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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2019	Apr. 19, 2019
Document and Entity Information [Abstract]
Entity Registrant Name	BIOGEN INC.
Entity Central Index Key	0000875045
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Document Type	10-Q
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Common Stock, Shares Outstanding		193,893,397

Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Total revenues	$ 3,489.8	$ 3,131.1
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	602.0	446.0
Research and development	563.7	496.7
Selling, general and administrative	567.7	501.3
Loss on assets and liabilities held for sale	115.5	0.0
Amortization and impairment of acquired intangible assets	68.2	103.9
Collaboration profit (loss) sharing	58.1	42.5
Acquired in-process research and development	0.0	10.0
Loss (gain) on fair value remeasurement of contingent consideration	11.5	(5.6)
Restructuring charges	0.4	1.6
Total cost and expenses	1,987.1	1,596.4
Income from operations	1,502.7	1,534.7
Other income (expense), net	357.3	(41.0)
Income before income tax expense and equity in loss of investee, net of tax	1,860.0	1,493.7
Income tax expense	422.5	322.5
Equity in loss of investee, net of tax	28.7	0.0
Net income	1,408.8	1,171.2
Net income (loss) attributable to noncontrolling interests, net of tax	0.0	(1.7)
Net income attributable to Biogen Inc.	$ 1,408.8	$ 1,172.9
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 7.17	$ 5.55
Diluted earnings per share attributable to Biogen Inc.	$ 7.15	$ 5.54
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	196.6	211.4
Diluted earnings per share attributable to Biogen Inc.	197.0	211.7
Product, net
Total revenues	$ 2,680.0	$ 2,523.5
Revenues from anti-CD20 therapeutic programs
Total revenues	517.4	443.2
Other
Total revenues	$ 292.4	$ 164.4

Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 1,408.8	$ 1,172.9
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	6.9	(2.2)
Unrealized gains (losses) on cash flow hedges, net of tax	16.9	(29.0)
Gain (losses) on net investment hedge	14.0	0.0
Unrealized gains (losses) on pension benefit obligation, net of tax	0.6	(0.5)
Currency translation adjustment	(17.8)	44.7
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	20.6	13.0
Comprehensive income attributable to Biogen Inc.	1,429.4	1,185.9
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	0.0	(1.7)
Comprehensive income	$ 1,429.4	$ 1,184.2

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 2,243.2	$ 1,224.6
Marketable securities	1,665.8	2,313.4
Accounts receivable, net	2,088.9	1,958.5
Due from anti-CD20 therapeutic programs	527.1	526.9
Inventory	770.2	929.9
Assets held for sale	682.0	0.0
Other current assets	965.4	687.6
Total current assets	8,942.6	7,640.9
Marketable securities	1,372.7	1,375.9
Property, plant and equipment, net	3,013.8	3,601.2
Operating lease assets	447.8	0.0
Intangible assets, net	3,056.2	3,120.0
Goodwill	5,639.7	5,706.4
Deferred tax asset	2,074.4	2,153.9
Investments and other assets	1,898.3	1,690.6
Total assets	26,445.5	25,288.9
Current liabilities:
Taxes payable	238.5	63.5
Accounts payable	378.0	370.5
Liabilities held for sale	97.2	0.0
Accrued expenses and other	2,435.0	2,861.2
Total current liabilities	3,148.7	3,295.2
Notes payable	5,943.2	5,936.5
Deferred tax liability	1,741.7	1,636.2
Long-term operating lease liabilities	436.1	0.0
Other long-term liabilities	1,353.8	1,389.4
Total liabilities	12,623.5	12,257.3
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	(219.8)	(240.4)
Retained earnings	17,026.7	16,257.0
Treasury stock, at cost	(2,977.1)	(2,977.1)
Total Biogen Inc. shareholders’ equity	13,829.9	13,039.6
Noncontrolling interests	(7.9)	(8.0)
Total equity	13,822.0	13,031.6
Total liabilities and equity	$ 26,445.5	$ 25,288.9

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
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, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net income	$ 1,408.8	$ 1,171.2
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation, amortization and impairments	121.1	168.9
Acquired in-process research and development	0.0	10.0
Share-based compensation	45.7	43.4
Contingent consideration	(11.5)	5.6
Loss on assets and liabilities held for sale	115.5	0.0
Deferred income taxes	228.0	53.1
Unrealized (gain) loss on strategic investments	(375.0)	0.0
Other	50.7	31.7
Changes in operating assets and liabilities, net:
Accounts receivable	(136.6)	(134.0)
Inventory	(129.0)	(2.6)
Accrued expenses and other current liabilities	138.4	121.8
Income tax assets and liabilities	170.3	257.6
Other changes in operating assets and liabilities, net	(171.1)	(20.0)
Net cash flows provided by operating activities	1,459.5	1,457.1
Cash flows from investing activities:
Proceeds from sales and maturities of marketable securities	1,489.2	4,068.9
Purchases of marketable securities	(825.0)	(1,919.2)
Contingent consideration paid related to Fumapharm AG acquisition	(300.0)	(600.0)
Purchases of property, plant and equipment	(127.1)	(194.7)
Acquired in-process research and development	0.0	(10.0)
Other	(1.7)	(1.6)
Net cash flows provided by investing activities	238.8	1,346.6
Cash flows from financing activities:
Purchases of treasury stock	(655.8)	(250.0)
Payments related to issuance of stock for share-based compensation arrangements, net	(32.2)	(21.2)
Other	8.7	2.6
Net cash flows used in financing activities	(679.3)	(268.6)
Net increase (decrease) in cash and cash equivalents	1,019.0	2,535.1
Effect of exchange rate changes on cash and cash equivalents	0.4	0.9
Cash and cash equivalents, beginning of the period	1,224.6	1,573.8
Cash and cash equivalents, end of the period	$ 2,243.2	$ 4,108.0

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	Noncontrolling interest	Parent	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	2016 Share Repurchase Program	2016 Share Repurchase Program Common stock	2016 Share Repurchase Program Additional paid-in capital	2016 Share Repurchase Program Retained earnings	2016 Share Repurchase Program Treasury stock	Accounting Standards Update 2016-01 Accumulated Other Comprehensive Income	Accounting Standards Update 2016-16 Retained earnings
Beginning balance at Dec. 31, 2017	$ 12,598.1	$ 0.0	$ 0.1	$ 97.8	$ (318.4)	$ 15,810.4	$ (2,977.1)
Beginning balance, shares at Dec. 31, 2017		0.0	(235.3)				(23.8)
Beginning balance at Dec. 31, 2017	12,612.8
Beginning balance at Dec. 31, 2017								$ (14.7)
Net income attributable to Biogen Inc.	1,172.9					1,172.9
Net income (loss) attributable to noncontrolling interest, net of tax	(1.7)							(1.7)
Net income	1,171.2
Other comprehensive income (loss), net of tax	13.0				13.0
Other comprehensive income (loss), net of tax	13.2							0.2
Repurchase of common stock, at cost															$ (250.0)				$ (250.0)
Repurchase of common stock, at cost, shares															(0.9)				(0.9)
Retirement of common stock pursuant to Share Repurchase Progams, at cost															$ 0.0	$ 0.0	$ (122.9)	$ (127.1)	$ (250.0)
Retirement of common stock pursuant to Share Repurchase Programs, at cost, shares																(0.9)			(0.9)
Issuance of common stock under stock option and stock purchase plans	16.2		$ 0.0	16.2
Issuance of common stock under stock option and stock purchase plans, shares			0.1
Issuance of common stock under award plan			0.3
Issuance of common stock under stock award plan	(37.8)		$ 0.0	(37.8)
Compensation related to share-based payments	46.7			46.7
Adoption of new accounting guidance	479.9																			$ 1.5	$ 478.4
Ending balance at Mar. 31, 2018	14,037.5	$ 0.0	$ 0.1	0.0	(303.9)	17,334.6	$ (2,977.1)
Ending balance at Mar. 31, 2018	14,053.7
Ending balance at Mar. 31, 2018								(16.2)
Ending balance, shares at Mar. 31, 2018		0.0	(234.8)				(23.8)
Beginning balance at Dec. 31, 2018	13,031.6	$ 0.0	$ 0.1	0.0	(240.4)	16,257.0	$ (2,977.1)
Beginning balance, shares at Dec. 31, 2018		0.0	(221.0)				(23.8)
Beginning balance at Dec. 31, 2018	13,039.6
Beginning balance at Dec. 31, 2018	(8.0)							(8.0)
Net income attributable to Biogen Inc.	1,408.8					1,408.8
Net income (loss) attributable to noncontrolling interest, net of tax	0.0							0.0
Net income	1,408.8
Other comprehensive income (loss), net of tax	20.6				20.6
Other comprehensive income (loss), net of tax	20.6							0.0
Noncontrolling interest, increase (decrease) other	0.1							0.1	$ 0.0
Repurchase of common stock, at cost										$ (655.8)				$ (655.8)
Repurchase of common stock, at cost, shares										(2.4)				(2.4)
Retirement of common stock pursuant to Share Repurchase Progams, at cost										$ 0.0	$ 0.0	$ (65.6)	$ (590.2)	$ (655.8)
Retirement of common stock pursuant to Share Repurchase Programs, at cost, shares											(2.4)			(2.4)
Issuance of common stock under stock option and stock purchase plans	16.6		$ 0.0	16.6
Issuance of common stock under stock option and stock purchase plans, shares			0.1
Issuance of common stock under award plan			0.3
Issuance of common stock under stock award plan	(48.9)		$ 0.0	0.0		(48.9)
Compensation related to share-based payments	49.0			49.0
Ending balance at Mar. 31, 2019	13,822.0	$ 0.0	$ 0.1	$ 0.0	$ (219.8)	$ 17,026.7	$ (2,977.1)
Ending balance at Mar. 31, 2019	13,829.9
Ending balance at Mar. 31, 2019	$ (7.9)							$ (7.9)
Ending balance, shares at Mar. 31, 2019		0.0	(219.0)				(23.8)

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Summary of significant accounting policies

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 (AD) and dementia, movement disorders, including Parkinson's disease, and neuromuscular disorders, including spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS). We are also focused on discovering, developing and delivering worldwide innovative therapies in our emerging growth areas of acute neurology, neurocognitive disorders, pain and ophthalmology. In addition, we are employing innovative technologies to discover potential treatments for rare and genetic disorders, including new ways of treating diseases through gene therapy in our core and emerging growth areas. We also commercialize biosimilars of advanced biologics.

Our marketed products include TECFIDERA, 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 also 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 (PPMS) and relapsing MS (RMS) 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 19, Collaborative and Other Relationships, to our consolidated financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2018 (2018 Form 10-K).

We support our drug discovery and development efforts through the commitment of significant resources to discovery, research and development programs and business development opportunities. For over two decades we have led in the research and development of new therapies to treat MS, resulting in our leading portfolio of MS treatments. Now our research is focused on additional improvements in the treatment of MS, such as the development of next generation therapies for MS, with a goal to reverse or possibly repair damage caused by the disease. We are also applying our scientific expertise to solve some of the most challenging and complex diseases, including Parkinson's disease, ALS, progressive supranuclear palsy, AD, stroke, epilepsy, cognitive impairment associated with schizophrenia and pain.

Our innovative drug development and commercialization activities are complemented by our biosimilar products that expand access to medicines and reduce the cost burden for healthcare systems. Through 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, FLIXABI, an infliximab biosimilar referencing REMICADE, and IMRALDI, an adalimumab biosimilar referencing HUMIRA, in the European Union (E.U.). For additional information on our collaboration arrangement with Samsung Bioepis, please read Note 17, 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 presentation 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 2018 Form 10-K. Our accounting policies are described in the “Notes to Consolidated Financial Statements” in our 2018 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, 2019, 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.

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% 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 an 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, revenues and expenses 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 revenues and expenses. Actual results may differ from these estimates.

Assets and Liabilities Held For Sale

Upon determining that a long-lived asset or disposal group meets the criteria to be classified as held for sale, we cease depreciation and separately present such assets and liabilities of the disposal group in our condensed consolidated balance sheet. We initially measure a long-lived asset or disposal group that is classified as held for sale at the lower of its carrying value or fair value less any costs to sell. Any loss resulting from this measurement is recognized in the period in which the held-for-sale criteria are met. Conversely, gains are not recognized on the sale of a long-lived asset or disposal group until the date of sale. We assess the fair value of a long-lived asset or disposal group less any costs to sell each reporting period it remains classified as held for sale and recognize any subsequent changes as an adjustment to the carrying value of the asset or disposal group, as long as the remeasured carrying value does not exceed the carrying value less costs to sell of the asset or disposal group at the time it was initially classified as held for sale.

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.

Leases

In February 2016 the FASB issued Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842), a new standard issued to increase transparency and comparability among organizations related to their leasing activities. This standard established a right-of-use model that requires all lessees to recognize right-of-use assets and lease liabilities on their balance sheet that arise from leases as well as provide disclosures with respect to certain qualitative and quantitative information related to a company's leasing arrangements to meet the objective of allowing users of financial statements to assess the amount, timing and uncertainty of cash flows arising from leases.

The FASB subsequently issued the following amendments to ASU 2016-02 that have the same effective date and transition date: ASU No. 2018-01, Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842, ASU No. 2018-10, Codification Improvements to Topic 842, Leases, ASU No. 2018-11, Leases (Topic 842): Targeted Improvements, ASU No. 2018-20, Narrow-Scope Improvement for Lessors, and ASU No. 2019-01, Leases (Topic 842): Codification Improvements. We adopted these amendments with ASU 2016-02 (collectively, the new leasing standards) effective January 1, 2019.

