share-based compensation expense included in total cost and expense	75.9		74.9
Income tax effect	(14.0)		(13.3)
Share-based compensation expense included in net income attributable to Biogen Inc.	$	61.9	$	61.6

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


	For the Three Months Ended March 31,
(In millions)	2021		2020
Market stock units	$	16.5	$	19.1
Time-vested restricted stock units	42.8		37.5
Cash settled performance units	—		(1.6)
Performance units	—		(0.1)
Performance stock units settled in stock	6.3		10.3
Performance stock units settled in cash	6.0		8.9
Employee stock purchase plan	6.9		2.3

Subtotal	78.5		76.4
Capitalized share-based compensation costs	(2.6)		(1.5)
Share-based compensation expense included in total cost and expense	$	75.9	$	74.9

We estimate the fair value of our obligations associated with our performance stock units settled in cash at the end of each reporting period through expected settlement. Cumulative adjustments to these obligations are recognized each quarter to reflect changes in the stock price and estimated outcome of the performance-related conditions.

Income Taxes

3 Months Ended

Mar. 31, 2021

Income Tax Disclosure [Abstract]

Income Taxes

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 18, Litigation, to these condensed 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 has significantly reduced our TECFIDERA revenue and is expected to have a substantial negative impact on our TECFIDERA revenue for as long as there is generic competition.

As of December 31, 2020, we assessed the realizability of our deferred tax assets that are dependent on future expected sales of TECFIDERA in the U.S. and reduced the value of certain deferred tax assets by

approximately $1.7 billion and reduced the value of deferred tax liabilities associated with global intangible low-taxed income (GILTI) and tax credits by approximately $1.6 billion. We continue to assess the realizability of these deferred tax assets and have not recorded a change to these deferred assets for the three months ended March 31, 2021.

Tax Rate

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


	For the Three Months Ended March 31,
	2021		2020
Statutory rate	21.0	%	21.0	%
State taxes	0.8		0.6
Taxes on foreign earnings	(10.9)		(4.1)
Tax credits	(3.7)		(1.0)
Purchased intangible assets	0.8		0.2



GILTI	0.9		0.6



Other	0.6		(0.1)
Effective tax rate	9.5	%	17.2	%

Changes in Tax Rate

For the three months ended March 31, 2021, compared to the same period in 2020, the decrease in our effective tax rate was primarily due to the change in the territorial mix of our profitability, which included the effect of generic competition for TECFIDERA in the U.S. market, and the non-cash tax effects of changes in the value of our equity investments, where we recorded a reduction of value in the first quarter of 2021. The tax effects of this change in value of our equity investments were recorded discretely, since changes in value of equity investments cannot be forecasted.

Accounting for Uncertainty in Income Taxes

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.

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.

- Definition

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

3 Months Ended

Mar. 31, 2021

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Other Consolidated Financial Statement Detail

Other Income (Expense), Net

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


	For the Three Months Ended March 31,
(In millions)	2021		2020
Interest income	$	2.9	$	24.4
Interest expense	(64.7)		(44.3)
Gain (loss) on investments, net	(436.6)		(77.3)
Foreign exchange gains (losses), net	(8.6)		(18.9)
Other, net	0.1		(4.4)
Total other income (expense), net	$	(506.9)	$	(120.5)

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 three months ended March 31, 2021, net unrealized losses and realized gains on our holdings in equity securities were approximately $442.3 million and $6.2 million, respectively, compared to net unrealized and realized losses of $60.9 million and zero, respectively, in the prior year comparative period. The net unrealized losses recognized during the three months ended March 31, 2021, primarily reflect a decrease in the fair value of Ionis, Sangamo, Denali and Sage common stock of approximately $442.0 million.

The following table summarizes our gain (loss) on investments, net that relates to our equity securities held as of March 31, 2021 and 2020:


	For the Three Months Ended March 31,
(In millions)	2021		2020
Net gains (losses) recognized during the period on equity securities	$	(436.1)	$	(60.9)
Less: Net gains (losses) recognized during the period on equity securities sold during the period	6.2		—
Unrealized gains (losses) recognized during the period on equity securities	$	(442.3)	$	(60.9)

Accrued Expense and Other

Accrued expense and other consists of the following:


(In millions)	As of March 31, 2021		As of December 31, 2020
Revenue-related reserves for discounts and allowances	$	993.0	$	1,080.6
Collaboration expense	390.7		389.9
Royalties and licensing fees	212.5		218.5
Employee compensation and benefits	207.4		333.8
Derivative liabilities	88.6		181.5
Current portion of contingent consideration obligations	—		149.6
Other	700.1		791.4
Total accrued expense and other	$	2,592.3	$	3,145.3

Other Long-term Liabilities

Other long-term liabilities were $1,411.8 million and $1,329.6 million as of March 31, 2021 and December 31, 2020, respectively, and included accrued income taxes totaling $725.2 million and $709.9 million, respectively.

Collaborative and Other Relationships

3 Months Ended

Mar. 31, 2021

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Collaborative and Other Relationships

Eisai Co., Ltd.

BAN2401 Collaboration

We have a collaboration agreement with Eisai Co., Ltd. (Eisai) to jointly develop and commercialize BAN2401 (lecanemab), 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 expense, 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, an anti-amyloid beta antibody candidate for the potential treatment of Alzheimer's disease (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 expense related to the BAN2401 Collaboration is as follows:


	For the Three Months Ended March 31,
(In millions)	2021		2020
Total development expense incurred by the collaboration related to the advancement of BAN2401 and elenbecestat	$	55.5	$	43.5
Biogen's share of BAN2401 and elenbecestat development expense reflected in research and development expense in our condensed consolidated statements of income	27.7		21.8
Total sales and marketing expense incurred by the BAN2401 collaboration	5.7		4.9
Biogen's share of BAN2401 and elenbecestat sales and marketing expense reflected in selling, general and administrative expense in our condensed consolidated statements of income	2.9		2.4

For additional information on our BAN2401 Collaboration, please read Note 18, Collaborative and Other Relationships, to our consolidated financial statements included in our 2020 Form 10-K.

Aducanumab Collaboration Agreement

Under the Aducanumab Collaboration Agreement, we lead the ongoing development of aducanumab and, if approved, we and Eisai will co-promote aducanumab with a region-based profit split. Beginning January 1, 2019, Eisai is reimbursing us for 45.0% of development costs incurred by the collaboration for the advancement of aducanumab (aducanumab development expense).

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 trial 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. 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.

Sales and marketing expense are shared in proportion to the same region-based profit split that will be utilized to co-promote aducanumab. A summary of development and sales and marketing expense related to the Aducanumab Collaboration Agreement is as follows:


	For the Three Months Ended March 31,
(In millions)	2021		2020
Total aducanumab development expense	$	47.0	$	19.1
Biogen's share of aducanumab development expense reflected in research and development expense in our condensed consolidated statements of income	25.8		10.5
Total aducanumab sales and marketing expense incurred by the Aducanumab Collaboration Agreement	111.8		22.7
Biogen's share of aducanumab sales and marketing expense reflected in selling, general and administrative expense in our condensed consolidated statements of income	60.3		12.3

For additional information on the Aducanumab Collaboration Agreement, please read Note 18, Collaborative and Other Relationships, to our consolidated financial statements included in our 2020 Form 10-K.

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 expense, 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 Three Months Ended March 31,
(In millions)	2021		2020
Total UCB collaboration development expense	$	16.9	$	19.0
Biogen's share of UCB development expense reflected in research and development expense in our condensed consolidated statements of income	8.4		9.5

Sage Therapeutics, Inc.

In November 2020 we entered into a collaboration and license agreement with Sage to jointly develop and commercialize BIIB125 (zuranolone) for the potential treatment of major depressive disorder and postpartum depression and BIIB124 (SAGE-324) for the potential treatment of essential tremor with potential in other neurological conditions such as epilepsy.

In connection with 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 condensed 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 condensed 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.

A summary of development and sales and marketing expense related to this collaboration is as follows:


	For the Three Months Ended March 31,
(In millions)	2021
Total Sage collaboration development expense	$	39.8
Biogen's share of Sage development expense reflected in research and development expense in our condensed consolidated statements of income	19.9
Total Sage sales and marketing expense incurred by the collaboration	5.3
Biogen's share of Sage sales and marketing expense reflected in selling, general and administrative expense in our condensed consolidated statements of income	2.7

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 the 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 condensed 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 condensed 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 LRRK2 are achieved. Under this collaboration, 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.

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 with the aim 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 condensed 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 condensed 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 may 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.

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. 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. As of March 31, 2021, 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 condensed 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.

Certain officers and affiliates of our joint venture partner, Samsung BioLogics, are currently subject to an 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 three months ended March 31, 2021 and 2020, we recognized net losses on our investment of $18.2 million and $14.8 million, respectively, reflecting our share of Samsung Bioepis' operating results and amortization of basis differences, net of tax.

As of March 31, 2021 and December 31, 2020, the carrying value of our investment in Samsung Bioepis totaled 653.4 billion South Korean won ($576.3 million) and 673.8 billion South Korean won ($620.2 million), respectively, which is classified as a component of investments and other assets in our condensed consolidated balance sheets.

2019 Development and Commercialization Agreement

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 condensed 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

We reflect revenue on sales of BENEPALI, IMRALDI and FLIXABI to third parties in product revenue, net in our condensed consolidated statements of income and record the related cost of revenue and sales and marketing expense in our condensed consolidated statements of income to their respective line items when these costs are incurred.

We share 50.0% of the profit or loss related to our commercial agreement with Samsung Bioepis, which is recognized in collaboration profit (loss) sharing in our condensed consolidated statements of income. For the three months ended March 31, 2021, we recognized net profit-sharing expense of $68.5 million to reflect Samsung Bioepis' 50.0% sharing of the net collaboration profits, compared to $71.8 million in the prior year comparative period. As discussed above, we have an option to extend this agreement by an additional five years, subject to the payment of an option exercise fee of $60.0 million.

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. Revenue related to these services are reflected in revenue from collaborative and other relationships as a component of other revenue in our condensed consolidated statements of income.

Amounts payable to Samsung Bioepis related to the agreements discussed above were $188.4 million and $99.0 million as of March 31, 2021 and December 31, 2020, respectively.

For additional information on our collaboration arrangements with Samsung Bioepis and our other significant collaboration arrangements, please read Note 18, Collaborative and Other Relationships, to our consolidated financial statements included in our 2020 Form 10-K.

Investments in Variable Interest Entities	3 Months Ended
Investments in Variable Interest Entities	Mar. 31, 2021
Investments in Variable Interest Entities [Abstract]
Investments in Variable Interest Entities	Consolidated Variable Interest Entities Our condensed 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. In addition, during the second quarter of 2020, we recognized net profit-sharing income of $33.8 million to reflect Eisai's 45.0% share of the $75.0 million milestone payment. Additionally, if aducanumab receives regulatory approval in the jurisdictions where we have submitted filings, we may pay up to approximately $150.0 million in milestones to Neurimmune in 2021, which includes $100.0 million if launched in the U.S. 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 condensed consolidated statements of income. During the three months ended March 31, 2021 and 2020, amounts reimbursed were immaterial. The assets and liabilities of Neurimmune are not significant to our condensed 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 16, Collaborative and Other Relationships, to these condensed 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 March 31, 2021 and December 31, 2020, the carrying value of our investments in certain biotechnology companies representing potential unconsolidated variable interest entities totaled $12.8 million. 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 condensed consolidated statements of income as they are incurred. We have provided no financing to these variable interest entities other than previously contractually required amounts. For additional information on our investments in Neurimmune and other variable interest entities, please read Note 19, Investments in Variable Interest Entities, to our consolidated financial statements included in our 2020 Form 10-K.

Litigation	3 Months Ended
Litigation	Mar. 31, 2021
Loss Contingency, Information about Litigation Matters [Abstract]
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 our consolidated financial statements included in our 2020 Form 10-K. 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 transferred to the U.S. District Court for the District of Massachusetts in March 2021. The action alleges violations of federal securities laws under 15 U.S.C §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5 and seeks 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. A second class action, filed by another shareholder on January 5, 2021, in the U.S. District Court for the District of Massachusetts, was dismissed in March 2021. 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. The request for preliminary injunction was denied in June 2019 and the decision was affirmed on appeal in February 2021. 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 commenced proceedings against Biogen GmbH in the Düsseldorf Regional Court alleging infringement of the German counterpart of European Patent No. 3 212 667, 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. Samsung BioLogics Arbitration In December 2020 we requested arbitration in the International Chamber of Commerce Court of International Arbitration against Samsung BioLogics seeking interpretation of certain provisions in the Joint Venture Agreement executed on December 6, 2011, as amended, by and between Biogen and Samsung BioLogics (the Joint Venture Agreement). Samsung BioLogics has asserted counterclaims, including breach of the Joint Venture Agreement, and seeks declaratory relief and unspecified damages. An estimate of the possible loss or range of loss cannot be made at this time. No hearing date has been set. 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. Hatch-Waxman Act Litigation relating to VUMERITY Orange-Book Listed Patents In March 2021 Biogen and Alkermes Pharma Ireland Limited filed patent infringement proceedings relating to VUMERITY Orange-Book listed patents (U.S. Patent Nos. 8,699,281, 9,090,558 and 10,080,733) pursuant to the Hatch-Waxman Act in the Delaware Court against Teva Pharmaceuticals Development, Inc. A trial date has not yet been set. 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 A/S' (Forward Pharma) 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. The Polish Patent Office dismissed the action in February 2021. In August 2020 a related entity, Polpharma Biologics S.A., also brought an action seeking to revoke the Polish ‘263 Patent in the Polish Patent Office. The action was suspended by the Polish Patent Office in April 2021. 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 and the court will announce its decision in that case on May 5, 2021. 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.

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Overview	Biogen is a global biopharmaceutical company 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 core growth areas include multiple sclerosis (MS) and neuroimmunology; Alzheimer’s disease and dementia; neuromuscular disorders, including spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS); movement disorders, including Parkinson's disease; ophthalmology; and neuropsychiatry. We are also focused on discovering, developing and delivering worldwide innovative therapies in our emerging growth areas of immunology; acute neurology; and neuropathic pain. In addition, we commercialize biosimilars of advanced biologics. We support our drug discovery and development efforts through the commitment of significant resources to discovery, research and development programs and business development opportunities. Our marketed products include TECFIDERA, VUMERITY, AVONEX, PLEGRIDY, TYSABRI and FAMPYRA for the treatment of MS; SPINRAZA for the treatment of SMA; and FUMADERM for the treatment of severe plaque psoriasis. We have certain business and financial rights with respect to RITUXAN for the treatment of non-Hodgkin's lymphoma, chronic lymphocytic leukemia (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 primary progressive MS and relapsing MS; and other potential anti-CD20 therapies pursuant to our collaboration arrangements with Genentech, Inc. (Genentech), a wholly-owned member of the Roche Group. For additional information on our collaboration arrangements with Genentech, please read Note 18, Collaborative and Other Relationships, to our consolidated financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2020 (2020 Form 10-K). Our innovative drug development and commercialization activities are complemented by our biosimilar business that expands access to medicines and reduces the cost burden for healthcare systems. Through our agreements with Samsung Bioepis Co., Ltd. (Samsung Bioepis), our joint venture with Samsung BioLogics Co., Ltd. (Samsung BioLogics), we market and sell BENEPALI, an etanercept biosimilar referencing ENBREL, IMRALDI, an adalimumab biosimilar referencing HUMIRA, and FLIXABI, an infliximab biosimilar referencing REMICADE, in certain countries in Europe and have an option to acquire exclusive rights to commercialize these products in China. Additionally, we have 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 United States (U.S.), Canada, Europe, Japan and Australia. For additional information on our collaboration arrangements with Samsung Bioepis, please read Note 16, Collaborative and Other Relationships, to these unaudited condensed consolidated financial statements (condensed consolidated financial statements).
Basis of presentation	In the opinion of management, our condensed consolidated financial statements include all adjustments, consisting of normal recurring accruals, necessary for a fair statement of our financial statements for interim periods in accordance with accounting principles generally accepted in the United States (U.S. GAAP). The information included in this quarterly report on Form 10-Q should be read in conjunction with our audited consolidated financial statements and the accompanying notes included in our 2020 Form 10-K. Our accounting policies are described in the Notes to Consolidated Financial Statements in our 2020 Form 10-K and updated, as necessary, in this report. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from our audited financial statements, but does not include all disclosures required by U.S. GAAP. The results of operations for the three months ended March 31, 2021, are not necessarily indicative of the operating results for the full year or for any other subsequent interim period. 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.
Consolidation	Our condensed consolidated financial statements reflect our financial statements, those of our wholly-owned subsidiaries and those of certain variable interest entities where we are the primary beneficiary. For consolidated entities where we own or are exposed to less than 100.0% of the economics, we record net income (loss) attributable to noncontrolling interests in our condensed consolidated statements of income equal to the percentage of the economic or ownership interest retained in such entities by the respective noncontrolling parties. Intercompany balances and transactions are eliminated in consolidation. In determining whether we are the primary beneficiary of a variable interest entity, we apply a qualitative approach that determines whether we have both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. These considerations impact the way we account for our existing collaborative relationships and other arrangements. We continuously assess whether we are the primary beneficiary of a variable interest entity as changes to existing relationships or future transactions may result in us consolidating or deconsolidating one or more of our collaborators or partners.
Use of estimates	The preparation of our condensed consolidated financial statements requires us to make estimates, judgments and assumptions that may affect the reported amounts of assets, liabilities, equity, revenue and expense and related disclosure of contingent assets and liabilities. On an ongoing basis we evaluate our estimates, judgments and methodologies. We base our estimates on historical experience and on various other assumptions that we believe are reasonable, the results of which form the basis for making judgments about the carrying values of assets, liabilities and equity and the amount of revenue and expense. Actual results may differ from these estimates.The length of time and full extent to which the COVID-19 pandemic directly or indirectly impacts our business, results of operations and financial condition, including sales, expense, reserves and allowances, manufacturing, clinical trials, research and development costs and employee-related amounts, depends on future developments that are highly uncertain, subject to change and are difficult to predict, including as a result of new information that may emerge concerning COVID-19 and the actions taken to contain or treat COVID-19 as well as the economic impact on local, regional, national and international customers and markets. We have made estimates of the impact of the COVID-19 pandemic within our condensed consolidated financial statements and there may be changes to those estimates in future periods.
New accounting pronouncements	From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (FASB) or other standard setting bodies that we adopt as of the specified effective date. Unless otherwise discussed below, we do not believe that the adoption of recently issued standards have or may have a material impact on our condensed consolidated financial statements or disclosures. Income Taxes In December 2019 the FASB issued Accounting Standards Update No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. This standard removes certain exceptions to the general principles in Topic 740 and simplifies certain other aspects of the accounting for income taxes. This standard became effective for us on January 1, 2021, and did not have a material impact on our condensed consolidated financial statements and related disclosures.