We adopted the new leasing standards using the modified retrospective transition approach, as of January 1, 2019, with no restatement of prior periods or cumulative adjustment to retained earnings. Upon adoption, we elected the package of transition practical expedients, which allowed us to carry forward prior conclusions related to whether any expired or existing contracts are or contain leases, the lease classification for any expired or existing leases and initial direct costs for existing leases. We also elected the practical expedient to not reassess certain land easements and made an accounting policy election to not recognize leases with an initial term of 12 months or less within our condensed consolidated balance sheets and to recognize those lease payments on a straight-line basis in our condensed consolidated statements of income over the lease term. Upon adoption of the new leasing standards we recognized an operating lease asset of approximately $463.0 million and a corresponding operating lease liability of approximately $526.0 million, which are included in our condensed consolidated balance sheet. The adoption of the new leasing standards did not have an impact on our condensed consolidated statements of income.

We determine if an arrangement is a lease at contract inception. Operating lease assets represent our right to use an underlying asset for the lease term and operating lease liabilities represent our obligation to make lease payments arising from the lease. Operating lease assets and liabilities are recognized at the commencement date of the lease based upon the present value of lease payments over the lease term. When determining the lease term, we include options to extend or terminate the lease when it is reasonably certain that we will exercise that option. We use the implicit rate when readily determinable and use our incremental borrowing rate when the implicit rate is not readily determinable based upon the information available at the commencement date in determining the present value of the lease payments. Our incremental borrowing rate is determined using a secured borrowing rate for the same currency and term as the associated lease.

The lease payments used to determine our operating lease assets may include lease incentives, stated rent increases and escalation clauses linked to rates of inflation when determinable and are recognized in our operating lease assets in our condensed consolidated balance sheets. In addition, our contracts contain lease and non-lease components. We separate lease payments for the identified assets from any non-lease payments included in the contract. For certain equipment leases, such as vehicles, we apply a portfolio approach to effectively account for the operating lease assets and liabilities.

Our operating leases are reflected in operating lease assets, accrued expenses and other and in long-term operating lease liabilities in our condensed consolidated balance sheets. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term.

We also have real estate lease agreements which are subleased to third parties. Operating leases for which we are the sublessor are included in accrued expenses and other and other long-term liabilities in our condensed consolidated balance sheets. We recognize sublease income on a straight-line basis over the lease term in our condensed consolidated statements of income.

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

Credit Losses

In June 2016 the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. This standard requires that credit losses be reported using an expected losses model rather than the incurred losses model that is currently used, and establishes additional disclosures related to credit risks. For available-for-sale debt securities with unrealized losses, this standard now requires allowances to be recorded instead of reducing the amortized cost of the investment. ASU 2016-13 limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and requires the reversal of previously recognized credit losses if fair value increases.

This standard will become effective for us on January 1, 2020, and requires adoption using a modified retrospective approach, with certain exceptions. Based on the composition of our investment portfolio, current market conditions and historical credit loss activity, the adoption of this standard is not expected to have a material impact on our consolidated financial position and results of operations and related disclosures.

Debt Securities

In March 2017 the FASB issued ASU No. 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities. This standard amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period to the earliest call date. This standard became effective for us on January 1, 2019, and was adopted using a modified retrospective transition approach. The adoption of this standard did not result in a significant adjustment to our marketable debt securities.

Fair Value Measurements

In August 2018 the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement. This standard modifies certain disclosure requirements on fair value measurements. This standard will become effective for us on January 1, 2020. We do not expect that the adoption of this standard will have a material impact on our disclosures.

Derivative Instruments and Hedging Activities

In October 2018 the FASB issued ASU No. 2018-16, Derivatives and Hedging (Topic 815): Inclusion of the Secured Overnight Financing Rate (SOFR) Overnight Index Swap (OIS) Rate as a Benchmark Interest Rate for Hedge Accounting Purposes. This standard permits the use of the OIS rate based on the SOFR as a United States (U.S.) benchmark interest rate for hedge accounting purposes under Accounting Standards Codification (ASC) 815, Derivative and Hedging. This standard became effective for us on January 1, 2019, and did not have an impact on our condensed consolidated results of operations or financial position.

Collaborative Arrangements

In November 2018 the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606. This standard makes targeted improvements for collaborative arrangements as follows:

•

Clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606, Revenue from Contracts with Customers, when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in ASC 606 should be applied, including recognition, measurement, presentation and disclosure requirements;


•	Adds unit-of-account guidance to ASC 808, Collaborative Arrangements, to align with the guidance in ASC 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of ASC 606; and


•	Precludes a company from presenting transactions with collaborative arrangement participants that are not directly related to sales to third parties with revenue recognized under ASC 606 if the collaborative arrangement participant is not a customer.

This standard will become effective for us on January 1, 2020; however, early adoption is permitted. A retrospective transition approach is required for either all contracts or only for contracts that are not completed at the date of initial application of ASC 606, with a cumulative adjustment to opening retained earnings, as of January 1, 2018. We are currently evaluating the potential impact that this standard may have on our consolidated financial position, results of operations and related disclosures.

Acquisitions	3 Months Ended
Acquisitions	Mar. 31, 2019
Business Combinations [Abstract]
Acquisitions	Acquisitions Proposed Acquisition of Nightstar Therapeutics plc In March 2019 we entered into an agreement to acquire Nightstar Therapeutics plc (NST), a clinical-stage gene therapy company focused on adeno-associated virus treatments for inherited retinal disorders. NST's lead asset is NSR-REP1 for the potential treatment of choroideremia, a rare, degenerative, X-linked inherited retinal disorder, which leads to blindness and has no approved treatments. NST’s second clinical program is NSR-RPGR for the potential treatment of X-linked retinitis pigmentosa, which is a rare inherited retinal disease with no approved treatments. Under the terms of the proposed acquisition, we would pay NST shareholders $25.50 in cash for each issued and outstanding NST share, which represents an expected total transaction value of approximately $800.0 million on a fully diluted basis, after expected transaction expenses and anticipated cash acquired at closing. We plan to fund the proposed acquisition of NST through available cash and to account for it as an acquisition of a business. It is intended that the proposed acquisition will be implemented by means of a U.K. Court-sanctioned scheme of arrangement under Part 26 of the U.K. Companies Act 2006. The proposed acquisition remains subject to customary closing conditions, including the approval by NST shareholders and the issuance of an order by the U.K. Court. We expect to complete the proposed acquisition by mid-year 2019.

Divestitures Divestitures

3 Months Ended

Mar. 31, 2019

Divestitures [Abstract]

Divestitures

Proposed Divestiture of Hillerød, Denmark Manufacturing Operations

In March 2019 we entered into a share purchase agreement with FUJIFILM Corporation (FUJIFILM) under which FUJIFILM will acquire all of the outstanding shares of our subsidiary that owns our biologics manufacturing operations in Hillerød, Denmark. Upon closing of the proposed transaction, we expect to receive up to $890.0 million in cash, subject to certain working capital adjustments and other contractual terms.

As part of the proposed transaction, we have provided FUJIFILM with certain minimum batch production commitment guarantees. There is a risk that the minimum contractual batch production commitments will not be met. Based upon current estimates we expect to incur an adverse commitment obligation of approximately $120.0 million associated with such guarantees. We may adjust this estimate based upon changes in business conditions, which may result in the recognition of additional losses. We are also obligated to indemnify FUJIFILM for liabilities that may exist relating to certain business activities incurred prior to the closing of the proposed transaction.

In addition, we may earn certain contingent payments based on future manufacturing activities at the Hillerød facility. For the disposition of a business, our policy is to recognize contingent consideration when the consideration is realizable. We currently believe the probability of earning these payments is remote and therefore we have not included these contingent payments in our estimate of the fair value of the operations.

As part of the proposed transaction, we also expect to enter into certain manufacturing services agreements with FUJIFILM pursuant to which FUJIFILM would use the Hillerød facility to produce commercial products for us, such as TYSABRI, as well as other third-party products.

We determined that the operations to be disposed of in the proposed transaction did not meet the criteria to be classified as discontinued operations under the applicable guidance.

In February 2019 the assets and liabilities related to our Hillerød, Denmark manufacturing operations met the criteria to be classified as held for sale. The following table presents information related to the carrying value of the major classes of assets and liabilities that were reclassified as held for sale in our condensed consolidated balance sheets:



(In millions)	As of March 31, 2019
Assets:
Inventory	$	27.0
Property, plant and equipment, net	629.7
Operating lease assets	2.5
Goodwill	69.5
Other assets	68.8
Valuation allowance on disposal group on assets held for sale	(115.5		)
Assets held for sale	$	682.0

Liabilities:
Accrued expenses and other liabilities	$	49.0
Long-term operating lease liabilities	1.7
Deferred tax liability	46.5
Liabilities held for sale	$	97.2

In the first quarter of 2019 we recorded a loss of approximately $174.6 million in our condensed consolidated statements of income. This estimated loss includes a pre-tax loss of $115.5 million reflecting our current estimated fair value of the assets and liabilities held for sale, adjusting for our expected costs to sell our Hillerød, Denmark manufacturing operations of approximately $10.0 million and our estimate of the fair value of an adverse commitment of approximately $120.0 million associated with the guarantee of future minimum batch production at the Hillerød facility. The value of this adverse commitment was determined using a probability-weighted estimate of future manufacturing activity. In addition, we recorded a tax expense of $59.1 million related to the proposed transaction. Our total estimated loss is based on current exchange rates and business conditions, and any changes to these factors through the closing date of the transaction will result in adjustments to the carrying values of the related assets and liabilities as well as a corresponding adjustment to the loss amount recognized on the sale.

Following the closing of the proposed transaction, the final purchase price will be adjusted by an amount equal to the difference between our current estimates of working capital and inventory balances that will be transferred to FUJIFILM and the amounts that are ultimately transferred.

Our estimate of the fair value of assets and liabilities expected to be sold to FUJIFILM is a Level 3 measurement and is based on the expected consideration from the sale, including the valuation of the adverse commitment, as discussed above.

The proposed transaction remains subject to customary closing conditions, including filings and clearances under the Danish Competition Act. We expect to complete the proposed transaction in the second half of 2019.

In addition, upon closing of the proposed transaction, we expect to separately sell certain raw materials remaining at the Hillerød facility to FUJIFILM at carrying value.

Revenues Revenues

3 Months Ended

Mar. 31, 2019

Revenue from Contract with Customer [Abstract]

Revenues

Product Revenues

Revenues by product are summarized as follows:



	For the Three Months Ended March 31,
(In millions)	2019						2018
	United States		Rest of World		Total		United States		Rest of World		Total
Multiple Sclerosis (MS):
TECFIDERA	$	717.7	$	281.1	$	998.8	$	728.9	$	258.0	$	986.9
Interferon*	327.3		173.6		500.9		371.4		178.9		550.3
TYSABRI	245.0		215.4		460.4		249.7		212.4		462.1
FAMPYRA	—		22.9		22.9		—		24.4		24.4
ZINBRYTA	—		—		—		—		1.4		1.4
Subtotal: MS product revenues	1,290.0		693.0		1,983.0		1,350.0		675.1		2,025.1

Spinal Muscular Atrophy:
SPINRAZA	223.3		295.2		518.5		188.0		175.9		363.9

Biosimilars:
BENEPALI	—		124.0		124.0		—		120.9		120.9
FLIXABI	—		14.7		14.7		—		6.6		6.6
IMRALDI	—		35.7		35.7		—		—		—
Subtotal: Biosimilar product revenues	—		174.4		174.4		—		127.5		127.5

Other:
FUMADERM	—		4.1		4.1		—		7.0		7.0
Total product revenues	$	1,513.3	$	1,166.7	$	2,680.0	$	1,538.0	$	985.5	$	2,523.5

*Interferon includes AVONEX and PLEGRIDY.

We recognized revenues from two wholesalers accounting for 31.3% and 14.2% of gross product revenues for the three months ended March 31, 2019, and 34.0% and 15.9% of gross product revenues for the three months ended March 31, 2018.

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



(In millions)	Discounts			Contractual Adjustments			Returns			Total
Balance, as of December 31, 2018	$	127.8		$	888.8		$	34.7		$	1,051.3
Current provisions relating to sales in current year	141.2			680.3			4.7			826.2
Adjustments relating to prior years	0.3			(25.1		)	0.3			(24.5		)
Payments/credits relating to sales in current year	(60.8		)	(233.9		)	(0.1		)	(294.8		)
Payments/credits relating to sales in prior years	(92.1		)	(378.1		)	(4.6		)	(474.8		)
Balance, as of March 31, 2019	$	116.4		$	932.0		$	35.0		$	1,083.4

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



(In millions)	As of March 31, 2019		As of December 31, 2018
Reduction of accounts receivable, net	$	175.8	$	176.6
Component of accrued expenses and other	907.6		874.7
Total revenue-related reserves	$	1,083.4	$	1,051.3

Revenues from Anti-CD20 Therapeutic Programs

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



	For the Three Months Ended March 31,
(In millions)	2019		2018
Biogen’s share of pre-tax profits in the U.S. for RITUXAN, RITUXAN HYCELA and GAZYVA	$	390.8	$	349.6
Other revenues from anti-CD20 therapeutic programs	126.6		93.6
Total revenues from anti-CD20 therapeutic programs	$	517.4	$	443.2

For additional information on our collaboration arrangements with Genentech, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2018 Form 10-K.