Revenues (Tables)

3 Months Ended

Mar. 31, 2021

Revenue from Contract with Customer [Abstract]

Revenues by product

Revenue by product is summarized as follows:


	For the Three Months Ended March 31,
	2021						2020
(In millions)	United States		Rest of World		Total		United States		Rest of World		Total
Multiple Sclerosis (MS):
Fumarate*	$	236.0	$	316.9	$	552.9	$	777.5	$	323.3	$	1,100.8
Interferon**	241.8		158.7		400.5		292.6		173.4		466.0
TYSABRI	273.3		230.0		503.3		277.7		244.7		522.4
FAMPYRA	—		26.6		26.6		—		28.3		28.3
Subtotal: MS product revenue	751.1		732.2		1,483.3		1,347.8		769.7		2,117.5

Spinal Muscular Atrophy:
SPINRAZA	148.7		371.8		520.5		235.4		329.6		565.0

Biosimilars:
BENEPALI	—		121.7		121.7		—		133.5		133.5
IMRALDI	—		57.9		57.9		—		61.6		61.6
FLIXABI	—		25.5		25.5		—		23.7		23.7
Subtotal: Biosimilar product revenue	—		205.1		205.1		—		218.8		218.8

Other:
FUMADERM	—		2.8		2.8		—		3.3		3.3
Total product revenue	$	899.8	$	1,311.9	$	2,211.7	$	1,583.2	$	1,321.4	$	2,904.6

*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:


(In millions)	Discounts		Contractual Adjustments		Returns		Total
Balance, December 31, 2020	$	141.4	$	1,093.0	$	41.6	$	1,276.0
Current provisions relating to sales in current year	185.1		775.7		3.8		964.6
Adjustments relating to prior years	0.9		(32.1)		1.0		(30.2)
Payments/credits relating to sales in current year	(111.8)		(284.8)		—		(396.6)
Payments/credits relating to sales in prior years	(82.8)		(562.9)		(1.7)		(647.4)
Balance, March 31, 2021	$	132.8	$	988.9	$	44.7	$	1,166.4

Total reserves included in consolidated balance sheets

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


(In millions)	As of March 31, 2021		As of December 31, 2020
Reduction of accounts receivable	$	173.4	$	195.4
Component of accrued expense and other	993.0		1,080.6
Total revenue-related reserves	$	1,166.4	$	1,276.0

Revenues from anti-CD20 therapeutic programs

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


	For the Three Months Ended March 31,
(In millions)	2021		2020
Biogen’s share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	$	174.1	$	341.3
Other revenue from anti-CD20 therapeutic programs	214.9		179.1
Total revenue from anti-CD20 therapeutic programs	$	389.0	$	520.4

Other revenues

Other revenue is summarized as follows:


	For the Three Months Ended March 31,
(In millions)	2021		2020
Revenue from collaborative and other relationships:
Revenue earned under our technical development agreement, manufacturing services agreements and royalty revenue on biosimilar products with Samsung Bioepis	$	3.9	$	3.7
Other revenue from collaborative and other relationships	—		0.2
Other royalty and corporate revenue:
Royalty	6.2		11.4
Other corporate	83.2		94.0
Total other revenue	$	93.3	$	109.3

Inventory (Tables)

3 Months Ended

Mar. 31, 2021

Inventory Disclosure [Abstract]

Components of inventory

The components of inventory are summarized as follows:


(In millions)	As of March 31, 2021		As of December 31, 2020
Raw materials	$	331.3	$	314.9
Work in process	634.8		544.5
Finished goods	205.7		209.2
Total inventory	$	1,171.8	$	1,068.6

Intangible Assets and Goodwill (Tables)

3 Months Ended

Mar. 31, 2021

Goodwill and Intangible Assets Disclosure [Abstract]

Intangible assets

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


		As of March 31, 2021						As of December 31, 2020
(In millions)	Estimated Life	Cost		Accumulated Amortization		Net		Cost		Accumulated Amortization		Net
Completed technology	4-28 years	$	7,394.3	$	(5,190.3)	$	2,204.0	$	7,394.3	$	(5,136.5)	$	2,257.8
In-process research and development	Indefinite until commercialization	720.1		—		720.1		762.5		—		762.5
Trademarks and trade names	Indefinite	64.0		—		64.0		64.0		—		64.0
Total intangible assets		$	8,178.4	$	(5,190.3)	$	2,988.1	$	8,220.8	$	(5,136.5)	$	3,084.3

Estimated future amortization for acquired 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 March 31, 2021
2021 (remaining nine months)	$	165.0
2022	215.0
2023	215.0
2024	225.0
2025	220.0
2026	200.0

Summary of roll forward of the changes in goodwill

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


(In millions)	As of March 31, 2021
Goodwill, beginning of period	$	5,762.1

Other	1.0
Goodwill, end of period	$	5,763.1

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2021

Fair Value Disclosures [Abstract]

Summary of assets and liabilities recorded at fair value


	As of March 31, 2021
(In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	745.8	$	—	$	745.8	$	—
Marketable debt securities:
Corporate debt securities	1,336.2		—		1,336.2		—
Government securities	677.4		—		677.4		—
Mortgage and other asset backed securities	128.3		—		128.3		—
Marketable equity securities	1,442.8		148.2		1,294.6		—
Derivative contracts	51.2		—		51.2		—
Plan assets for deferred compensation	31.1		—		31.1		—
Total	$	4,412.8	$	148.2	$	4,264.6	$	—
Liabilities:
Derivative contracts	$	88.6	$	—	$	88.6	$	—
Contingent consideration obligations	226.0		—		—		226.0
Total	$	314.6	$	—	$	88.6	$	226.0


	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

Summary of fair and carrying value of debt instruments

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


	As of March 31, 2021				As of December 31, 2020
(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value

3.625% Senior Notes due September 15, 2022	$	1,044.2	$	998.2	$	1,054.1	$	997.9
4.050% Senior Notes due September 15, 2025	1,940.8		1,741.5		2,003.1		1,741.2
2.250% Senior Notes due May 1, 2030	1,455.3		1,491.4		1,557.2		1,491.1
5.200% Senior Notes due September 15, 2045 (1)	1,394.8		1,099.7		2,365.1		1,723.4
3.150% Senior Notes due May 1, 2050	1,369.8		1,472.7		1,536.4		1,472.6
3.250% Senior Notes due February 15, 2051 (1)	654.3		463.7		—		—
Total	$	7,859.2	$	7,267.2	$	8,515.9	$	7,426.2

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:


	For the Three Months Ended March 31,
(In millions)	2021		2020
Fair value, beginning of period	$	259.8	$	346.1
Changes in fair value	(33.8)		(4.6)

Fair value, end of period	$	226.0	$	341.5

Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2021

Investments, Debt and Equity Securities [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 condensed consolidated balance sheets:


(In millions)	As of March 31, 2021		As of December 31, 2020
Commercial paper	$	50.7	$	61.1
Overnight reverse repurchase agreements	93.7		37.4
Money market funds	541.5		505.1
Short-term debt securities	59.9		23.3
Total	$	745.8	$	626.9

Marketable debt and equity securities

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


	As of March 31, 2021
(In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Marketable debt securities
Corporate debt securities:
Current	$	979.7	$	0.2	$	(0.4)	$	979.5
Non-current	356.0		0.8		(0.1)		356.7
Government securities:
Current	340.5		—		—		340.5
Non-current	336.5		0.4		—		336.9
Mortgage and other asset backed securities:
Current	—		—		—		—
Non-current	128.2		0.2		(0.1)		128.3
Total marketable debt securities	$	2,140.9	$	1.6	$	(0.6)	$	2,141.9

Marketable equity securities

Marketable equity securities, non-current	$	1,168.9	$	351.0	$	(77.1)	$	1,442.8
Total marketable equity securities	$	1,168.9	$	351.0	$	(77.1)	$	1,442.8


	As of December 31, 2020
(In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Marketable debt securities
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
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
Total marketable equity securities	$	1,239.5	$	749.7	$	(14.9)	$	1,974.3

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 March 31, 2021				As of December 31, 2020
(In millions)	Estimated Fair Value		Amortized Cost		Estimated Fair Value		Amortized Cost
Due in one year or less	$	1,320.0	$	1,320.2	$	1,278.9	$	1,278.6
Due after one year through five years	781.2		780.4		722.6		721.3
Due after five years	40.7		40.3		49.5		49.2
Total marketable debt securities	$	2,141.9	$	2,140.9	$	2,051.0	$	2,049.1

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 Three Months Ended March 31,
(In millions)	2021		2020
Proceeds from maturities and sales	$	819.2	$	2,389.3
Realized gains	0.2		5.7
Realized losses	(0.7)		(19.1)

Derivative Instruments (Tables)

3 Months Ended

Mar. 31, 2021

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 revenue and operating expense is summarized as follows:


	Notional Amount
(In millions)	As of March 31, 2021		As of December 31, 2020
Euro	$	2,657.6	$	2,979.1
British pound	190.4		250.6
Swiss franc	159.4		—
Japanese yen	98.5		—
Canadian dollar	80.5		—
Total foreign currency forward contracts	$	3,186.4	$	3,229.7

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

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


For the Three Months Ended March 31,
Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)					Net Gains/(Losses) Recognized in Operating Income (in millions)
Location	2021		2020		Location	2021		2020
Revenue	$	(23.1)	$	27.0	Revenue	$	(3.0)	$	9.3
Operating expense	(0.4)		(0.1)		Operating expense	(0.1)		(0.9)

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 hedge in our condensed consolidated financial statements:


For the Three Months Ended March 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	2021		2020		Location	2021		2020		Location	2021		2020
Gains (losses) on net investment hedge	$	23.8	$	24.2	Gains (losses) on net investment hedge	$	(1.4)	$	(0.3)	Other income (expense)	$	0.1	$	0.9

Summary of fair value and presentation of derivatives


(In millions)	Balance Sheet Location	As of March 31, 2021		As of December 31, 2020
Cash Flow Hedging Instruments:
Asset derivative instruments	Other current assets	$	20.6	$	—
	Investments and other assets	15.2		—
Liability derivative instruments	Accrued expense and other	60.8		157.1
	Other long-term liabilities	—		35.7

Net Investment Hedging Instruments:
Asset derivative instruments	Other current assets	2.7		—
Liability derivative instruments	Accrued expense and other	—		19.7





Other Derivative Instruments:
Asset derivative instruments	Other current assets	12.7		20.5
Liability derivative instruments	Accrued expense and other	27.8		4.7

Accumulated Other Comprehensive Income (Loss) (Tables)

3 Months Ended

Mar. 31, 2021

Equity [Abstract]

Schedule of Accumulated Other Comprehensive Income (Loss)