Other Revenues

Other revenues are summarized as follows:



	For the Three Months Ended March 31,
(In millions)	2019			2018
Revenues from collaborative and other relationships:
(Loss) profit earned under our 50% share of the co-promotion losses on ZINBRYTA in the U.S. with AbbVie	$	(0.4	)	$	(4.7	)
Revenues earned under our technical development agreement, manufacturing services agreements and royalty revenues on biosimilar products with Samsung Bioepis	24.8			17.9
Other royalty and corporate revenues:
Royalty	3.9			10.6
Other corporate	264.1			140.6
Total other revenues	$	292.4		$	164.4

Other corporate revenues primarily reflect amounts earned under contract manufacturing agreements with our strategic partners, including Bioverativ Inc. (Bioverativ). During the three months ended March 31, 2019 and 2018, we recognized $206.8 million and $47.0 million, respectively, in revenues under the manufacturing and supply agreement with Bioverativ entered into in connection with the spin-off of our hemophilia business.

For additional information on our collaboration arrangement with Samsung Bioepis, please read Note 17, Collaborative and Other Relationships, to these condensed consolidated financial statements. For additional information on our collaboration arrangement with AbbVie Inc. (AbbVie), please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2018 Form 10-K. For additional information on our contract manufacturing agreements with Bioverativ, please read Note 3, Hemophilia Spin-Off, to our consolidated financial statements included in our 2018 Form 10-K.

Inventory

3 Months Ended

Mar. 31, 2019

Inventory Disclosure [Abstract]

Inventory

The components of inventory are summarized as follows:



(In millions)	As of March 31, 2019		As of December 31, 2018
Raw materials	$	199.7	$	196.3
Work in process	462.3		606.7
Finished goods	108.2		133.5
Total inventory	$	770.2	$	936.5

Balance Sheet Classification:
Inventory	$	770.2	$	929.9
Investments and other assets	—		6.6
Total inventory	$	770.2	$	936.5

During the three months ended March 31, 2019, we sold hemophilia related inventory to Bioverativ with a cost basis totaling $173.5 million pursuant to the terms of the manufacture and supply agreement with Bioverativ entered into in connection with the spin-off of our hemophilia business.

Long-term inventory, which primarily consists of work in process, is included in investments and other assets in our condensed consolidated balance sheets.

Proposed Divestiture of Hillerød, Denmark Manufacturing Operations

In March 2019 we entered into a share purchase agreement with FUJIFILM under which FUJIFILM will acquire all of the outstanding shares of our subsidiary that owns our biologics manufacturing operations in Hillerød, Denmark. Upon closing of the proposed transaction, we expect to receive up to $890.0 million in cash, subject to certain working capital adjustments and other contractual terms. As a result, $27.0 million of work in process inventory was reclassified to assets held for sale in our condensed consolidated balance sheets as of March 31, 2019. Following the closing of the proposed transaction, the final purchase price will be adjusted by an amount equal to the difference between our current estimates of working capital and inventory balances that will be transferred to FUJIFILM and the amounts that are ultimately transferred. In addition, upon closing of the proposed transaction, we expect to separately sell certain raw materials remaining at the Hillerød facility to FUJIFILM at carrying value.

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

Intangible Assets and Goodwill

3 Months Ended

Mar. 31, 2019

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, 2019							As of December 31, 2018
(In millions)	Estimated Life	Cost		Accumulated Amortization			Net		Cost		Accumulated Amortization			Net
Out-licensed patents	13-23 years	$	543.3	$	(542.4	)	$	0.9	$	543.3	$	(542.3	)	$	1.0
Developed technology	15-23 years	3,005.3		(2,744.1		)	261.2		3,005.3		(2,734.8		)	270.5
In-process research and development	Indefinite until commercialization	480.5		—			480.5		476.0		—			476.0
Trademarks and tradenames	Indefinite	64.0		—			64.0		64.0		—			64.0
Acquired and in-licensed rights and patents	4-18 years	3,638.7		(1,389.1		)	2,249.6		3,638.7		(1,330.2		)	2,308.5
Total intangible assets		$	7,731.8	$	(4,675.6	)	$	3,056.2	$	7,727.3	$	(4,607.3	)	$	3,120.0

For the three months ended March 31, 2019, amortization and impairment of acquired intangible assets totaled $68.2 million, compared to $103.9 million in the prior year comparative period. The decrease in amortization and impairment of acquired intangible asset was primarily due to a net overall decrease in our expected rate of amortization for acquired intangible assets. This was primarily due to lower amortization subsequent to the impairment in the fourth quarter of 2018 of the U.S. license to Forward Pharma A/S' (Forward Pharma) intellectual property, including Forward Pharma's intellectual property related to TECFIDERA, and higher expected lifetime revenues of TYSABRI. For the three months ended March 31, 2019 and 2018, we had no impairment charges.

Developed Technology

Developed technology primarily relates to our AVONEX product, which was recorded in connection with the merger of Biogen, Inc. and IDEC Pharmaceuticals Corporation in 2003. The net book value of this asset as of March 31, 2019, was $256.2 million.

Acquired and In-licensed Rights and Patents

Acquired and in-licensed rights and patents primarily relate to our acquisition of all remaining rights to TYSABRI from Elan Pharma International Ltd., an affiliate of Elan Corporation plc. Acquired and in-licensed rights and patents also includes our rest of world license to Forward Pharma's intellectual property, including Forward Pharma's intellectual property related to TECFIDERA, and other amounts related to our other marketed products and other programs acquired through business combinations. The net book value of the TYSABRI asset as of March 31, 2019, was $1,980.8 million and the net book value of the TECFIDERA asset as of March 31, 2019, was $63.1 million. For additional information on our TECFIDERA license rights, please read Note 7, Intangible Assets and Goodwill, to our consolidated financial statements included in our 2018 Form 10-K.

Estimated Future Amortization of Intangible Assets

Our amortization expense is based on the economic consumption and impairment of intangible assets. Our most significant intangible assets are related to our TYSABRI, AVONEX, SPINRAZA and TECFIDERA products and other programs acquired through business combinations. Annually, during our long-range planning cycle, we perform an analysis of anticipated lifetime revenues of our TYSABRI, AVONEX, SPINRAZA and TECFIDERA products. This analysis is also updated whenever events or changes in circumstances would significantly affect the anticipated lifetime revenues of any of these products. Impairments are recorded in the period in which they are incurred.

Our most recent long-range planning cycle was completed in the third quarter of 2018. Based upon this most recent analysis, the estimated future amortization of acquired intangible assets for the next five years is expected to be as follows:



(In millions)	As of March 31, 2019
2019 (remaining nine months)	$	200.0
2020	290.0
2021	250.0
2022	250.0
2023	230.0
2024	190.0

Goodwill

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



(In millions)	As of March 31, 2019
Goodwill, beginning of period	$	5,706.4
Reclassification of goodwill to assets held for sale	(69.5		)
Other	2.8
Goodwill, end of period	$	5,639.7

The reclassification of goodwill to assets held for sale relates to an allocation based upon the relative fair value of the proposed divestiture of our Hillerød, Denmark manufacturing operations.

During the three months ended March 31, 2019, goodwill was reviewed for impairment due to the proposed divestiture of our Hillerød, Denmark manufacturing operations, and based upon this review, no impairments were recognized. As of March 31, 2019, we had no accumulated impairment losses related to goodwill.

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

Other includes changes related to foreign currency exchange rate fluctuations.

Fair Value Measurements

3 Months Ended

Mar. 31, 2019

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, 2019 (In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	1,881.9	$	—	$	1,881.9	$	—
Marketable debt securities:
Corporate debt securities	1,835.7		—		1,835.7		—
Government securities	921.3		—		921.3		—
Mortgage and other asset backed securities	281.5		—		281.5		—
Marketable equity securities	904.0		19.5		884.5		—
Derivative contracts	104.4		—		104.4		—
Plan assets for deferred compensation	29.4		—		29.4		—
Total	$	5,958.2	$	19.5	$	5,938.7	$	—
Liabilities:
Derivative contracts	$	24.1	$	—	$	24.1	$	—
Contingent consideration obligations	421.3		—		—		421.3
Total	$	445.4	$	—	$	24.1	$	421.3



As of December 31, 2018 (In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	705.5	$	—	$	705.5	$	—
Marketable debt securities:
Corporate debt securities	2,459.2		—		2,459.2		—
Government securities	969.6		—		969.6		—
Mortgage and other asset backed securities	260.5		—		260.5		—
Marketable equity securities	615.4		51.7		563.7		—
Derivative contracts	66.9		—		66.9		—
Plan assets for deferred compensation	25.4		—		25.4		—
Total	$	5,102.5	$	51.7	$	5,050.8	$	—
Liabilities:
Derivative contracts	$	24.6	$	—	$	24.6	$	—
Contingent consideration obligations	409.8		—		—		409.8
Total	$	434.4	$	—	$	24.6	$	409.8

There have been no impairments of our assets measured and carried at fair value during the three months ended March 31, 2019. In addition, there were no changes in valuation techniques or inputs utilized or transfers between fair value measurement levels during the three months ended March 31, 2019. The fair value of Level 2 instruments classified as cash equivalents, marketable debt securities and our marketable equity security investment in Ionis Pharmaceuticals, Inc. (Ionis) were determined through third-party pricing services or an option pricing valuation model. For additional information on our agreement with Ionis, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2018 Form 10-K. 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 2018 Form 10-K.

Debt Instruments

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



	As of March 31, 2019				As of December 31, 2018
(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value
2.900% Senior Notes due September 15, 2020	$	1,500.7	$	1,486.6	$	1,489.5	$	1,480.8
3.625% Senior Notes due September 15, 2022	1,022.0		995.8		1,000.4		995.5
4.050% Senior Notes due September 15, 2025	1,795.1		1,738.2		1,745.1		1,737.8
5.200% Senior Notes due September 15, 2045	1,838.0		1,722.6		1,802.6		1,722.4
Total	$	6,155.8	$	5,943.2	$	6,037.6	$	5,936.5

The fair values of each of our series of Senior Notes were determined through market, observable and corroborated sources. For additional information on our debt instruments, please read Note 12, Indebtedness, to our consolidated financial statements included in our 2018 Form 10-K.

Contingent Consideration Obligations

In connection with our acquisitions of Convergence Pharmaceuticals Ltd., Stromedix Inc. and Biogen International Neuroscience GmbH in 2015, 2012 and 2010, respectively, 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)	2019		2018
Fair value, beginning of period	$	409.8	$	523.6
Changes in fair value	11.5		(5.6		)
Payments	—		(20.0		)
Fair value, end of period	$	421.3	$	498.0

As of March 31, 2019 and December 31, 2018, $274.0 million and $265.0 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 expenses and other.

For the three months ended March 31, 2019, changes in the fair value of our contingent consideration obligations were primarily due to changes in the expected timing of the achievement of certain remaining development milestones, a decrease in interest rates used to revalue our contingent consideration liabilities and the passage of time.

For the three months ended March 31, 2018, changes in the fair value of our contingent consideration obligations were primarily due to an increase in the interest rate used to revalue our contingent consideration liabilities during the period.

- 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, 2019

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, 2019		As of December 31, 2018
Commercial paper	$	446.0	$	231.2
Overnight reverse repurchase agreements	192.7		—
Money market funds	1,074.4		279.5
Short-term debt securities	168.8		194.8
Total	$	1,881.9	$	705.5

The carrying values of our commercial paper, including accrued interest, overnight reverse repurchase agreements, money market funds and 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:



As of March 31, 2019 (In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Corporate debt securities
Current	$	981.0	$	0.5	$	(0.1	)	$	981.4
Non-current	852.0		2.8		(0.5		)	854.3
Government securities
Current	683.6		0.2		(0.1		)	683.7
Non-current	237.5		0.3		(0.2		)	237.6
Mortgage and other asset backed securities
Current	0.7		—		—			0.7
Non-current	280.3		0.8		(0.3		)	280.8
Total marketable debt securities	$	3,035.1	$	4.6	$	(1.2	)	$	3,038.5
Marketable equity securities, non-current	489.3		421.7		(7.0		)	904.0



As of December 31, 2018 (In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Corporate debt securities
Current	$	1,608.4	$	—	$	(0.9	)	$	1,607.5
Non-current	854.9		0.7		(3.9		)	851.7
Government securities
Current	706.1		0.1		(0.4		)	705.8
Non-current	264.0		0.1		(0.3		)	263.8
Mortgage and other asset backed securities
Current	0.1		—		—			0.1
Non-current	260.5		0.4		(0.5		)	260.4
Total marketable debt securities	$	3,694.0	$	1.3	$	(6.0	)	$	3,689.3
Marketable equity securities, non-current	496.2		127.7		(8.5		)	615.4

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, 2019				As of December 31, 2018
(In millions)	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Due in one year or less	$	1,665.3	$	1,665.8	$	2,314.6	$	2,313.4
Due after one year through five years	1,234.8		1,237.8		1,235.9		1,232.7
Due after five years	135.0		134.9		143.5		143.2
Total available-for-sale securities	$	3,035.1	$	3,038.5	$	3,694.0	$	3,689.3

The average maturity of our marketable debt securities available-for-sale as of March 31, 2019 and December 31, 2018, were approximately 12 months.