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


(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		Unfunded Status of Postretirement Benefit Plans, Net of Tax		Currency Translation Adjustments		Total
Balance, December 31, 2020	$	1.4	$	(179.0)	$	(8.5)	$	(66.3)	$	(46.6)	$	(299.0)
Other comprehensive income (loss) before reclassifications	(1.2)		128.6		22.4		2.0		(48.5)		103.3
Amounts reclassified from accumulated other comprehensive income (loss)	0.4		21.0		—		—		—		21.4
Net current period other comprehensive income (loss)	(0.8)		149.6		22.4		2.0		(48.5)		124.7
Balance, March 31, 2021	$	0.6	$	(29.4)	$	13.9	$	(64.3)	$	(95.1)	$	(174.3)


(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		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	(20.8)		60.7		23.9		0.8		(63.9)		0.7
Amounts reclassified from accumulated other comprehensive income (loss)	13.0		(26.9)		(0.9)		—		—		(14.8)
Net current period other comprehensive income (loss)	(7.8)		33.8		23.0		0.8		(63.9)		(14.1)
Balance, March 31, 2020	$	(3.6)	$	41.6	$	48.1	$	(32.0)	$	(203.4)	$	(149.3)

Reclassification out of Accumulated Other Comprehensive Income

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


(In millions)	Income Statement Location	Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)
		For the Three Months Ended March 31,
		2021		2020
Gains (losses) on securities available for sale	Other income (expense)	$	(0.5)	$	(16.4)
	Income tax benefit (expense)	0.1		3.4

Gains (losses) on cash flow hedges	Revenue	(23.1)		27.0
	Operating expense	(0.4)		(0.1)
	Other income (expense)	0.2		0.1
	Income tax benefit (expense)	2.3		(0.1)

Gains (losses) on net investment hedge	Other income (expense)	—		0.9


Total reclassifications, net of tax		$	(21.4)	$	14.8

Earnings per Share (Tables)

3 Months Ended

Mar. 31, 2021

Earnings Per Share [Abstract]

Basic and diluted earnings per share

Basic and diluted earnings per share are calculated as follows:


	For the Three Months Ended March 31,
(In millions)	2021		2020
Numerator:
Net income attributable to Biogen Inc.	$	410.2	$	1,399.1
Denominator:
Weighted average number of common shares outstanding	151.9		172.8
Effect of dilutive securities:

Time-vested restricted stock units	0.2		0.1
Market stock units	0.1		0.1
Performance stock units settled in stock	0.1		0.1
Dilutive potential common shares	0.4		0.3
Shares used in calculating diluted earnings per share	152.3		173.1

Share-Based Payments (Tables)

3 Months Ended

Mar. 31, 2021

Share-based Payment Arrangement [Abstract]

Share-based compensation expense included in condensed consolidated statements of income

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


	For the Three Months Ended March 31,
(In millions)	2021		2020
Research and development	$	33.6	$	33.3
Selling, general and administrative	44.9		43.1
Subtotal	78.5		76.4
Capitalized share-based compensation costs	(2.6)		(1.5)
Share-based compensation expense included in total cost and expense	75.9		74.9
Income tax effect	(14.0)		(13.3)
Share-based compensation expense included in net income attributable to Biogen Inc.	$	61.9	$	61.6

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 Three Months Ended March 31,
(In millions)	2021		2020
Market stock units	$	16.5	$	19.1
Time-vested restricted stock units	42.8		37.5
Cash settled performance units	—		(1.6)
Performance units	—		(0.1)
Performance stock units settled in stock	6.3		10.3
Performance stock units settled in cash	6.0		8.9
Employee stock purchase plan	6.9		2.3

Subtotal	78.5		76.4
Capitalized share-based compensation costs	(2.6)		(1.5)
Share-based compensation expense included in total cost and expense	$	75.9	$	74.9

Income Taxes (Tables)

3 Months Ended

Mar. 31, 2021

Income Tax Disclosure [Abstract]

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 Three Months Ended March 31,
	2021		2020
Statutory rate	21.0	%	21.0	%
State taxes	0.8		0.6
Taxes on foreign earnings	(10.9)		(4.1)
Tax credits	(3.7)		(1.0)
Purchased intangible assets	0.8		0.2



GILTI	0.9		0.6



Other	0.6		(0.1)
Effective tax rate	9.5	%	17.2	%

Other Consolidated Financial Statement Detail (Tables)

3 Months Ended

Mar. 31, 2021

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Other income (expense), net

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


	For the Three Months Ended March 31,
(In millions)	2021		2020
Interest income	$	2.9	$	24.4
Interest expense	(64.7)		(44.3)
Gain (loss) on investments, net	(436.6)		(77.3)
Foreign exchange gains (losses), net	(8.6)		(18.9)
Other, net	0.1		(4.4)
Total other income (expense), net	$	(506.9)	$	(120.5)

Gain (loss) on investments in equity securities

The following table summarizes our gain (loss) on investments, net that relates to our equity securities held as of March 31, 2021 and 2020:


	For the Three Months Ended March 31,
(In millions)	2021		2020
Net gains (losses) recognized during the period on equity securities	$	(436.1)	$	(60.9)
Less: Net gains (losses) recognized during the period on equity securities sold during the period	6.2		—
Unrealized gains (losses) recognized during the period on equity securities	$	(442.3)	$	(60.9)

Accrued expenses and other

Accrued expense and other consists of the following:


(In millions)	As of March 31, 2021		As of December 31, 2020
Revenue-related reserves for discounts and allowances	$	993.0	$	1,080.6
Collaboration expense	390.7		389.9
Royalties and licensing fees	212.5		218.5
Employee compensation and benefits	207.4		333.8
Derivative liabilities	88.6		181.5
Current portion of contingent consideration obligations	—		149.6
Other	700.1		791.4
Total accrued expense and other	$	2,592.3	$	3,145.3

Collaborative and Other Relationships (Tables)

3 Months Ended

Mar. 31, 2021

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Summary of Activity Related to BAN2401 and Elenbecestat Collaboration


	For the Three Months Ended March 31,
(In millions)	2021		2020
Total development expense incurred by the collaboration related to the advancement of BAN2401 and elenbecestat	$	55.5	$	43.5
Biogen's share of BAN2401 and elenbecestat development expense reflected in research and development expense in our condensed consolidated statements of income	27.7		21.8
Total sales and marketing expense incurred by the BAN2401 collaboration	5.7		4.9
Biogen's share of BAN2401 and elenbecestat sales and marketing expense reflected in selling, general and administrative expense in our condensed consolidated statements of income	2.9		2.4

Summary of Activity Related to Aducanumab Collaboration

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


	For the Three Months Ended March 31,
(In millions)	2021		2020
Total aducanumab development expense	$	47.0	$	19.1
Biogen's share of aducanumab development expense reflected in research and development expense in our condensed consolidated statements of income	25.8		10.5
Total aducanumab sales and marketing expense incurred by the Aducanumab Collaboration Agreement	111.8		22.7
Biogen's share of aducanumab sales and marketing expense reflected in selling, general and administrative expense in our condensed consolidated statements of income	60.3		12.3

Summary of Significant Accounting Policies (Details)	3 Months Ended
Summary of Significant Accounting Policies (Details)	Mar. 31, 2021 product segment
Accounting Policies [Abstract]
Number of potential products, exclusive rights \| product	2
Number of reportable segments \| segment	1
Interest in subsidiary (less than given percentage)	100.00%

Business Acquisition (Details Textual) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Business Acquisition [Line Items]
Acquired in-process research and development		$ 0.0	$ 75.0
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

Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended
Revenues by Product (Details) - USD ($) $ in Millions	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Product revenues	$ 2,694.0	$ 3,534.3
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	552.9	1,100.8
Fumarate \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	236.0	777.5
Fumarate \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	316.9	323.3
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	400.5	466.0
Interferon \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	241.8	292.6
Interferon \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	158.7	173.4
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	503.3	522.4
TYSABRI \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	273.3	277.7
TYSABRI \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	230.0	244.7
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	26.6	28.3
FAMPYRA \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0
FAMPYRA \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	26.6	28.3
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,483.3	2,117.5
MS Product Revenues \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	751.1	1,347.8
MS Product Revenues \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	732.2	769.7
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	520.5	565.0
SPINRAZA \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	148.7	235.4
SPINRAZA \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	371.8	329.6
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	121.7	133.5
BENEPALI \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0
BENEPALI \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	121.7	133.5
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	57.9	61.6
IMRALDI \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0
IMRALDI \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	57.9	61.6
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	25.5	23.7
FLIXABI \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0
FLIXABI \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	25.5	23.7
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	205.1	218.8
Biosimilars \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0
Biosimilars \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	205.1	218.8
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	2.8	3.3
FUMADERM \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0
FUMADERM \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	2.8	3.3
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,211.7	2,904.6
Product, net \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	899.8	1,583.2
Product, net \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,311.9	$ 1,321.4

Reserves for Discounts and Allowances (Details 1) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	$ 1,276.0
Current Provisions Relating To Sales In Current Year	964.6
Adjustments Relating To Prior Years	(30.2)
Payments/Returns Relating To Sales in Current Year	(396.6)
Payments/Returns Relating To Sales in Prior Year	(647.4)
Ending Balance	1,166.4
Discounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	141.4
Current Provisions Relating To Sales In Current Year	185.1
Adjustments Relating To Prior Years	0.9
Payments/Returns Relating To Sales in Current Year	(111.8)
Payments/Returns Relating To Sales in Prior Year	(82.8)
Ending Balance	132.8
Contractual adjustments
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	1,093.0
Current Provisions Relating To Sales In Current Year	775.7
Adjustments Relating To Prior Years	(32.1)
Payments/Returns Relating To Sales in Current Year	(284.8)
Payments/Returns Relating To Sales in Prior Year	(562.9)
Ending Balance	988.9
Returns
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	41.6
Current Provisions Relating To Sales In Current Year	3.8
Adjustments Relating To Prior Years	1.0
Payments/Returns Relating To Sales in Current Year	0.0
Payments/Returns Relating To Sales in Prior Year	(1.7)
Ending Balance	$ 44.7