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)	2019			2018
Proceeds from maturities and sales	$	1,489.2		$	4,068.9
Realized gains	$	0.6		$	1.8
Realized losses	$	(0.3	)	$	(9.4	)

Strategic Investments

As of March 31, 2019, our strategic investment portfolio was comprised of investments totaling $1,049.7 million, of which $90.1 million was reflected as a component of other current assets in our condensed consolidated balance sheet, with the remaining balance included in investments and other assets. As of December 31, 2018, our strategic investment portfolio was comprised of investments totaling $676.3 million, which is included in investments and other assets in our condensed consolidated balance sheet.

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

Our investments in equity securities include approximately 11.5 million shares of Ionis' common stock, acquired in June 2018 at a cost of approximately $625.0 million, which is remeasured each reporting period and carried at fair value. This investment is classified as a Level 2 marketable security due to certain holding period restrictions. The remainder of our investments in equity securities of certain publicly-traded biotechnology companies are regularly measured and carried at fair value and classified as Level 1. The effect of the holding period restrictions on our Ionis stock valuation 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 Ionis' common stock and a dividend yield of zero based upon the fact that Ionis and similar companies generally have not historically granted cash dividends.

The increase in our strategic investment portfolio primarily reflects an increase in the fair value in our investment in Ionis' common stock as well as an increase in the value of a non-marketable equity security.

For additional information on our June 2018 investment in Ionis' common stock, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2018 Form 10-K.

Derivative Instruments

3 Months Ended

Mar. 31, 2019

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Derivative Instruments

Foreign Currency Forward Contracts - Hedging Instruments

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

Foreign currency forward contracts in effect as of March 31, 2019 and December 31, 2018, had durations of 1 to 12 months. 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 revenues when the sale of product in the currency being hedged is recognized and in operating expenses when the expense in the currency being hedged is recorded. We recognize all cash flow hedge reclassifications from accumulated other comprehensive income 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 revenues and operating expenses is summarized as follows:



	Notional Amount
(In millions)	As of March 31, 2019		As of December 31, 2018
Euro	$	1,722.2	$	1,701.4
British pound	170.9		215.3
Swiss franc	99.2		131.4
Japanese yen	79.5		98.8
Canadian dollar	72.2		92.2
Total foreign currency forward contracts	$	2,144.0	$	2,239.1

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 $44.3 million and $27.3 million as of March 31, 2019 and December 31, 2018, respectively. We expect the net gains of $44.3 million to be settled over the next 12 months, with any amounts in accumulated other comprehensive income (loss) to be reported as an adjustment to revenues or operating expenses. We consider the impact of our and our counterparties’ credit risk on the fair value of the contracts as well as the ability of each party to execute its contractual obligations. As of March 31, 2019 and December 31, 2018, credit risk did not 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	2019			2018			Location	2019			2018
Revenues	$	14.8		$	(32.9	)	Revenues	$	3.7		$	(0.9	)
Operating expenses	$	(0.5	)	$	1.3		Operating expenses	$	(0.9	)	$	(0.3	)

Interest Rate Contracts - Hedging Instruments

We have entered into interest rate swap contracts on certain borrowing transactions to manage our exposure to interest rate changes.

In connection with the issuance of our 2.90% Senior Notes, we entered into interest rate swaps with an aggregate notional amount of $675.0 million, which expire on September 15, 2020. The interest rate swap contracts are 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 March 31 2019 and December 31, 2018, includes approximately $9.3 million and $14.5 million, respectively, related to changes in the fair value of these interest rate swap contracts. Since the specific terms and notional amount of the swaps match the debt being hedged, it is assumed to be a highly effective hedge and all changes in the fair value of the swaps are recorded as a component of our 2.90% Senior Notes with no net impact recorded in income. Any net interest payments made or received on the interest rate swap contracts are recorded as a component of interest expense in our condensed consolidated statements of income.

Net Investment Hedges - Hedging Instruments

In February 2012 we entered into a joint venture agreement with Samsung BioLogics, establishing an entity, Samsung Bioepis, to develop, manufacture and market biosimilar products. In June 2018 we exercised our option under our joint venture agreement to increase our ownership percentage in Samsung Bioepis from approximately 5% 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, 2019, had remaining durations 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 $8.0 million and net losses of $3.8 million as of March 31, 2019 and December 31, 2018, 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 $9.5 million and $7.3 million as of March 31, 2019 and December 31, 2018, 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	2019		2018		Location	2019		2018		Location	2019		2018
Gains (losses) on net investment hedge	$	11.8	$	—	Gains (losses) on net investment hedge	$	4.4	$	—	Other income (expense)	$	2.2	$	—

For additional information on our collaboration arrangement with Samsung Bioepis, please read Note 17, Collaborative and Other Relationships, to these condensed consolidated financial statements.

Foreign Currency Forward Contracts - Other Derivatives

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 $895.0 million and $735.1 million as of March 31, 2019 and December 31, 2018, respectively. Net losses of $4.8 million related to these contracts were recognized as a component of other income (expense), net for the three months ended March 31, 2019, compared to net losses of $5.6 million in the prior year comparative period.

Summary of Derivatives

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, 2019		As of December 31, 2018
Cash Flow Hedging Instruments:
Asset derivative instruments	Other current assets	$	101.4	$	65.8
Liability derivative instruments	Accrued expenses and other	$	6.6	$	6.9
Fair Value Hedging Instruments:
Liability derivative instruments	Other long-term liabilities	$	9.3	$	14.5
Other Derivatives:
Asset derivative instruments	Other current assets	$	3.0	$	1.1
Liability derivative instruments	Accrued expenses and other	$	8.2	$	3.2

Property, Plant and Equipment Property, Plant and Equipment	3 Months Ended
	Mar. 31, 2019
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	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 $1,841.0 million and $1,797.4 million as of March 31, 2019 and December 31, 2018, respectively. For the three months ended March 31, 2019, depreciation expense totaled $52.9 million, compared to $65.0 million in the prior year comparative period. Solothurn, Switzerland Facility We are building a large-scale biologics manufacturing facility in Solothurn, Switzerland. We expect this facility to be operational by the end of 2020. Upon completion, the 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, 2019 and December 31, 2018, we had approximately $1.7 billion and $1.6 billion, respectively, capitalized as construction in progress related to this facility. As of March 31, 2019, we had contractual commitments of approximately $92.0 million outstanding related to the construction of this facility. Proposed Divestiture of Hillerød, Denmark Manufacturing Operations In March 2019 we entered into a share purchase agreement with FUJIFILM under which FUJIFILM will acquire all of the outstanding shares of our subsidiary that owns our biologics manufacturing operations in Hillerød, Denmark. As a result, $629.7 million of property, plant and equipment, which is primarily comprised of $312.8 million for buildings and $286.5 million for machinery and equipment, was reclassified to assets held for sale in our condensed consolidated balance sheets as of March 31, 2019. Additionally, we ceased recording depreciation on these assets as depreciation is not recorded during the period in which a long-lived asset or disposal group is classified as held for sale, even if the asset or disposal group continues to generate revenue during the period. For additional information on the proposed divestiture of our Hillerød, Denmark manufacturing operations, please read Note 3, Divestitures, to these condensed consolidated financial statements.

Leases Leases

3 Months Ended

Mar. 31, 2019

Leases [Abstract]

Leases

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

Our leases have remaining lease terms ranging from less than 1 year to 10 years. Certain leases include one or more options to renew, exercised at our sole discretion, with renewal terms that can extend the lease term from one year to six years.

In addition, we sublease certain real estate to third parties. Our sublease portfolio consists of operating leases, with remaining lease terms ranging from less than 1 year to 10 years. Our subleases do not include an option to renew as they are coterminous with our operating leases.

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



(In millions)	Balance sheet location	As of March 31, 2019
Assets:
Operating lease assets	Operating lease assets	$	447.8

Liabilities
Current operating lease liabilities	Accrued expenses and other	$	72.5
Non-current operating lease liabilities	Long-term operating lease liabilities	436.1
Total operating lease liabilities		$	508.6

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



		For the Three Months Ended March 31,
(In millions)	Income Statement Location	2019
Operating lease cost	Research and development	$	0.3
	Selling, general and administrative	23.5
Sublease income	Selling, general and administrative	(7.1		)
	Other (income) expense, net	(1.0		)
Net lease cost		$	15.7

The minimum lease payments, net of income from subleases, for the next five years and thereafter is expected to be as follows:



(In millions)	As of March 31, 2019
2019 (remaining nine months)	$	69.4
2020	81.7
2021	76.0
2022	69.2
2023	68.6
2024	66.1
Thereafter	146.2
Total lease payments	$	577.2
Less: interest	68.6
Present value of operating lease liabilities	$	508.6

Under the prior lease guidance minimum rental commitments under non-cancelable leases, net of income from subleases, for each of the next five years and total thereafter as of December 31, 2018, were as follows:



(In millions)	2019			2020			2021			2022			2023			Thereafter			Total
Minimum lease payments	$	87.0		$	80.7		$	75.9		$	71.7		$	71.0		$	215.3		$	601.6
Less: income from subleases (1)	(26.8		)	(25.6		)	(23.7		)	(24.0		)	(24.3		)	(58.4		)	(182.8		)
Net minimum lease payments	$	60.2		$	55.1		$	52.2		$	47.7		$	46.7		$	156.9		$	418.8


(1)	Represents sublease income expected to be received for the vacated manufacturing facility in Cambridge, MA, the vacated portion of our Weston, MA facility and other facilities throughout the world.

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



	As of March 31, 2019
Weighted average remaining lease term in years	7.88
Weighted average discount rate	3.3	%

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



	For the Three Months Ended March 31,
(In millions)	2019
Cash paid for amounts included in the measurement of lease liabilities	$	19.4
Operating lease assets obtained in exchange for lease obligations	$	5.4

Proposed Divestiture of Hillerød, Denmark Manufacturing Operations

In March 2019 we entered into a share purchase agreement with FUJIFILM under which FUJIFILM will acquire all of the outstanding shares of our subsidiary that owns our biologics manufacturing operations in Hillerød, Denmark. As a result, $2.5 million of operating lease assets and $2.5 million of operating lease liabilities were reclassified to assets held for sale in our condensed consolidated balance sheets as of March 31, 2019. For additional information on the proposed divestiture of our Hillerød, Denmark manufacturing operations, please read Note 3, Divestitures, to these condensed consolidated financial statements.

Equity Equity (Notes)

3 Months Ended

Mar. 31, 2019

Equity [Abstract]

Equity

Share Repurchases

In March 2019 our Board of Directors authorized a program to repurchase up to $5.0 billion of our common stock (2019 Share Repurchase Program). Our 2019 Share Repurchase Program does not have an expiration date. All share repurchases under our 2019 Share Repurchase Program will be retired. We did not repurchase any shares of our common stock under our 2019 Share Repurchase Program during the three months ended March 31, 2019.

In August 2018 our Board of Directors authorized a program to repurchase up to $3.5 billion of our common stock (2018 Share Repurchase Program). Our 2018 Share Repurchase Program does not have an expiration date. All share repurchases under our 2018 Share Repurchase Program will be retired. Under our 2018 Share Repurchase Program, we repurchased and retired approximately 2.4 million shares of our common stock at a cost of approximately $655.8 million during the three months ended March 31, 2019.

From April 1, 2019 through April 24, 2019, we repurchased and retired approximately 2.1 million shares of our common stock at a cost of approximately $491.6 million under our 2018 Share Repurchase Program. Approximately $1.0 billion remained available under our 2018 Share Repurchase Program as of April 24, 2019.

In July 2016 our Board of Directors authorized a program to repurchase up to $5.0 billion of our common stock (2016 Share Repurchase Program), which was completed as of June 30, 2018. All share repurchases under our 2016 Share Repurchase Program were retired. Under our 2016 Share Repurchase Program, we repurchased and retired approximately 0.9 million shares of our common stock at a cost of approximately $250.0 million during the three months ended March 31, 2018.

Accumulated Other Comprehensive Income (Loss)

The following table summarizes 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, 2018	$	(4.0	)	$	34.7		$	3.5		$	(31.3	)	$	(243.3	)	$	(240.4	)
Other comprehensive income (loss) before reclassifications	7.1			31.2			16.2			0.6			(17.8		)	37.3
Amounts reclassified from accumulated other comprehensive income (loss)	(0.2		)	(14.3		)	(2.2		)	—			—			(16.7		)
Net current period other comprehensive income (loss)	6.9			16.9			14.0			0.6			(17.8		)	20.6
Balance, March 31, 2019	$	2.9		$	51.6		$	17.5		$	(30.7	)	$	(261.1	)	$	(219.8	)



(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, 2017	$	(1.6	)	$	(104.5	)	$	—	$	(36.8	)	$	(175.5	)	$	(318.4	)
Amounts reclassified, net of tax, upon adoption of ASU No. 2016-01	1.5			—			—		—			—			1.5
Balance, January 1, 2018	(0.1		)	(104.5		)	—		(36.8		)	(175.5		)	(316.9		)
Other comprehensive income (loss) before reclassifications	(8.2		)	(60.4		)	—		(0.5		)	44.7			(24.4		)
Amounts reclassified from accumulated other comprehensive income (loss)	6.0			31.4			—		—			—			37.4
Net current period other comprehensive income (loss)	(2.2		)	(29.0		)	—		(0.5		)	44.7			13.0
Balance, March 31, 2018	$	(2.3	)	$	(133.5	)	$	—	$	(37.3	)	$	(130.8	)	$	(303.9	)

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



(In millions)	Income Statement Location	Amounts Reclassified from Accumulated Other Comprehensive Income
		For the Three Months Ended March 31,
		2019			2018
Gains (losses) on securities available for sale	Other income (expense)	$	0.3		$	(7.6	)
	Income tax benefit (expense)	(0.1		)	1.6

Gains (losses) on cash flow hedges	Revenues	14.8			(32.9		)
	Operating expenses	(0.5		)	1.3
	Other income (expense)	0.1			0.1
	Income tax benefit (expense)	(0.1		)	0.1

Gains (losses) on net investment hedge	Other income (expense)	2.2			—
	Income tax benefit (expense)	—			—

Total reclassifications, net of tax		$	16.7		$	(37.4	)

Earnings per Share

3 Months Ended

Mar. 31, 2019

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)	2019		2018
Numerator:
Net income attributable to Biogen Inc.	$	1,408.8	$	1,172.9
Denominator:
Weighted average number of common shares outstanding	196.6		211.4
Effect of dilutive securities:
Stock options and employee stock purchase plan	—		—
Time-vested restricted stock units	0.3		0.2
Market stock units	0.1		0.1
Performance stock units settled in stock	—		—
Dilutive potential common shares	0.4		0.3
Shares used in calculating diluted earnings per share	197.0		211.7

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, 2019

Disclosure of Compensation Related Costs, Share-based Payments [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)	2019			2018
Research and development	$	21.7		$	21.9
Selling, general and administrative	27.8			28.5
Subtotal	49.5			50.4
Capitalized share-based compensation costs	(3.3		)	(3.4		)
Share-based compensation expense included in total cost and expenses	46.2			47.0
Income tax effect	(7.4		)	(7.6		)
Share-based compensation expense included in net income attributable to Biogen Inc.	$	38.8		$	39.4

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)	2019			2018
Market stock units	$	7.7		$	6.1
Time-vested restricted stock units	34.7			36.1
Cash settled performance units	(1.0		)	4.3
Performance units	0.5			(0.8		)
Performance stock units settled in stock	2.0			0.7
Performance stock units settled in cash	1.0			0.1
Employee stock purchase plan	4.6			3.9
Subtotal	49.5			50.4
Capitalized share-based compensation costs	(3.3		)	(3.4		)
Share-based compensation expense included in total cost and expenses	$	46.2		$	47.0

We estimate the fair value of our obligations associated with our performance units, cash settled performance units and 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, 2019

Income Tax Disclosure [Abstract]

Income Taxes

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,
	2019		2018
Statutory rate	21.0	%	21.0	%
State taxes	0.4		0.9
Taxes on foreign earnings	(4.6	)	(2.0	)
Credits and net operating loss utilization	(0.8	)	(0.8	)
Purchased intangible assets	0.3		0.6
Denmark assets held for sale	4.3		—
Global Intangible Low-Taxed Income (GILTI)	1.9		1.3
Other permanent items	0.4		0.4
Other	(0.2	)	0.2
Effective tax rate	22.7	%	21.6	%

Changes in Tax Rate

For the three months ended March 31, 2019, compared to the same period in 2018, the increase in our effective tax rate was primarily due to a $59.1 million tax expense related to the proposed divestiture of our subsidiary that owns our Hillerød, Denmark manufacturing operations. Although we are recognizing a loss on the proposed divestiture of such subsidiary, the proposed divestiture requires us to write off certain deferred tax assets upon the classification of our Hillerød, Denmark manufacturing operations as held for sale and results in a taxable gain in certain jurisdictions. The effect of this increase to our 2019 effective tax rate was partially offset by a higher effective tax rate in 2018 resulting from the sale of inventory, the tax effect of which had been included within prepaid taxes at December 31, 2017, at a higher effective tax rate.

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

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 2013 or state, local or non-U.S. income tax examinations for years before 2010.

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.

International Uncertain Tax Positions

We have made payments totaling approximately $60.0 million to the Danish Tax Authority (SKAT) for assessments received for 2009, 2011 and 2013 regarding withholding taxes on certain payments made by our subsidiary that owns our biologics manufacturing operations in Hillerød, Denmark. We continue to dispute the assessments for all of these periods and believe that the tax positions taken related to these payments are valid. Any amount refunded by SKAT associated with this withholding tax receivable will be paid to our subsidiary that owns our biologics manufacturing operations in Hillerød, Denmark.

In March 2019 we entered into a share purchase agreement with FUJIFILM under which FUJIFILM will acquire all of the outstanding shares of our subsidiary that owns our biologics manufacturing operations in Hillerød, Denmark. This withholding tax receivable from SKAT will be included within the assets that will be transferred to FUJIFILM as part of the proposed transaction. Under the share purchase agreement, FUJIFILM is required to remit any future proceeds refunded by SKAT to us. We have assessed the collectability of the receivable from FUJIFILM and regard it as a contingent gain, which does not meet the probable threshold for recognition under ASC 450, Contingencies, and therefore we have recorded a pre-tax charge of $60.0 million to write the asset down to zero as a component of the loss on assets and liabilities held for sale in the first quarter of 2019. For additional information on the proposed divestiture of our Hillerød, Denmark manufacturing operations, please read Note 3, Divestitures, to these condensed consolidated financial statements.

Federal and State Uncertain Tax Positions

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

Proposed Divestiture of Hillerød, Denmark Manufacturing Operations

In March 2019 we entered into a share purchase agreement with FUJIFILM under which FUJIFILM will acquire all of the outstanding shares of our subsidiary that owns our biologics manufacturing operations in Hillerød, Denmark. As a result, $46.5 million of our deferred tax liability was reclassified to liabilities held for sale in our condensed consolidated balance sheets as of March 31, 2019. For additional information on the proposed divestiture of our Hillerød, Denmark manufacturing operations, please read Note 3, Divestitures, to these condensed consolidated financial statements.

Other Consolidated Financial Statement Detail

3 Months Ended

Mar. 31, 2019

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)	2019			2018
Interest income	$	31.2		$	26.7
Interest expense	(47.9		)	(50.5		)
Gain (loss) on investments, net	376.4			(14.4		)
Foreign exchange gains (losses), net	(2.2		)	(1.0		)
Other, net	(0.2		)	(1.8		)
Total other income (expense), net	$	357.3		$	(41.0	)

For the three months ended March 31, 2019, gain (loss) on investments, net, as reflected in the table above, substantially relate to marketable equity securities held at March 31, 2019. The increase in gain (loss) on investments, net, compared to the prior year period, was primarily due to an increase in the fair value in our investment in Ionis' common stock as well as an increase in the value of a non-marketable equity security.

Accrued Expenses and Other

Accrued expenses and other consists of the following:



(In millions)	As of March 31, 2019		As of December 31, 2018
Revenue-related reserves for discounts and allowances	$	907.6	$	874.7
Collaboration expenses	216.6		261.6
Royalties and licensing fees	195.0		224.7
Employee compensation and benefits	160.8		320.9
Current portion of contingent consideration obligations	147.3		444.8
Construction in progress	99.7		125.2
Other	708.0		609.3
Total accrued expenses and other	$	2,435.0	$	2,861.2

Other Long-term Liabilities

Other long-term liabilities were $1,353.8 million and $1,389.4 million as of March 31, 2019 and December 31, 2018, respectively, and included accrued income taxes totaling $796.1 million and $791.4 million, respectively.

Collaborative and Other Relationships

3 Months Ended

Mar. 31, 2019

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Collaborative and Other Relationships

Eisai Co., Ltd.

BAN2401 and Elenbecestat Collaboration

We have a collaboration agreement with Eisai Co., Ltd. (Eisai) to jointly develop and commercialize BAN2401, a monoclonal antibody that targets amyloid beta aggregates, and elenbecestat, the oral BACE (base amyloid cleaving enzyme) inhibitor, two Eisai product candidates for the treatment of AD (the BAN2401 and Elenbecestat Collaboration).

The BAN2401 and Elenbecestat Collaboration also provided Eisai with an option to jointly develop and commercialize aducanumab (Aducanumab Option), and an option to jointly develop and commercialize one of our anti-tau monoclonal antibodies (Anti-Tau Option). In October 2017 Eisai exercised its Aducanumab Option and we entered into a new collaboration agreement for the joint development and commercialization of aducanumab (Aducanumab Collaboration Agreement). Eisai has not yet exercised its Anti-Tau Option.

For additional information on our BAN2401 and Elenbecestat Collaboration, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2018 Form 10-K.

For the three months ended March 31, 2019 and 2018, sales and marketing expenses related to the BAN2401 and Elenbecestat Collaboration were immaterial.

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



	For the Three Months Ended March 31,
(In millions)	2019		2018
Total development expense incurred by the collaboration related to the advancement of BAN2401 and Elenbecestat	$	68.0	$	52.2
Biogen's share of BAN2401 and Elenbecestat development expense reflected in research and development expense in our condensed consolidated statements of income	$	34.0	$	26.1

Aducanumab Collaboration Agreement

For the period through March 31, 2018, we were responsible for 100% of development costs incurred by the collaboration for the advancement of aducanumab (aducanumab development expense). For the period April 1, 2018 through December 31, 2018, Eisai reimbursed us for 15% of aducanumab development expense incurred and, beginning January 1, 2019, is reimbursing us for 45% of aducanumab development expense incurred.

In March 2019, following the results of a futility analysis conducted by an independent data monitoring committee, we and Eisai announced the decision to discontinue the global Phase 3 trials, ENGAGE and EMERGE, designed to evaluate the efficacy and safety of aducanumab in patients with mild cognitive impairment due to AD and mild AD dementia. As a result of this decision, in the first quarter of 2019, we accrued approximately $45.0 million related to the termination of various clinical trials and research and development contracts net of the expected 45% Eisai reimbursement of development costs incurred by the collaboration for the advancement of aducanumab.

Sales and marketing expense incurred were shared in proportion to the same region-based profit split that would have been utilized to co-promote aducanumab had it been commercialized. For additional information on the Aducanumab Collaboration Agreement, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2018 Form 10-K.

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



	For the Three Months Ended March 31,
(In millions)	2019		2018
Total aducanumab development expense	$	162.5	$	63.6
Biogen's share of aducanumab development expense reflected in research and development expense in our condensed consolidated statements of income	$	89.4	$	63.6

Total aducanumab sales and marketing expense incurred by the collaboration	$	20.9	$	7.1
Biogen's share of aducanumab sales and marketing expense reflected in selling, general and administrative expense our condensed consolidated statements of income	$	11.6	$	3.9

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.

Other Research and Discovery Arrangements

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

Skyhawk Therapeutics, Inc.

In January 2019 we entered into a collaboration and research and development services agreement with Skyhawk Therapeutics, Inc. (Skyhawk) pursuant to which the companies will leverage Skyhawk's SkySTAR technology platform with the goal of discovering innovative small molecule treatments for patients with neurological diseases, including MS and SMA. We will be responsible for the development and potential commercialization of any therapies resulting from this collaboration and we may also pay Skyhawk up to a total of approximately $2.0 billion in additional milestone payments as well as potential royalties on net commercial sales.

In connection with this agreement, we made an upfront payment of $74.0 million to Skyhawk, of which $38.5 million was recorded as research and development expense in our condensed consolidated statements of income and $35.5 million was recorded as prepaid research and development expenditures within investments and other assets in our condensed consolidated balance sheets and will be expensed as the services are provided.

Samsung Bioepis

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% 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, 2019, 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. During the three months ended March 31, 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 million, consisting of approximately $115 million attributed to inventory, approximately $615 million attributed to developed technology and approximately $170 million attributed to in-process research and development. A deferred tax liability of $225 million was established for the acquired assets that had no tax basis. The basis differences related to inventory and developed technology will be amortized, net of tax, over their estimated useful lives of 1.5 years and 15 years, respectively, one quarter in arrears.

For the three months ended March 31, 2019, we recognized a loss on our investment of $28.7 million, reflecting our share of losses for the fourth quarter of 2018 totaling $14.0 million and amortization of basis differences of $14.7 million.

As of March 31, 2019 and December 31, 2018, the carrying value of our investment in Samsung Bioepis totaled 729.9 billion South Korean won ($642.1 million) and 759.5 billion South Korean won ($680.6 million), respectively, which is classified as a component of investments and other assets in our condensed consolidated balance sheets.

Commercial Agreement

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

In August 2017 the European Commission granted a marketing authorization in the E.U. for IMRALDI. In April 2018 we and Samsung Bioepis entered into an agreement with AbbVie for the commercialization of IMRALDI. Under the terms of the agreement, AbbVie granted us and Samsung Bioepis patent licenses for the use and sale of IMRALDI in Europe, on a country-by-country basis, and we make royalty payments to AbbVie on behalf of Samsung Bioepis. Royalty payments to AbbVie on sales of IMRALDI are recognized in cost of sales in our condensed consolidated statements of income. We began to recognize revenues on sales of IMRALDI to third parties in the E.U. in the fourth quarter of 2018.

We share 50% of the profit or loss related to our commercial agreement with Samsung Bioepis, which is recognized in collaboration profit (loss) sharing in our condensed consolidated statements of income. For the three months ended March 31, 2019, we recognized net profit-sharing expense of $58.1 million to reflect Samsung Bioepis' 50% sharing of the net collaboration profits, compared to $43.8 million in the prior year comparative period.