Reserves for Discounts and Allowances (Details 2) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Total Reserves	$ 1,166.4	$ 1,276.0
Reduction of accounts receivable
Total Reserves	173.4	195.4
Component of accrued expenses and other
Total Reserves	$ 993.0	$ 1,080.6

Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 2,694.0	$ 3,534.3
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	174.1	341.3
Other revenues from anti-CD20 therapeutic programs	214.9	179.1
Revenue from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 389.0	$ 520.4

- Definition

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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 40
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9031-108599

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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-Section 50
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-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8906-108599

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 41
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9038-108599

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 42
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9054-108599

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(g)(1)(ii))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 235
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-05(b)(2))
-URI http://asc.fasb.org/extlink&oid=120399901&loc=d3e537907-122884

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(1))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

+ Details

Other Revenues (Details) - USD ($) $ in Millions	3 Months Ended
Other Revenues (Details) - USD ($) $ in Millions	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,694.0	$ 3,534.3
Royalty
Disaggregation of Revenue [Line Items]
Total revenue	6.2	11.4
Other corporate revenues
Disaggregation of Revenue [Line Items]
Total revenue	83.2	94.0
Other
Disaggregation of Revenue [Line Items]
Total revenue	93.3	109.3
Collaborative arrangement \| Samsung Bioepis
Disaggregation of Revenue [Line Items]
Total revenue	0.0	0.2
Collaborative arrangement \| ZINBRYTA
Disaggregation of Revenue [Line Items]
Total revenue	$ 3.9	$ 3.7

- Definition

+ References

+ Details

Revenues (Details Textual) $ in Millions	3 Months Ended
Revenues (Details Textual) $ in Millions	Mar. 31, 2021 USD ($) wholesaler	Mar. 31, 2020 USD ($)
Disaggregation of Revenue [Line Items]
Other Revenues	$ 2,694.0	$ 3,534.3
Number of wholesalers \| wholesaler	2
Distributor One
Disaggregation of Revenue [Line Items]
Percentage of revenues from major distributors	30.00%	30.00%
Distributor Two
Disaggregation of Revenue [Line Items]
Percentage of revenues from major distributors	9.30%	14.60%
Other corporate revenues
Disaggregation of Revenue [Line Items]
Other Revenues	$ 83.2	$ 94.0
Other
Disaggregation of Revenue [Line Items]
Other Revenues	$ 93.3	$ 109.3

- Definition

+ References

+ Details

Inventory (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Components of inventories
Raw materials	$ 331.3	$ 314.9
Work in process	634.8	544.5
Finished goods	205.7	209.2
Total inventory	$ 1,171.8	$ 1,068.6

Intangible Assets and Goodwill (Details) - USD ($)	3 Months Ended
Intangible Assets and Goodwill (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Impairment of Intangible Assets (Excluding Goodwill)	$ 0		$ 0
Intangible assets
Contingent consideration impairment	33,800,000		4,600,000
Total intangible assets, gross	8,178,400,000	$ 8,220,800,000
Accumulated Amortization	(5,190,300,000)	(5,136,500,000)
Intangible assets, net	2,988,100,000	3,084,300,000
Amortization and impairment of acquired intangible assets	98,100,000		71,500,000
Expected future amortization expense, 2019 (remaining three months)	165,000,000.0
Expected future amortization expense, 2020	215,000,000.0
Expected future amortization expense, 2021	215,000,000.0
Expected future amortization expense, 2022	225,000,000.0
Expected future amortization expense, 2023	220,000,000.0
Expected future amortization expense, 2024	200,000,000.0
Out-licensed patents
Intangible assets
Cost	7,394,300,000	7,394,300,000
Net	2,204,000,000.0	2,257,800,000
Accumulated Amortization	$ 5,190,300,000	5,136,500,000
Out-licensed patents \| Minimum
Intangible assets
Estimated life, (in years)	13 years
Out-licensed patents \| Maximum
Intangible assets
Estimated life, (in years)	23 years
Developed technology \| Minimum
Intangible assets
Estimated life, (in years)	15 years
Developed technology \| Maximum
Intangible assets
Estimated life, (in years)	28 years
Acquired and in-licensed rights and patents \| Minimum
Intangible assets
Estimated life, (in years)	4 years
Acquired and in-licensed rights and patents \| Maximum
Intangible assets
Estimated life, (in years)	18 years
In Process Research and Development
Intangible assets
Indefinite lived intangible assets useful life	Indefinite until commercialization
Cost and Net	$ 720,100,000	762,500,000
Accumulated Amortization	$ 0	0
Trademarks and Trade Names
Intangible assets
Indefinite lived intangible assets useful life	Indefinite
Cost and Net	$ 64,000,000.0	64,000,000.0
Accumulated Amortization	0	0
TGN [Member] \| In Process Research and Development
Intangible assets
Cost and Net	135,100,000
Vixotrigine
Intangible assets
Amortization and impairment of acquired intangible assets	$ 44,300,000		$ 0
BIIB111 \| Nightstar
Intangible assets
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)		$ 115,000,000.0

Intangible Assets and Goodwill (Details 1) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Summary of roll forward of the changes in goodwill
Goodwill, beginning of period	$ 5,762.1
Other	1.0
Goodwill, end of period	5,763.1
Accumulated impairment losses related to goodwill	0.0
Amortization and impairment of acquired intangible assets	$ 98.1	$ 71.5

Fair Value Measurements (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Assets:
Marketable debt securities	$ 2,141.9	$ 2,051.0
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	745.8	626.9
Marketable equity securities	1,442.8	1,974.3
Derivative contracts	51.2	20.5
Plan assets for deferred compensation	31.1	28.2
Total	4,412.8	4,700.9
Liabilities:
Derivative contracts	88.6	217.2
Contingent consideration obligations	226.0	259.8	$ 341.5	$ 346.1
Total	314.6	477.0
Fair Value, Measurements, Recurring \| Corporate debt securities
Assets:
Marketable debt securities	1,336.2	1,301.5
Fair Value, Measurements, Recurring \| Government securities
Assets:
Marketable debt securities	677.4	627.1
Fair Value, Measurements, Recurring \| Mortgage and other asset backed securities
Assets:
Marketable debt securities	128.3	122.4
Quoted Prices in Active Markets (Level 1) \| Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0.0	0.0
Marketable equity securities	148.2	271.1
Derivative contracts	0.0	0.0
Plan assets for deferred compensation	0.0	0.0
Total	148.2	271.1
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:
Marketable debt securities	0.0	0.0
Quoted Prices in Active Markets (Level 1) \| Fair Value, Measurements, Recurring \| Government securities
Assets:
Marketable debt securities	0.0	0.0
Quoted Prices in Active Markets (Level 1) \| Fair Value, Measurements, Recurring \| Mortgage and other asset backed securities
Assets:
Marketable debt securities	0.0	0.0
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring
Assets:
Cash equivalents	745.8	626.9
Marketable equity securities	1,294.6	1,703.2
Derivative contracts	51.2	20.5
Plan assets for deferred compensation	31.1	28.2
Total	4,264.6	4,429.8
Liabilities:
Derivative contracts	88.6	217.2
Contingent consideration obligations	0.0	0.0
Total	88.6	217.2
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Corporate debt securities
Assets:
Marketable debt securities	1,336.2	1,301.5
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Government securities
Assets:
Marketable debt securities	677.4	627.1
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Mortgage and other asset backed securities
Assets:
Marketable debt securities	128.3	122.4
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	226.0	259.8
Total	226.0	259.8
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Corporate debt securities
Assets:
Marketable debt securities	0.0	0.0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Government securities
Assets:
Marketable debt securities	0.0	0.0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Mortgage and other asset backed securities
Assets:
Marketable debt securities	$ 0.0	$ 0.0

Fair Value Measurements (Details Textual) - USD ($)	3 Months Ended
Fair Value Measurements (Details Textual) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Business Acquisition, Contingent Consideration [Line Items]
Asset impairment charges	$ 0
Contingent consideration obligations	$ 226,000,000.0	$ 110,300,000
Technological and Regulatory Success, Probability	21.70%
2.900% Senior Notes due 2020
Business Acquisition, Contingent Consideration [Line Items]
Senior notes interest rate	2.90%
3.625% Senior Notes due 2022
Business Acquisition, Contingent Consideration [Line Items]
Senior notes interest rate	3.625%
4.050% Senior Notes due 2025
Business Acquisition, Contingent Consideration [Line Items]
Senior notes interest rate	4.05%
5.200% Senior Notes due 2045
Business Acquisition, Contingent Consideration [Line Items]
Senior notes interest rate	5.20%

Fair Value Measurements Fair Value Measurements - Contingent Consideration (Details) - Discount rate	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate	0.77	0.0060
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate	0.0083
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate	0.0118
Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate	0.0077	0.0060