Other Services

Simultaneous with the formation of Samsung Bioepis, we also entered into a license agreement, a technical development services agreement and a manufacturing agreement with Samsung Bioepis. For the three months ended March 31, 2019, we recognized $24.8 million in revenues in relation to these services, which is reflected in collaborative and other relationships revenues as a component of other revenues in our condensed consolidated statements of income, compared to $17.9 million in the prior year comparative period.

For additional information on our collaboration arrangement with Samsung Bioepis and our other significant collaboration arrangements, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2018 Form 10-K.

Investments in Variable Interest Entities	3 Months Ended
Investments in Variable Interest Entities	Mar. 31, 2019
Investments in Variable Interest Entities [Abstract]
Investments in Variable Interest Entities	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 treatment of AD, including aducanumab. We are responsible for the development, manufacturing and commercialization of all collaboration products. This 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 any product using such a licensed compound. 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% of the research and development costs incurred in support of the collaboration. In March 2019, following the results of a futility analysis conducted by an independent data monitoring committee, we and Eisai announced the decision to discontinue the global Phase 3 trials, ENGAGE and EMERGE, designed to evaluate the efficacy and safety of aducanumab in patients with mild cognitive impairment due to AD and mild AD dementia. Research and development costs for which we reimbursed Neurimmune are reflected in research and development expense in our condensed consolidated statements of income. During the three months ended March 31, 2019 and 2018, 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. Unconsolidated Variable Interest Entities We have relationships with 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, 2019 and December 31, 2018, the carrying value of our investments in certain biotechnology companies representing potential unconsolidated variable interest entities totaled $112.2 million and $28.7 million, respectively. Our maximum exposure to loss related to these variable interest entities is limited to the carrying value of our investments. We have also entered into research collaboration agreements with certain variable interest entities where we are required to fund certain development activities. These development activities are included in research and development expense in our 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 20, Investments in Variable Interest Entities, to our consolidated financial statements included in our 2018 Form 10-K.

Litigation	3 Months Ended
Litigation	Mar. 31, 2019
Loss Contingency, Information about Litigation Matters [Abstract]
Litigation	Litigation We are currently involved in various claims and legal proceedings, including the matters described below. For information as to our accounting policies relating to claims and legal proceedings, including use of estimates and contingencies, please read Note 1, Summary of Significant Accounting Policies, to our consolidated financial statements included in our 2018 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 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 commercializes 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. 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 the Danish Utility Models. In November 2018 Fresenius Kabi commenced infringement proceedings for damages and injunctive relief against Biogen Italia S.R.L. in the District Court of Milan relating to the Italian counterpart of the ‘510 Patent, and against Biogen GmbH in the Dusseldorf Regional Court relating to the German counterpart of the ‘510 Patent. A hearing has been scheduled for May 2019 in the proceeding in Denmark. No hearing has been scheduled in the other proceedings. In August 2018 Biogen Idec Ltd. (Biogen UK) and Samsung Bioepis UK Limited filed an action in the United Kingdom Patents Court to revoke the United Kingdom counterpart of the ‘510 Patent. Fresenius Kabi has filed a counterclaim asserting infringement of the United Kingdom counterpart of the ‘510 Patent and seeking damages and an injunction to restrain infringement if the patent is found valid and infringed. A trial has been set for July 2019. In December 2018 Biogen B.V. and Samsung Bioepis UK Limited filed an action in the District Court of the Hague, Netherlands to revoke the Dutch counterpart of the ‘510 Patent. A trial has been set for October 2019. An estimate of the possible loss or range of loss in the above matters cannot be made at this time. In October 2018 Gedeon Richter PLC asserted to Biogen and Samsung Bioepis UK Limited that IMRALDI infringes European Patent No. 3 212 667, which was issued in September 2018 and expires in October 2035. We dispute the assertion. An estimate of the possible loss or range of loss 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. Our motion to dismiss was denied in part. 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. In July 2018 we and certain other drug manufacturers and pharmacy benefit managers were served with a qui tam action filed by John Borzilleri on behalf of the U.S. and certain states in the U.S. District Court for the District of Rhode Island. The case was filed under seal in January 2014 and unsealed in April 2018 after the U.S. declined to intervene. The case alleges agreements with pharmacy benefit managers 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. We, the other defendants and the U.S. have moved to dismiss the case and the motions are pending. No trial date has been set. An estimate of the possible loss or range of loss cannot be made at this time. Securities Litigation We and certain current and former officers are defendants in an action filed by a shareholder in October 2016 in the U.S. District Court for the District of Massachusetts alleging violations of federal securities laws under 15 U.S.C §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5 and seeking a declaration of the action as a class action and an award of damages, interest and attorneys' fees. In March 2018 the court dismissed the complaint with prejudice. The plaintiff's appeal is pending. An estimate of the possible loss or range of loss cannot be made at this time. Other Matters Hatch-Waxman Act Litigation relating to TECFIDERA Orange-Book Listed Patents In 2017, 2018 and 2019 we initiated patent infringement proceedings against multiple parties pursuant to the Drug Price Competition and Patent Term Restoration Act of 1984, commonly known as the Hatch-Waxman Act, in the U.S. District Courts. Patent infringement proceedings pursuant to the Hatch-Waxman Act are pending against Accord Healthcare Inc., Alkem Laboratories Ltd., Amneal Pharmaceuticals LLC, Aurobindo Pharma U.S.A., Inc., Banner Life Sciences LLC, Cipla Limited, Glenmark Pharmaceuticals Ltd., 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., Sawai USA, Inc., Shipla Medicare Limited, Slayback Pharma LLC, Torrent Pharmaceuticals Ltd., TWi Pharmaceuticals, Inc., Windlas Healthcare Pvt. Ltd. and Zydus Pharmaceuticals (USA) Inc. in the U.S. District Court for the District of Delaware and against Mylan Pharmaceuticals Inc. in the U.S. District Court for the Northern District of West Virginia. A trial date has not been set in the Delaware actions against Banner Life Sciences LLC or Zydus Pharmaceuticals (USA) Inc. A December 2019 trial date has been set for the other Delaware actions, and a trial date has been set for February 2020 in the West Virginia action against Mylan Pharmaceuticals Inc. Petition for Inter Partes Review In July 2018 Mylan Pharmaceuticals Inc. filed a petition with the U.S. Patent Trial and Appeal Board seeking inter partes review of our U.S. Patent No. 8,399,514 (the '514 Patent). The '514 Patent includes claims covering the treatment of MS with 480 mg of dimethyl fumarate per day as provided for in our TECFIDERA label. On February 6, 2019, the U.S. Patent Trial and Appeal Board instituted inter partes review of the '514 Patent. European Patent Office Oppositions In 2016 the European Patent Office (EPO) revoked our European patent number 2 137 537 (the '537 Patent), which includes claims covering 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 the appeal is pending. A hearing has been set for March 2020. In March 2018 the EPO revoked Forward Pharma’s European Patent No. 2 801 355, which expires in October 2025. Forward Pharma has filed an appeal to the Technical Boards of Appeal of the EPO and the appeal is pending. The settlement and license agreement that we entered with Forward Pharma in January 2017 did not resolve the issues pending in this proceeding and we and Forward Pharma intend to permit the Technical Boards of Appeal and the Enlarged Board of Appeal, if applicable, to make a final determination. 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 Number 215263 (the Polish ‘263 Patent), the Polish patent corresponding to our European Patent Number 1 485 127 (the EU ‘127 Patent) (“Administration of agents to treat inflammation”). The Polish ‘263 Patent concerns administration of natalizumab (TYSABRI) to treat MS. The Polish ‘263 Patent expires in February 2023. A hearing has been scheduled for May 2019. Swiss Pharma International AG, also affiliated with the Polpharma Group, filed actions in the District Court of The Hague (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 of the EU '127 Patent, which also concerns administration of natalizumab (TYSABRI) to treat MS and expires in February 2023. The Dutch and German counterparts were ruled invalid and we have appealed. No date for a hearing on the merits has been set in the Italian action. '755 Patent Litigation In May 2010 Biogen MA Inc. (formerly Biogen Idec MA Inc.) filed a complaint in the U.S. District Court for the District of New Jersey alleging infringement by Bayer Healthcare Pharmaceuticals Inc. (Bayer) (manufacturer, marketer and seller of BETASERON and manufacturer of EXTAVIA), EMD Serono, Inc. (EMD Serono) (manufacturer, marketer and seller of REBIF), Pfizer Inc. (Pfizer) (co-marketer of REBIF) and Novartis Pharmaceuticals Corp. (Novartis) (marketer and seller of EXTAVIA) of our U.S. Patent No. 7,588,755 ('755 Patent), which claims the use of interferon beta for immunomodulation or treating a viral condition, viral disease, cancers or tumors. The complaint seeks monetary damages, including lost profits and royalties. Bayer, Pfizer, Novartis and EMD Serono filed counterclaims seeking declaratory judgments of patent invalidity and non-infringement and seeking monetary relief in the form of costs and attorneys' fees. Bayer had previously filed a complaint against us in the same court, on May 27, 2010, seeking a declaratory judgment that it does not infringe the '755 Patent and that the '755 Patent is invalid, and seeking monetary relief in the form of attorneys' fees, costs and expenses. In September 2018 the trial court entered judgment against EMD Serono and Pfizer that the '755 Patent is infringed and valid and ordered a new trial on damages. In October 2018 EMD Serono and Pfizer filed an appeal from the judgment in the U.S. Court of Appeals for the Federal Circuit, which is pending. The trial court has not yet scheduled the new damages trial or a trial against Bayer and Novartis. Government Matters We have learned that state and federal governmental authorities are investigating our sales and promotional practices and have received related subpoenas. We are cooperating with the government. We have received subpoenas and other requests from the federal government for documents and information relating to our relationship with non-profit organizations that assist patients taking drugs sold by Biogen and Biogen's co-pay assistance programs. We are cooperating with the government. In July 2016 we received civil investigative demands from the federal government for documents and information relating to our treatment of certain service agreements with wholesalers when calculating and reporting Average Manufacturer Prices in connection with the Medicaid Drug Rebate Program. We are cooperating with the government. In July 2017 we learned that the Prosecution Office of Milan was investigating our interactions with certain healthcare providers in Italy. In January 2019 the Prosecution Office dismissed the proceeding. We consider the matter closed. Tax Matter In the second quarter of 2018 the State Treasury of Goias, Brazil issued tax assessments for the period 2013 through February 2018 relating to tax on the circulation of goods and totaling approximately $70.0 million including interest and penalties. We dispute the assessments and have filed defenses with the Administrative Court of Appeals for the State of Goias, which are pending. We have not formed an opinion that an unfavorable outcome of the dispute is either probable or remote. 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, 2019

Accounting Policies [Abstract]

Overview

Business Overview

Basis of presentation

We operate as one operating segment, focused on discovering, developing and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases.

Consolidation

Use of estimates

Assets and liabilities held for sale

Assets and Liabilities Held For Sale

New accounting pronouncements

Leases

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

Credit Losses

Debt Securities

Fair Value Measurements

Derivative Instruments and Hedging Activities

Collaborative Arrangements

•


•	Adds unit-of-account guidance to ASC 808, Collaborative Arrangements, to align with the guidance in ASC 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of ASC 606; and


•	Precludes a company from presenting transactions with collaborative arrangement participants that are not directly related to sales to third parties with revenue recognized under ASC 606 if the collaborative arrangement participant is not a customer.

Divestitures Divestitures (Tables)

3 Months Ended

Mar. 31, 2019

Divestitures [Abstract]

Assets and liabilities held for sale

The following table presents information related to the carrying value of the major classes of assets and liabilities that were reclassified as held for sale in our condensed consolidated balance sheets:



(In millions)	As of March 31, 2019
Assets:
Inventory	$	27.0
Property, plant and equipment, net	629.7
Operating lease assets	2.5
Goodwill	69.5
Other assets	68.8
Valuation allowance on disposal group on assets held for sale	(115.5		)
Assets held for sale	$	682.0

Liabilities:
Accrued expenses and other liabilities	$	49.0
Long-term operating lease liabilities	1.7
Deferred tax liability	46.5
Liabilities held for sale	$	97.2

Revenues Revenues (Tables)

3 Months Ended

Mar. 31, 2019

Revenue from Contract with Customer [Abstract]

Revenues by product

Revenues by product are summarized as follows:



	For the Three Months Ended March 31,
(In millions)	2019						2018
	United States		Rest of World		Total		United States		Rest of World		Total
Multiple Sclerosis (MS):
TECFIDERA	$	717.7	$	281.1	$	998.8	$	728.9	$	258.0	$	986.9
Interferon*	327.3		173.6		500.9		371.4		178.9		550.3
TYSABRI	245.0		215.4		460.4		249.7		212.4		462.1
FAMPYRA	—		22.9		22.9		—		24.4		24.4
ZINBRYTA	—		—		—		—		1.4		1.4
Subtotal: MS product revenues	1,290.0		693.0		1,983.0		1,350.0		675.1		2,025.1

Spinal Muscular Atrophy:
SPINRAZA	223.3		295.2		518.5		188.0		175.9		363.9

Biosimilars:
BENEPALI	—		124.0		124.0		—		120.9		120.9
FLIXABI	—		14.7		14.7		—		6.6		6.6
IMRALDI	—		35.7		35.7		—		—		—
Subtotal: Biosimilar product revenues	—		174.4		174.4		—		127.5		127.5

Other:
FUMADERM	—		4.1		4.1		—		7.0		7.0
Total product revenues	$	1,513.3	$	1,166.7	$	2,680.0	$	1,538.0	$	985.5	$	2,523.5

*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, as of December 31, 2018	$	127.8		$	888.8		$	34.7		$	1,051.3
Current provisions relating to sales in current year	141.2			680.3			4.7			826.2
Adjustments relating to prior years	0.3			(25.1		)	0.3			(24.5		)
Payments/credits relating to sales in current year	(60.8		)	(233.9		)	(0.1		)	(294.8		)
Payments/credits relating to sales in prior years	(92.1		)	(378.1		)	(4.6		)	(474.8		)
Balance, as of March 31, 2019	$	116.4		$	932.0		$	35.0		$	1,083.4

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, 2019		As of December 31, 2018
Reduction of accounts receivable, net	$	175.8	$	176.6
Component of accrued expenses and other	907.6		874.7
Total revenue-related reserves	$	1,083.4	$	1,051.3

Revenues from anti-CD20 therapeutic programs

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



	For the Three Months Ended March 31,
(In millions)	2019		2018
Biogen’s share of pre-tax profits in the U.S. for RITUXAN, RITUXAN HYCELA and GAZYVA	$	390.8	$	349.6
Other revenues from anti-CD20 therapeutic programs	126.6		93.6
Total revenues from anti-CD20 therapeutic programs	$	517.4	$	443.2

Other revenues

Other revenues are summarized as follows:



	For the Three Months Ended March 31,
(In millions)	2019			2018
Revenues from collaborative and other relationships:
(Loss) profit earned under our 50% share of the co-promotion losses on ZINBRYTA in the U.S. with AbbVie	$	(0.4	)	$	(4.7	)
Revenues earned under our technical development agreement, manufacturing services agreements and royalty revenues on biosimilar products with Samsung Bioepis	24.8			17.9
Other royalty and corporate revenues:
Royalty	3.9			10.6
Other corporate	264.1			140.6
Total other revenues	$	292.4		$	164.4

Inventory (Tables)

3 Months Ended

Mar. 31, 2019

Inventory Disclosure [Abstract]

Components of inventory

The components of inventory are summarized as follows:



(In millions)	As of March 31, 2019		As of December 31, 2018
Raw materials	$	199.7	$	196.3
Work in process	462.3		606.7
Finished goods	108.2		133.5
Total inventory	$	770.2	$	936.5

Balance Sheet Classification:
Inventory	$	770.2	$	929.9
Investments and other assets	—		6.6
Total inventory	$	770.2	$	936.5

Intangible Assets and Goodwill (Tables)

3 Months Ended

Mar. 31, 2019

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, 2019							As of December 31, 2018
(In millions)	Estimated Life	Cost		Accumulated Amortization			Net		Cost		Accumulated Amortization			Net
Out-licensed patents	13-23 years	$	543.3	$	(542.4	)	$	0.9	$	543.3	$	(542.3	)	$	1.0
Developed technology	15-23 years	3,005.3		(2,744.1		)	261.2		3,005.3		(2,734.8		)	270.5
In-process research and development	Indefinite until commercialization	480.5		—			480.5		476.0		—			476.0
Trademarks and tradenames	Indefinite	64.0		—			64.0		64.0		—			64.0
Acquired and in-licensed rights and patents	4-18 years	3,638.7		(1,389.1		)	2,249.6		3,638.7		(1,330.2		)	2,308.5
Total intangible assets		$	7,731.8	$	(4,675.6	)	$	3,056.2	$	7,727.3	$	(4,607.3	)	$	3,120.0

Estimated future amortization for acquired intangible assets

Based upon this most recent analysis, the estimated future amortization of acquired intangible assets for the next five years is expected to be as follows:



(In millions)	As of March 31, 2019
2019 (remaining nine months)	$	200.0
2020	290.0
2021	250.0
2022	250.0
2023	230.0
2024	190.0

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, 2019
Goodwill, beginning of period	$	5,706.4
Reclassification of goodwill to assets held for sale	(69.5		)
Other	2.8
Goodwill, end of period	$	5,639.7

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2019

Fair Value Disclosures [Abstract]

Summary of assets and liabilities recorded at fair value



As of March 31, 2019 (In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	1,881.9	$	—	$	1,881.9	$	—
Marketable debt securities:
Corporate debt securities	1,835.7		—		1,835.7		—
Government securities	921.3		—		921.3		—
Mortgage and other asset backed securities	281.5		—		281.5		—
Marketable equity securities	904.0		19.5		884.5		—
Derivative contracts	104.4		—		104.4		—
Plan assets for deferred compensation	29.4		—		29.4		—
Total	$	5,958.2	$	19.5	$	5,938.7	$	—
Liabilities:
Derivative contracts	$	24.1	$	—	$	24.1	$	—
Contingent consideration obligations	421.3		—		—		421.3
Total	$	445.4	$	—	$	24.1	$	421.3



As of December 31, 2018 (In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	705.5	$	—	$	705.5	$	—
Marketable debt securities:
Corporate debt securities	2,459.2		—		2,459.2		—
Government securities	969.6		—		969.6		—
Mortgage and other asset backed securities	260.5		—		260.5		—
Marketable equity securities	615.4		51.7		563.7		—
Derivative contracts	66.9		—		66.9		—
Plan assets for deferred compensation	25.4		—		25.4		—
Total	$	5,102.5	$	51.7	$	5,050.8	$	—
Liabilities:
Derivative contracts	$	24.6	$	—	$	24.6	$	—
Contingent consideration obligations	409.8		—		—		409.8
Total	$	434.4	$	—	$	24.6	$	409.8

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, 2019				As of December 31, 2018
(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value
2.900% Senior Notes due September 15, 2020	$	1,500.7	$	1,486.6	$	1,489.5	$	1,480.8
3.625% Senior Notes due September 15, 2022	1,022.0		995.8		1,000.4		995.5
4.050% Senior Notes due September 15, 2025	1,795.1		1,738.2		1,745.1		1,737.8
5.200% Senior Notes due September 15, 2045	1,838.0		1,722.6		1,802.6		1,722.4
Total	$	6,155.8	$	5,943.2	$	6,037.6	$	5,936.5

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)	2019		2018
Fair value, beginning of period	$	409.8	$	523.6
Changes in fair value	11.5		(5.6		)
Payments	—		(20.0		)
Fair value, end of period	$	421.3	$	498.0

Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2019

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, 2019		As of December 31, 2018
Commercial paper	$	446.0	$	231.2
Overnight reverse repurchase agreements	192.7		—
Money market funds	1,074.4		279.5
Short-term debt securities	168.8		194.8
Total	$	1,881.9	$	705.5

Marketable debt and equity securities

The following tables summarize our marketable debt and equity securities:



As of March 31, 2019 (In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Corporate debt securities
Current	$	981.0	$	0.5	$	(0.1	)	$	981.4
Non-current	852.0		2.8		(0.5		)	854.3
Government securities
Current	683.6		0.2		(0.1		)	683.7
Non-current	237.5		0.3		(0.2		)	237.6
Mortgage and other asset backed securities
Current	0.7		—		—			0.7
Non-current	280.3		0.8		(0.3		)	280.8
Total marketable debt securities	$	3,035.1	$	4.6	$	(1.2	)	$	3,038.5
Marketable equity securities, non-current	489.3		421.7		(7.0		)	904.0



As of December 31, 2018 (In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Corporate debt securities
Current	$	1,608.4	$	—	$	(0.9	)	$	1,607.5
Non-current	854.9		0.7		(3.9		)	851.7
Government securities
Current	706.1		0.1		(0.4		)	705.8
Non-current	264.0		0.1		(0.3		)	263.8
Mortgage and other asset backed securities
Current	0.1		—		—			0.1
Non-current	260.5		0.4		(0.5		)	260.4
Total marketable debt securities	$	3,694.0	$	1.3	$	(6.0	)	$	3,689.3
Marketable equity securities, non-current	496.2		127.7		(8.5		)	615.4

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, 2019				As of December 31, 2018
(In millions)	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Due in one year or less	$	1,665.3	$	1,665.8	$	2,314.6	$	2,313.4
Due after one year through five years	1,234.8		1,237.8		1,235.9		1,232.7
Due after five years	135.0		134.9		143.5		143.2
Total available-for-sale securities	$	3,035.1	$	3,038.5	$	3,694.0	$	3,689.3

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)	2019			2018
Proceeds from maturities and sales	$	1,489.2		$	4,068.9
Realized gains	$	0.6		$	1.8
Realized losses	$	(0.3	)	$	(9.4	)

Derivative Instruments (Tables)

3 Months Ended

Mar. 31, 2019

Derivative Instruments and Hedging Activities Disclosure [Abstract]

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

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



	Notional Amount
(In millions)	As of March 31, 2019		As of December 31, 2018
Euro	$	1,722.2	$	1,701.4
British pound	170.9		215.3
Swiss franc	99.2		131.4
Japanese yen	79.5		98.8
Canadian dollar	72.2		92.2
Total foreign currency forward contracts	$	2,144.0	$	2,239.1

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	2019			2018			Location	2019			2018
Revenues	$	14.8		$	(32.9	)	Revenues	$	3.7		$	(0.9	)
Operating expenses	$	(0.5	)	$	1.3		Operating expenses	$	(0.9	)	$	(0.3	)

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	2019		2018		Location	2019		2018		Location	2019		2018
Gains (losses) on net investment hedge	$	11.8	$	—	Gains (losses) on net investment hedge	$	4.4	$	—	Other income (expense)	$	2.2	$	—

Summary of fair value and presentation of derivatives



(In millions)	Balance Sheet Location	As of March 31, 2019		As of December 31, 2018
Cash Flow Hedging Instruments:
Asset derivative instruments	Other current assets	$	101.4	$	65.8
Liability derivative instruments	Accrued expenses and other	$	6.6	$	6.9
Fair Value Hedging Instruments:
Liability derivative instruments	Other long-term liabilities	$	9.3	$	14.5
Other Derivatives:
Asset derivative instruments	Other current assets	$	3.0	$	1.1
Liability derivative instruments	Accrued expenses and other	$	8.2	$	3.2

Leases Leases (Tables)

3 Months Ended

Mar. 31, 2019

Leases [Abstract]

Schedule of operating leases

The following table summarizes the presentation in our condensed consolidated balance sheets of our operating leases:



(In millions)	Balance sheet location	As of March 31, 2019
Assets:
Operating lease assets	Operating lease assets	$	447.8

Liabilities
Current operating lease liabilities	Accrued expenses and other	$	72.5
Non-current operating lease liabilities	Long-term operating lease liabilities	436.1
Total operating lease liabilities		$	508.6

Operating lease cost

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



		For the Three Months Ended March 31,
(In millions)	Income Statement Location	2019
Operating lease cost	Research and development	$	0.3
	Selling, general and administrative	23.5
Sublease income	Selling, general and administrative	(7.1		)
	Other (income) expense, net	(1.0		)
Net lease cost		$	15.7

Operating lease liability maturity

The minimum lease payments, net of income from subleases, for the next five years and thereafter is expected to be as follows:



(In millions)	As of March 31, 2019
2019 (remaining nine months)	$	69.4
2020	81.7
2021	76.0
2022	69.2
2023	68.6
2024	66.1
Thereafter	146.2
Total lease payments	$	577.2
Less: interest	68.6
Present value of operating lease liabilities	$	508.6

Minimum lease payments



(In millions)	2019			2020			2021			2022			2023			Thereafter			Total
Minimum lease payments	$	87.0		$	80.7		$	75.9		$	71.7		$	71.0		$	215.3		$	601.6
Less: income from subleases (1)	(26.8		)	(25.6		)	(23.7		)	(24.0		)	(24.3		)	(58.4		)	(182.8		)
Net minimum lease payments	$	60.2		$	55.1		$	52.2		$	47.7		$	46.7		$	156.9		$	418.8


(1)	Represents sublease income expected to be received for the vacated manufacturing facility in Cambridge, MA, the vacated portion of our Weston, MA facility and other facilities throughout the world.