Fair Value Measurements (Details 1) - USD ($)	1 Months Ended
Fair Value Measurements (Details 1) - USD ($)	Feb. 28, 2021	Mar. 31, 2021	Dec. 31, 2020	Apr. 30, 2020
Debt Instrument [Line Items]
Notes payable, carrying value		$ 7,267,200,000	$ 7,426,200,000
Debt instruments, fair value		$ 7,859,200,000	8,515,900,000
2.900% Senior Notes due 2020
Debt Instrument [Line Items]
Senior notes interest rate		2.90%
3.625% Senior Notes due 2022
Debt Instrument [Line Items]
Notes payable, fair value		$ 1,044,200,000	1,054,100,000
Notes payable, carrying value		$ 998,200,000	997,900,000
Senior notes interest rate		3.625%
4.050% Senior Notes due 2025
Debt Instrument [Line Items]
Notes payable, fair value		$ 1,940,800,000	2,003,100,000
Notes payable, carrying value		$ 1,741,500,000	1,741,200,000
Senior notes interest rate		4.05%
2.250% Senior Notes due May 1, 2030
Debt Instrument [Line Items]
Notes payable, fair value		$ 1,455,300,000	1,557,200,000
Notes payable, carrying value		$ 1,491,400,000	1,491,100,000
Senior notes interest rate		2.25%
5.200% Senior Notes due 2045
Debt Instrument [Line Items]
Notes payable, fair value		$ 1,394,800,000	2,365,100,000
Notes payable, carrying value		$ 1,099,700,000	1,723,400,000
Senior notes interest rate		5.20%
Long Term Debt, Exchanged, Amount	$ 624,600,000
3.150% Senior Notes due May 1, 2050
Debt Instrument [Line Items]
Notes payable, fair value		$ 1,369,800,000	1,536,400,000
Notes payable, carrying value		$ 1,472,700,000	1,472,600,000
Senior notes interest rate		3.15%
3.250% Senior Notes, Due February 15, 2051
Debt Instrument [Line Items]
Notes payable, fair value		$ 654,300,000	0
Notes payable, carrying value		$ 463,700,000	$ 0
Senior notes interest rate		3.25%
Long Term Debt, Exchanged, Amount	$ 700,700,000
Senior Notes
Debt Instrument [Line Items]
Face amount				$ 3,000,000,000.0
Senior Notes \| 2.250% Senior Notes due May 1, 2030
Debt Instrument [Line Items]
Senior notes interest rate				2.25%
Face amount				$ 1,500,000,000
Senior Notes \| 3.150% Senior Notes due May 1, 2050
Debt Instrument [Line Items]
Senior notes interest rate				3.15%
Face amount				$ 1,500,000,000

Fair Value Measurements (Details 2) - USD ($) $ in Millions	3 Months Ended
Fair Value Measurements (Details 2) - USD ($) $ in Millions	Mar. 31, 2021	Mar. 31, 2020
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration impairment	$ 33.8	$ 4.6
Fair Value, Measurements, Recurring
Business Acquisition, Contingent Consideration [Line Items]
Fair value, beginning of period	259.8	346.1
Fair value, end of period	$ 226.0	$ 341.5

Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 745.8	$ 626.9
Commercial paper
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	50.7	61.1
Overnight reverse repurchase agreements
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	93.7	37.4
Money market funds
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	541.5	505.1
Short-term debt securities
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 59.9	$ 23.3

Financial Instruments (Details 1) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 2,140.9	$ 2,049.1
Gross unrealized gains	1.6	2.3
Gross unrealized losses	(0.6)	(0.4)
Fair value	2,141.9	2,051.0
Marketable equity securities, fair value	2,141.9	2,051.0
Equity Securities, FV-NI, Cost		1,239.5
Equity Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		749.7
Equity Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, Before Tax		(14.9)
Equity Securities, FV-NI		1,974.3
Corporate debt securities Current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	979.7	897.8
Gross unrealized gains	0.2	0.4
Gross unrealized losses	(0.4)	(0.2)
Fair value	979.5	898.0
Corporate debt securities Non-current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	356.0	402.5
Gross unrealized gains	0.8	1.1
Gross unrealized losses	(0.1)	(0.1)
Fair value	356.7	403.5
Government securities Current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	340.5	380.6
Gross unrealized gains	0.0	0.1
Gross unrealized losses	0.0	0.0
Fair value	340.5	380.7
Government securities Non-current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	336.5	245.9
Gross unrealized gains	0.4	0.5
Gross unrealized losses	0.0	0.0
Fair value	336.9	246.4
Mortgage and other asset backed securities Current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	0.0	0.2
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Fair value	0.0	0.2
Mortgage and other asset backed securities Non-current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	128.2	122.1
Gross unrealized gains	0.2	0.2
Gross unrealized losses	(0.1)	(0.1)
Fair value	128.3	122.2
Equity Securities, Current
Debt Securities, Available-for-sale [Line Items]
Equity Securities, FV-NI, Cost	1,168.9	70.6
Equity Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	351.0	15.9
Equity Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, Before Tax	(77.1)	0.0
Equity Securities, FV-NI	1,442.8	86.5
Equity Securities, Non-Current
Debt Securities, Available-for-sale [Line Items]
Equity Securities, FV-NI, Cost	1,168.9	1,168.9
Equity Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	351.0	733.8
Equity Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, Before Tax	(77.1)	(14.9)
Equity Securities, FV-NI	$ 1,442.8	$ 1,887.8

Financial Instruments (Details 2) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, amortized cost	$ 1,320.0	$ 1,278.9
Due in one year or less, estimated fair value	1,320.2	1,278.6
Due after one year through five years, amortized cost	781.2	722.6
Due after one year through five years, estimated fair value	780.4	721.3
Due after five years, amortized cost	40.7	49.5
Due after five years, estimated fair value	40.3	49.2
Amortized cost	2,140.9	2,049.1
Available-for-sale Securities	$ 2,141.9	$ 2,051.0

Financial Instruments (Details 3) - USD ($) $ in Millions	3 Months Ended
Financial Instruments (Details 3) - USD ($) $ in Millions	Mar. 31, 2021	Mar. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Proceeds from maturities and sales	$ 819.2	$ 2,389.3
Realized gains	0.2	5.7
Realized losses	$ (0.7)	$ (19.1)

Financial Instruments (Details Textual)	3 Months Ended	6 Months Ended
Financial Instruments (Details Textual)	Mar. 31, 2021	Jun. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Average maturity of marketable securities, months	12 months	11 months

Financial Instruments Financial Instruments (Details Textual 2) - USD ($) $ / shares in Units, shares in Millions, number in Millions, $ in Millions	1 Months Ended			3 Months Ended
	Nov. 30, 2020	Apr. 30, 2020	Feb. 29, 2020	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Business Acquisition [Line Items]
Proceeds from sales of strategic investments				$ 91.2	$ 0.5
Sage Therapeutics Inc.
Business Acquisition [Line Items]
Global Licensing Collaboration Agreement, Amount, Shares Purchased	$ 650.0
Global Licensing Collaboration Agreement, Shares, Purchased	6.2
Global Licensing Collaboration Agreement, Purchase Price Per Share	$ 104.14
Dividend yield percentage				0.00%
Sangamo Therapeutics, Inc. Agreement
Business Acquisition [Line Items]
Global Licensing Collaboration Agreement, Amount, Shares Purchased			$ 225.0
Global Licensing Collaboration Agreement, Shares, Purchased			24.0
Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio				$ 1,493.1
Other noncurrent assets \| Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio						$ 2,024.6
Denali Therapeutics
Business Acquisition [Line Items]
Investment in common stock, shares purchased				13.0
Dividend yield percentage				0.00%
Sangamo
Business Acquisition [Line Items]
Dividend yield percentage		0.00%

Derivative Instruments (Details) $ in Millions, ₩ in Billions	1 Months Ended		3 Months Ended			6 Months Ended	9 Months Ended
	Nov. 30, 2018 USD ($)	Nov. 30, 2018 KRW (₩)	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2020	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Derivatives [Line Items]
Gain/Loss on fair value of foreign currency forward contracts			$ 41.5					$ (212.5)
Net gains (losses) of other income (expense) related to foreign currency forward contracts			(17.4)	$ (2.4)
Unrealized Gross Losses on Foreign Currency Derivatives, Net, before Tax			73.5
Unrealized Gross Gains on Foreign Currency Derivatives, Net, before Tax			32.0
Gains (losses) on net investment hedges			$ 22.4	23.0
Minimum
Derivatives [Line Items]
Range of durations of foreign currency forward contracts			1 month			1 month
Maximum
Derivatives [Line Items]
Range of durations of foreign currency forward contracts			21 months			24 months
Not designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount			$ 1,358.8					1,158.0
Other current assets \| Designated as hedging instrument
Derivatives [Line Items]
Derivative asset, fair value			20.6					0.0
Other current assets \| Not designated as hedging instrument
Derivatives [Line Items]
Derivative asset, fair value			12.7					20.5
Other noncurrent assets \| Designated as hedging instrument
Derivatives [Line Items]
Derivative asset, fair value			15.2					0.0
Accrued expenses and other \| Designated as hedging instrument
Derivatives [Line Items]
Derivative liability, fair value			60.8					157.1
Accrued expenses and other \| Not designated as hedging instrument
Derivatives [Line Items]
Derivative liability, fair value			27.8					4.7
Foreign exchange contract \| Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount			3,186.4					3,229.7
Interest rate swap \| Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount			675.0
Cash Flow Hedging [Member] \| Foreign exchange contract \| Other long-term liabilities
Derivatives [Line Items]
Derivative liability, fair value			$ 0.0					35.7
Fair Value Hedging [Member] \| Interest rate swap \| Accrued expenses and other
Derivatives [Line Items]
Derivative liability, fair value								2.3
Net Investment Hedging
Derivatives [Line Items]
Remaining duration of Net Investment Hedges			7 months
Unrealized gain (loss) on net investment hedges in AOCI			$ 2.7				$ 21.2
Derivative qualifying as net investment hedge, excluded component			0.1					0.2
Gains (losses) on net investment hedges			23.8	24.2
Gains (losses) on net investment hedge, excluded component			(1.4)	(0.3)
Net Investment Hedging \| Foreign exchange contract \| Other current assets
Derivatives [Line Items]
Derivative asset, fair value			2.7					0.0
Net Investment Hedging \| Foreign exchange contract \| Accrued expenses and other
Derivatives [Line Items]
Derivative liability, fair value			0.0					19.7
Interest Expense \| Interest rate swap
Derivatives [Line Items]
Gain on interest rate swap			3.3
Revenue \| Foreign exchange contract
Derivatives [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax			(23.1)	27.0
Revenue \| Cash flows, revenue \| Foreign exchange contract
Derivatives [Line Items]
Gain on interest rate swap			(3.0)	9.3
Operating expense \| Foreign exchange contract
Derivatives [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax			(0.4)	(0.1)
Operating expense \| Cash flows, operating expenses \| Foreign exchange contract
Derivatives [Line Items]
Gain on interest rate swap			(0.1)	(0.9)
Other income (expense) \| Net Investment Hedging
Derivatives [Line Items]
Other Comprehensive Income (Loss), Derivative, Excluded Component, Increase (Decrease), before Adjustments and Tax			0.1	$ 0.9
Euro \| Foreign exchange contract \| Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount			2,657.6					2,979.1
British pound \| Foreign exchange contract \| Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount			98.5					0.0
Swiss franc \| Foreign exchange contract \| Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount			190.4					250.6
Japan, Yen \| Foreign exchange contract \| Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount			80.5					0.0
Canadian dollar \| Foreign exchange contract \| Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount			$ 159.4					$ 0.0
2.900% Senior Notes due 2020
Derivatives [Line Items]
Senior notes interest rate			2.90%
Samsung Bioepis
Derivatives [Line Items]
Percentage of stake in entity				5.00%
Equity Method Investment, Ownership Percentage			49.90%
Payments to acquire additional investment in equity method investment	$ 676.6	₩ 759.5			$ 676.6
Short-term derivative
Derivatives [Line Items]
Gain/Loss on fair value of foreign currency forward contracts			$ 54.3
Range of durations of foreign currency forward contracts			12 months

Property, Plant and Equipment (Details) $ in Millions	3 Months Ended
Property, Plant and Equipment (Details) $ in Millions	Mar. 31, 2021 USD ($) ft²	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Property, Plant and Equipment [Line Items]
Accumulated depreciation \| $	$ 1,827.6		$ 1,782.3
Depreciation \| $	$ 48.8	$ 48.4
Biologics Manufacturing
Property, Plant and Equipment [Line Items]
Number of square feet \| ft²	393,000
Warehouse, Utilities and Support Space
Property, Plant and Equipment [Line Items]
Number of square feet \| ft²	290,000
Administrative Space
Property, Plant and Equipment [Line Items]
Number of square feet \| ft²	51,000
Solothurn, Switzerland
Property, Plant and Equipment [Line Items]
Construction in progress \| $	$ 1,900.0		$ 1,800.0

Indebtedness (Details) - USD ($)	1 Months Ended	3 Months Ended
Indebtedness (Details) - USD ($)	Feb. 28, 2021	Mar. 31, 2021	Mar. 31, 2020	Apr. 30, 2020	Sep. 15, 2015
Debt Instrument [Line Items]
Payments of financing costs		$ 24,400,000
Repayments of Senior Debt	$ 151,800,000
Repayments of borrowings and premiums paid on debt exchange		169,300,000	$ 0
Interest Expense		$ 64,700,000	$ 44,300,000
2.250% Senior Notes due May 1, 2030
Debt Instrument [Line Items]
Senior notes interest rate		2.25%
3.150% Senior Notes due May 1, 2050
Debt Instrument [Line Items]
Senior notes interest rate		3.15%
5.200% Senior Notes due 2045
Debt Instrument [Line Items]
Senior notes interest rate		5.20%
Repayments of Senior Debt	12,100,000
Debt Instrument, Redemption, Amount	8,900,000
Long Term Debt, Exchanged, Amount	624,600,000
Gain (Loss) on Extinguishment of Debt		$ 3,200,000
Repayments of borrowings and premiums paid on debt exchange		13,800,000
Interest Expense		$ 6,100,000
3.250% Senior Notes, Due February 15, 2051
Debt Instrument [Line Items]
Senior notes interest rate		3.25%
Long Term Debt, Exchanged, Amount	$ 700,700,000
Senior Notes
Debt Instrument [Line Items]
Face amount				$ 3,000,000,000.0
Senior Notes \| 2.250% Senior Notes due May 1, 2030
Debt Instrument [Line Items]
Face amount				$ 1,500,000,000
Senior notes interest rate				2.25%
Percentage par value of senior notes				99.973%
Senior Notes \| 3.150% Senior Notes due May 1, 2050
Debt Instrument [Line Items]
Face amount				$ 1,500,000,000
Senior notes interest rate				3.15%
Percentage par value of senior notes				99.174%
Senior Notes \| 2.90% Senior Notes Due Sept 15, 2020
Debt Instrument [Line Items]
Face amount					$ 1,500,000,000
Senior notes interest rate					2.90%
Percentage par value of senior notes					99.792%
Payment of early call premium and write off of remaining unamortized debt issuance costs		$ 12,700,000
Interest Expense \| Senior Notes \| 2.90% Senior Notes Due Sept 15, 2020
Debt Instrument [Line Items]
Pre-tax charge, payment of early call premium and write off of remaining unamortized debt issuance costs, net of derivative gain		9,400,000
Interest Expense \| Interest rate swap
Debt Instrument [Line Items]
Derivative, gain on derivative		$ 3,300,000

Share Repurchases (Details) - USD ($) shares in Millions	3 Months Ended
Share Repurchases (Details) - USD ($) shares in Millions	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Oct. 31, 2020
Class of Stock [Line Items]
Payments for repurchase of common stock	$ 600,000,000.0	$ 2,220,200,000
December 2019 Share Repurchase Program
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount			$ 5,000,000,000.0
Repurchase of common stock, at cost, shares		3.2
Payments for repurchase of common stock		$ 941,100,000
2019 Share Repurchase Program
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount		$ 5,000,000,000.0
Repurchase of common stock, at cost, shares		4.1
Payments for repurchase of common stock		$ 1,300,000,000
October 2020 Share Repurchase Program
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount				$ 5,000,000,000.0
Repurchase of common stock, at cost, shares	2.2
Payments for repurchase of common stock	$ 600,000,000.0
Amount remaining under 2019 Share Repurchase Program	$ 4,000,000,000.0

Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	$ (299.0)	$ (135.2)
Balance, January 1, 2018	(299.0)	(149.3)
Other comprehensive income (loss), before reclassifications, net of tax	103.3	0.7
Amounts reclassified from accumulated other comprehensive income, net of tax	21.4	(14.8)
Other comprehensive income (loss), net of tax	125.4	(13.5)
Accumulated other comprehensive income (loss), net of tax ending balance	(174.3)	(149.3)
Other comprehensive income (loss), net of tax	124.7	(14.1)
Unrealized gains (losses) on securities available for sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	1.4	4.2
Balance, January 1, 2018	0.6	(3.6)
Other comprehensive income (loss), before reclassifications, net of tax	(1.2)	(20.8)
Amounts reclassified from accumulated other comprehensive income, net of tax	0.4	13.0
Other comprehensive income (loss), net of tax	(0.8)	(7.8)
Accumulated other comprehensive income (loss), net of tax ending balance	0.6	(3.6)
Unrealized gains (losses) on cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	(179.0)	7.8
Balance, January 1, 2018	(29.4)	41.6
Other comprehensive income (loss), before reclassifications, net of tax	128.6	60.7
Amounts reclassified from accumulated other comprehensive income, net of tax	21.0	(26.9)
Other comprehensive income (loss), net of tax	149.6	33.8
Accumulated other comprehensive income (loss), net of tax ending balance	(29.4)	41.6
Gains (losses) on net investment hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	(8.5)	25.1
Balance, January 1, 2018	13.9	48.1
Other comprehensive income (loss), before reclassifications, net of tax	22.4	23.9
Amounts reclassified from accumulated other comprehensive income, net of tax	0.0	(0.9)
Other comprehensive income (loss), net of tax	22.4	23.0
Accumulated other comprehensive income (loss), net of tax ending balance	13.9	48.1
Unfunded status of postretirement benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	(66.3)	(32.8)
Balance, January 1, 2018	(64.3)	(32.0)
Other comprehensive income (loss), before reclassifications, net of tax	2.0	0.8
Amounts reclassified from accumulated other comprehensive income, net of tax	0.0	0.0
Other comprehensive income (loss), net of tax	2.0	0.8
Accumulated other comprehensive income (loss), net of tax ending balance	(64.3)	(32.0)
Currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	(46.6)	(139.5)
Balance, January 1, 2018	(95.1)	(203.4)
Other comprehensive income (loss), before reclassifications, net of tax	(48.5)	(63.9)
Amounts reclassified from accumulated other comprehensive income, net of tax	0.0	0.0
Other comprehensive income (loss), net of tax	(48.5)	(63.9)
Accumulated other comprehensive income (loss), net of tax ending balance	$ (95.1)	$ (203.4)

Reclassification out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Nonoperating Income (Expense)	$ (506.9)	$ (120.5)
Income Tax Expense (Benefit)	44.2	292.0
Product revenues	(2,694.0)	(3,534.3)
Net income attributable to Biogen Inc.	410.2	1,399.1
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income attributable to Biogen Inc.	(21.4)	14.8
Unrealized 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]
Nonoperating Income (Expense)	(0.5)	(16.4)
Income Tax Expense (Benefit)	0.1	3.4
Unrealized 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]
Nonoperating Income (Expense)	0.2	0.1
Income Tax Expense (Benefit)	2.3	(0.1)
Product revenues	23.1	27.0
Operating Expenses	(0.4)	(0.1)
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]
Nonoperating Income (Expense)	$ 0.0	$ 0.9

Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator:
Net income attributable to Biogen Inc.	$ 410.2	$ 1,399.1
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	151.9	172.8
Effect of dilutive securities:
Dilutive potential common shares	0.4	0.3
Shares used in calculating diluted earnings per share	152.3	173.1
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.2	0.1
Market stock units
Effect of dilutive securities:
Stock units	0.1	0.1
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	0.1	0.1

Share-Based Payments (Details) - USD ($) $ in Millions	3 Months Ended
Share-Based Payments (Details) - USD ($) $ in Millions	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	$ (72.6)	$ (69.2)
Subtotal	78.5	76.4
Capitalized share-based compensation costs	(2.6)	(1.5)
Share-based compensation expense included in total costs and expenses	75.9	74.9
Income tax effect	(14.0)	(13.3)
Research and development
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	33.6	33.3
Selling, general and administrative
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	44.9	43.1
Total share-based compensation expense, net of tax
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	$ 61.9	$ 61.6

Share-Based Payments (Details 1) - USD ($) $ in Millions	3 Months Ended
Share-Based Payments (Details 1) - USD ($) $ in Millions	Mar. 31, 2021	Mar. 31, 2020
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	$ (72.6)	$ (69.2)
Subtotal	78.5	76.4
Capitalized share-based compensation costs	(2.6)	(1.5)
Share-based compensation expense included in total costs and expenses	75.9	74.9
Market stock units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	16.5	19.1
Time-vested restricted stock units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	42.8	37.5
Cash settled performance shares
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	0.0	(1.6)
Performance units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	0.0	(0.1)
Performance stock units settled in stock
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	6.3	10.3
Performance stock units settled in cash
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	6.0	8.9
Employee stock purchase plan
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	$ 6.9	$ 2.3

Income Taxes (Details Textual) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Income Tax Contingency [Line Items]
Decrease in unrecognized tax benefits is reasonably possible		$ 25.0
TECFIDERA
Income Tax Contingency [Line Items]
Deferred tax assets, decrease in value	$ 1,700.0
Deferred tax liabilities, decrease in value	$ 1,600.0

Income Taxes (Details)	3 Months Ended
Income Taxes (Details)	Mar. 31, 2021	Mar. 31, 2020
Reconciliation between the U.S. federal statutory tax rate and effective tax rate
Statutory rate	21.00%	21.00%
State taxes	0.80%	0.60%
Taxes on foreign earnings	(10.90%)	(4.10%)
Credits and net operating loss utilization	(3.70%)	(1.00%)
Purchased intangible assets	0.80%	0.20%
GILTI	0.90%	0.60%
Other	0.60%	(0.10%)
Effective tax rate	9.50%	17.20%

Other Consolidated Financial Statement Detail (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Other Income (Expense), Net
Interest income	$ 2.9	$ 24.4
Interest expense	(64.7)	(44.3)
Gain (loss) on investments, net	(436.6)	(77.3)
Foreign exchange gains (losses), net	(8.6)	(18.9)
Other, net	0.1	(4.4)
Total other income (expense), net	(506.9)	(120.5)
Equity Securities, FV-NI, Gain (Loss), Alternative [Abstract]
Net gains (losses) recognized during the period on equity securities	(436.1)	(60.9)
Less: Net gains (losses) recognized during the period on equity securities sold during the period	6.2	0.0
Unrealized gains (losses) recognized during the period on equity securities	(442.3)	$ (60.9)
Accrued Expenses and Other
Revenue-related reserves for discounts and allowances	1,166.4		$ 1,276.0
Collaboration expense	207.4		333.8
Royalties and licensing fees	212.5		218.5
Collaboration expense	390.7		389.9
Current portion of contingent consideration obligations	0.0		149.6
Derivative liabilities	88.6		181.5
Other	700.1		791.4
Total accrued expense and other	2,592.3		3,145.3
Component of accrued expenses and other
Accrued Expenses and Other
Revenue-related reserves for discounts and allowances	$ 993.0		$ 1,080.6

Other Consolidated Financial Statement (Details Textual) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Class of Stock [Line Items]
Gain (loss) on investments, net	$ (436.6)	$ (77.3)
Other long-term liabilities	1,411.8		$ 1,329.6
Accrued income taxes	725.2		$ 709.9
Net gains recognized on the increase in fair value of equity securities	(442.3)	(60.9)
Proceeds from sale of available-for-sale securities, equity	91.2	$ 0.5
Ionis, Sangamo, Denali and Sage
Class of Stock [Line Items]
Net gains recognized on the increase in fair value of equity securities	$ 442.0

Collaborative and Other Relationships - Collaborations (Details) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended		3 Months Ended
	Nov. 30, 2020 USD ($) $ / shares shares	Feb. 29, 2020 USD ($) $ / shares shares	Mar. 31, 2021 USD ($) product $ / shares	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jan. 31, 2020 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Loss on equity method investment			$ (18.2)	$ (16.8)
Eisai
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of product candidates \| product			2
Expense Incurred By Collaboration			$ 55.5	43.5
Expense reflected within statements of income			$ 27.7	21.8
Percentage of future development costs related to Eisai			45.00%
Loss on research and development contracts terminated with Eisai					$ 45.0
Loss on equity method investment			$ 33.8
Denali Therapeutics
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Global Licensing Collaboration Agreement, Amount, Shares Purchased			$ 465.0
Global Licensing Collaboration Agreement, Shares Purchased Per Share \| $ / shares			$ 34.94
Upfront and milestone payments made to collaborative partner			$ 560.0
Global Licensing Collaboration Agreement, Research, Development, Regulatory and Commercial Milestone Payments			1,100.0
Research and development expense			$ 41.3
Samsung Bioepis
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront and milestone payments made to collaborative partner				100.0
Collaboration agreement term			5 years
Research and development expense				63.0
Prepaid research and development expenses						$ 37.0
Sage Therapeutics Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Global Licensing Collaboration Agreement, Amount, Shares Purchased	$ 650.0
Global Licensing Collaboration Agreement, Shares, Purchased \| shares	6.2
Upfront and milestone payments made to collaborative partner	$ 875.0
Global Licensing Collaboration Agreement, Research, Development, Regulatory and Commercial Milestone Payments	1,600.0
Research and development expense	$ 209.0
Global Licensing Collaboration Agreement, Purchase Price Per Share \| $ / shares	$ 104.14
Sangamo Therapeutics, Inc. Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Global Licensing Collaboration Agreement, Amount, Shares Purchased		$ 225.0
Global Licensing Collaboration Agreement, Shares, Purchased \| shares		24.0
Global Licensing Collaboration Agreement, Shares Purchased Per Share \| $ / shares		$ 9.21
Upfront and milestone payments made to collaborative partner		$ 125.0
Global Licensing Collaboration Agreement, Research, Development, Regulatory and Commercial Milestone Payments		2,400.0
Global Licensing Collaboration Agreement, Payment, Selection of Targets		80.0
Global Licensing Collaboration Agreement, Payment, Achievement of First Commercial Sale Milestones		1,900.0
Global Licensing Collaboration Agreement, Payment, Achievement of Specific Sales Based Milestones		380.0
Research and development expense		$ 83.0
Term of collaboration agreement			5 years
Research and development \| Eisai \| Aducanumab
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			$ 47.0	19.1
Expense reflected within statements of income			25.8	10.5
Research and development \| UCB Pharma S.A. [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			16.9	19.0
Expense reflected within statements of income			8.4	9.5
Selling, general and administrative \| Eisai \| BAN2401 and Elenbecestat
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			5.7	4.9
Expense reflected within statements of income			2.9	2.4
Selling, general and administrative \| Eisai \| Aducanumab
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			111.8	22.7
Expense reflected within statements of income			$ 60.3	$ 12.3

Collaborative and Other Relationships - Equity Method Investments (Details) $ in Millions, ₩ in Billions	1 Months Ended		3 Months Ended
	Nov. 30, 2018 USD ($)	Nov. 30, 2018 KRW (₩)	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2021 KRW (₩)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 KRW (₩)	Jan. 31, 2020 USD ($)	Nov. 07, 2018 USD ($)
Schedule of Equity Method Investments [Line Items]
Our share of Samsung Bioepis gains (losses)			$ 18.2	$ 16.8
Collaboration profit (loss) sharing			68.5	71.8
Total revenue			$ 2,694.0	$ 3,534.3
Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Percentage of stake in entity				5.00%
Percentage of stake in entity maximum			49.90%				49.90%
Payments to acquire additional investment in equity method investment	$ 676.6	₩ 759.5				$ 676.6
Amortization of basis differences											$ 675.0
Loss recorded on Samsung Bioepis joint venture			$ 18.2	$ 14.8
Investment in Samsung Bioepis			$ 576.3				₩ 653.4	$ 620.2	₩ 673.8
Biogen share of co-promotion profits or losses			50.00%
Collaboration profit (loss) sharing			$ 68.5	71.8
Eisai
Schedule of Equity Method Investments [Line Items]
Loss on research and development contracts terminated with Eisai					$ 45.0
Our share of Samsung Bioepis gains (losses)			(33.8)
Additional Milestone Payment			75.0
Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Upfront and milestone payments made to collaborative partner				100.0
Research and development expense				63.0
Prepaid research and development expenses										$ 37.0
Estimated additional payments upon achievement of development and commercial milestones			195.0
Additional Milestone Payment			15.0
Contract Option Exercise Fee				$ 60.0
Due to Related Parties			$ 188.4					$ 99.0
Inventory \| Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Equity method investment basis difference amortization period			1 year 6 months
Developed technology \| Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Equity method investment basis difference amortization period			15 years

Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended
	May 31, 2018	Oct. 31, 2017	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Variable Interest Entity [Line Items]
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities					$ 12.8
Loss on equity method investment			$ (18.2)	$ (16.8)
Neurimmune
Variable Interest Entity [Line Items]
Collaboration agreement term			12 years
Research and development costs, percentage			100.00%
Payments to Noncontrolling Interests	$ 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%
Twelve months
Variable Interest Entity [Line Items]
Potential Future Milestone Payments Commitment To Third Party Approximately			$ 150.0
Regulatory Milestones [Member] \| Neurimmune
Variable Interest Entity [Line Items]
Potential Future Milestone Payments Commitment To Third Party Approximately					$ 75.0
Eisai
Variable Interest Entity [Line Items]
Loss on equity method investment			$ 33.8
Percentage of future development costs related to Eisai			45.00%
Aducanumab \| Regulatory Milestones [Member] \| JAPAN
Variable Interest Entity [Line Items]
Potential Future Milestone Payments Commitment To Third Party Approximately			$ 50.0
Neurimmune \| Regulatory Milestones [Member]
Variable Interest Entity [Line Items]
Potential Future Milestone Payments Commitment To Third Party Approximately			$ 100.0

Litigation (Details) $ in Millions	Dec. 31, 2020 USD ($)
Loss Contingency, Information about Litigation Matters [Abstract]
Loss Contingency, Estimate of Possible Loss	$ 200.0