Operating lease weighted average remaining term and discount rate

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



	As of March 31, 2019
Weighted average remaining lease term in years	7.88
Weighted average discount rate	3.3	%

Operating lease supplemental cash flow disclosure



	For the Three Months Ended March 31,
(In millions)	2019
Cash paid for amounts included in the measurement of lease liabilities	$	19.4
Operating lease assets obtained in exchange for lease obligations	$	5.4

Equity Accumulated Other Comprehensive Income (Loss) (Tables)

3 Months Ended

Mar. 31, 2019

Equity [Abstract]

Schedule of Accumulated Other Comprehensive Income (Loss)

The following table summarizes 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, 2018	$	(4.0	)	$	34.7		$	3.5		$	(31.3	)	$	(243.3	)	$	(240.4	)
Other comprehensive income (loss) before reclassifications	7.1			31.2			16.2			0.6			(17.8		)	37.3
Amounts reclassified from accumulated other comprehensive income (loss)	(0.2		)	(14.3		)	(2.2		)	—			—			(16.7		)
Net current period other comprehensive income (loss)	6.9			16.9			14.0			0.6			(17.8		)	20.6
Balance, March 31, 2019	$	2.9		$	51.6		$	17.5		$	(30.7	)	$	(261.1	)	$	(219.8	)



(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, 2017	$	(1.6	)	$	(104.5	)	$	—	$	(36.8	)	$	(175.5	)	$	(318.4	)
Amounts reclassified, net of tax, upon adoption of ASU No. 2016-01	1.5			—			—		—			—			1.5
Balance, January 1, 2018	(0.1		)	(104.5		)	—		(36.8		)	(175.5		)	(316.9		)
Other comprehensive income (loss) before reclassifications	(8.2		)	(60.4		)	—		(0.5		)	44.7			(24.4		)
Amounts reclassified from accumulated other comprehensive income (loss)	6.0			31.4			—		—			—			37.4
Net current period other comprehensive income (loss)	(2.2		)	(29.0		)	—		(0.5		)	44.7			13.0
Balance, March 31, 2018	$	(2.3	)	$	(133.5	)	$	—	$	(37.3	)	$	(130.8	)	$	(303.9	)

Reclassification out of Accumulated Other Comprehensive Income

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



(In millions)	Income Statement Location	Amounts Reclassified from Accumulated Other Comprehensive Income
		For the Three Months Ended March 31,
		2019			2018
Gains (losses) on securities available for sale	Other income (expense)	$	0.3		$	(7.6	)
	Income tax benefit (expense)	(0.1		)	1.6

Gains (losses) on cash flow hedges	Revenues	14.8			(32.9		)
	Operating expenses	(0.5		)	1.3
	Other income (expense)	0.1			0.1
	Income tax benefit (expense)	(0.1		)	0.1

Gains (losses) on net investment hedge	Other income (expense)	2.2			—
	Income tax benefit (expense)	—			—

Total reclassifications, net of tax		$	16.7		$	(37.4	)

Earnings per Share (Tables)

3 Months Ended

Mar. 31, 2019

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)	2019		2018
Numerator:
Net income attributable to Biogen Inc.	$	1,408.8	$	1,172.9
Denominator:
Weighted average number of common shares outstanding	196.6		211.4
Effect of dilutive securities:
Stock options and employee stock purchase plan	—		—
Time-vested restricted stock units	0.3		0.2
Market stock units	0.1		0.1
Performance stock units settled in stock	—		—
Dilutive potential common shares	0.4		0.3
Shares used in calculating diluted earnings per share	197.0		211.7

Share-Based Payments (Tables)

3 Months Ended

Mar. 31, 2019

Disclosure of Compensation Related Costs, Share-based Payments [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)	2019			2018
Research and development	$	21.7		$	21.9
Selling, general and administrative	27.8			28.5
Subtotal	49.5			50.4
Capitalized share-based compensation costs	(3.3		)	(3.4		)
Share-based compensation expense included in total cost and expenses	46.2			47.0
Income tax effect	(7.4		)	(7.6		)
Share-based compensation expense included in net income attributable to Biogen Inc.	$	38.8		$	39.4

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)	2019			2018
Market stock units	$	7.7		$	6.1
Time-vested restricted stock units	34.7			36.1
Cash settled performance units	(1.0		)	4.3
Performance units	0.5			(0.8		)
Performance stock units settled in stock	2.0			0.7
Performance stock units settled in cash	1.0			0.1
Employee stock purchase plan	4.6			3.9
Subtotal	49.5			50.4
Capitalized share-based compensation costs	(3.3		)	(3.4		)
Share-based compensation expense included in total cost and expenses	$	46.2		$	47.0

Income Taxes (Tables)

3 Months Ended

Mar. 31, 2019

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,
	2019		2018
Statutory rate	21.0	%	21.0	%
State taxes	0.4		0.9
Taxes on foreign earnings	(4.6	)	(2.0	)
Credits and net operating loss utilization	(0.8	)	(0.8	)
Purchased intangible assets	0.3		0.6
Denmark assets held for sale	4.3		—
Global Intangible Low-Taxed Income (GILTI)	1.9		1.3
Other permanent items	0.4		0.4
Other	(0.2	)	0.2
Effective tax rate	22.7	%	21.6	%

Other Consolidated Financial Statement Detail (Tables)

3 Months Ended

Mar. 31, 2019

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)	2019			2018
Interest income	$	31.2		$	26.7
Interest expense	(47.9		)	(50.5		)
Gain (loss) on investments, net	376.4			(14.4		)
Foreign exchange gains (losses), net	(2.2		)	(1.0		)
Other, net	(0.2		)	(1.8		)
Total other income (expense), net	$	357.3		$	(41.0	)

Accrued expenses and other

Accrued expenses and other consists of the following:



(In millions)	As of March 31, 2019		As of December 31, 2018
Revenue-related reserves for discounts and allowances	$	907.6	$	874.7
Collaboration expenses	216.6		261.6
Royalties and licensing fees	195.0		224.7
Employee compensation and benefits	160.8		320.9
Current portion of contingent consideration obligations	147.3		444.8
Construction in progress	99.7		125.2
Other	708.0		609.3
Total accrued expenses and other	$	2,435.0	$	2,861.2

Collaborative and Other Relationships Collaborative and Other Relationships (Tables)

3 Months Ended

Mar. 31, 2019

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Summary of Activity Related to BAN2401 and Elenbecestat Collaboration

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



	For the Three Months Ended March 31,
(In millions)	2019		2018
Total development expense incurred by the collaboration related to the advancement of BAN2401 and Elenbecestat	$	68.0	$	52.2
Biogen's share of BAN2401 and Elenbecestat development expense reflected in research and development expense in our condensed consolidated statements of income	$	34.0	$	26.1

Summary of Activity Related to Aducanumab Collaboration

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



	For the Three Months Ended March 31,
(In millions)	2019		2018
Total aducanumab development expense	$	162.5	$	63.6
Biogen's share of aducanumab development expense reflected in research and development expense in our condensed consolidated statements of income	$	89.4	$	63.6

Total aducanumab sales and marketing expense incurred by the collaboration	$	20.9	$	7.1
Biogen's share of aducanumab sales and marketing expense reflected in selling, general and administrative expense our condensed consolidated statements of income	$	11.6	$	3.9

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) segment	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
Operating lease assets	$ 447.8		$ 0.0
Operating lease liabilities	$ 508.6
Number of reportable segments \| segment	1
Interest in subsidiary (less than given percentage)	100.00%
Accounting Standards Update 2016-02
Operating lease assets		$ 463.0
Operating lease liabilities		$ 526.0

Acquisitions Business Acquisition (Details Textual) - Nightstar $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares
Business Acquisition [Line Items]
Price per share \| $ / shares	$ 25.50
Total transaction value \| $	$ 800.0

Divestitures Divestitures (Details) - USD ($) $ in Millions	3 Months Ended
Divestitures Divestitures (Details) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Long Lived Assets and Liabilities Held-for-sale [Line Items]
Loss on assets and liabilities held for sale	$ (115.5)	$ 0.0
Denmark Manufacturing Operations
Long Lived Assets and Liabilities Held-for-sale [Line Items]
Inventory	27.0
Property, plant and equipment	629.7
Operating lease assets	2.5
Goodwill	69.5
Other assets	68.8
Loss on assets and liabilities held for sale	(115.5)
Assets held for sale	682.0
Accrued expenses and other liabilities	49.0
Operating lease liabilities	2.5
Deferred tax liability	46.5
Liabilities held for sale	97.2
Long-term operating lease liabilities \| Denmark Manufacturing Operations
Long Lived Assets and Liabilities Held-for-sale [Line Items]
Operating lease liabilities	$ 1.7

Divestitures Divestitures (Details Textual) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss on assets and liabilities held for sale	$ 115.5	$ 0.0
Expected costs to sell disposal group	10.0
Denmark Manufacturing Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration expected to be received for sale of Denmark Manufacturing Operations	890.0
Loss on assets and liabilities held for sale, net of tax	174.6
Loss on assets and liabilities held for sale	115.5
Future minimum batch production for Denmark Manufacturing Operations	120.0
Tax expense on disposal group	$ 59.1

- Definition

Amount of tax expense (benefit) related to a discontinued operation. Includes, but is not limited to, tax expense (benefit) related to income (loss) from operations during the phase-out period, tax expense (benefit) related to gain (loss) on disposal, tax expense (benefit) related to gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and tax expense (benefit) related to adjustments of a prior period gain (loss) on disposal.

+ References + Details

Revenues Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Product revenues	$ 3,489.8	$ 3,131.1
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	998.8	986.9
TECFIDERA \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	717.7	728.9
TECFIDERA \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	281.1	258.0
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	500.9	550.3
Interferon \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	327.3	371.4
Interferon \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	173.6	178.9
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	460.4	462.1
TYSABRI \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	245.0	249.7
TYSABRI \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	215.4	212.4
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	22.9	24.4
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	22.9	24.4
ZINBRYTA
Disaggregation of Revenue [Line Items]
Product revenues	0.0	1.4
ZINBRYTA \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0
ZINBRYTA \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.0	1.4
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,983.0	2,025.1
MS Product Revenues \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,290.0	1,350.0
MS Product Revenues \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	693.0	675.1
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	518.5	363.9
SPINRAZA \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	223.3	188.0
SPINRAZA \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	295.2	175.9
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	124.0	120.9
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	124.0	120.9
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	14.7	6.6
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	14.7	6.6
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	35.7	0.0
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	35.7	0.0
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	174.4	127.5
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	174.4	127.5
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	4.1	7.0
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	4.1	7.0
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,680.0	2,523.5
Product, net \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,513.3	1,538.0
Product, net \| Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,166.7	$ 985.5

Revenues Reserves for Discounts and Allowances (Details 1) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	$ 1,051.3
Current Provisions Relating To Sales In Current Year	826.2
Adjustments Relating To Prior Years	(24.5)
Payments/Returns Relating To Sales in Current Year	(294.8)
Payments/Returns Relating To Sales in Prior Year	(474.8)
Ending Balance	1,083.4
Discounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	127.8
Current Provisions Relating To Sales In Current Year	141.2
Adjustments Relating To Prior Years	0.3
Payments/Returns Relating To Sales in Current Year	(60.8)
Payments/Returns Relating To Sales in Prior Year	(92.1)
Ending Balance	116.4
Contractual adjustments
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	888.8
Current Provisions Relating To Sales In Current Year	680.3
Adjustments Relating To Prior Years	(25.1)
Payments/Returns Relating To Sales in Current Year	(233.9)
Payments/Returns Relating To Sales in Prior Year	(378.1)
Ending Balance	932.0
Returns
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	34.7
Current Provisions Relating To Sales In Current Year	4.7
Adjustments Relating To Prior Years	0.3
Payments/Returns Relating To Sales in Current Year	(0.1)
Payments/Returns Relating To Sales in Prior Year	(4.6)
Ending Balance	$ 35.0

Revenues Reserves for Discounts and Allowances (Details 2) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Total Reserves	$ 1,083.4	$ 1,051.3
Reduction of accounts receivable
Total Reserves	175.8	176.6
Component of accrued expenses and other
Total Reserves	$ 907.6	$ 874.7

Revenues Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 3,489.8	$ 3,131.1
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	390.8	349.6
Other revenues from anti-CD20 therapeutic programs	126.6	93.6
Revenues from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 517.4	$ 443.2

Revenues Other Revenues (Details) - USD ($) $ in Millions	3 Months Ended
Revenues Other Revenues (Details) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ (3,489.8)	$ (3,131.1)
Royalty
Disaggregation of Revenue [Line Items]
Total revenues	(3.9)	(10.6)
Other corporate revenues
Disaggregation of Revenue [Line Items]
Total revenues	(264.1)	(140.6)
Other
Disaggregation of Revenue [Line Items]
Total revenues	(292.4)	(164.4)
Collaborative arrangement \| AbbVie
Disaggregation of Revenue [Line Items]
Total revenues	(0.4)	(4.7)
Collaborative arrangement \| Samsung Bioepis
Disaggregation of Revenue [Line Items]
Total revenues	$ (24.8)	$ (17.9)

Revenues Revenues (Details Textual) - USD ($) $ in Millions	3 Months Ended
Revenues Revenues (Details Textual) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Other Revenues	$ 3,489.8	$ 3,131.1
Distributor One
Disaggregation of Revenue [Line Items]
Percentage of revenues from major distributors	31.30%	34.00%
Distributor Two
Disaggregation of Revenue [Line Items]
Percentage of revenues from major distributors	14.20%	15.90%
Other
Disaggregation of Revenue [Line Items]
Other Revenues	$ 292.4	$ 164.4
Other \| Bioverativ
Disaggregation of Revenue [Line Items]
Other Revenues	$ 206.8	$ 47.0

Inventory (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Components of inventories
Raw materials	$ 199.7	$ 196.3
Work in process	462.3	606.7
Finished goods	108.2	133.5
Total inventory	770.2	936.5
Inventory, current	770.2	929.9
Inventory, Noncurrent	$ 0.0	$ 6.6

Inventory Inventory (Details Textual) $ in Millions	3 Months Ended
Inventory Inventory (Details Textual) $ in Millions	Mar. 31, 2019 USD ($)
Bioverativ
Inventory [Line Items]
Inventory sold to Bioverativ, cost	$ 173.5

Inventory Proposed Divestiture of Denmark Manufacturing Operations (Details) $ in Millions	Mar. 31, 2019 USD ($)
Denmark Manufacturing Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration expected to be received for sale of Denmark Manufacturing Operations	$ 890.0
Inventory expected to be sold to FUJIFILM	27.0
Work-in-process
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Inventory expected to be sold to FUJIFILM	$ 27.0

Intangible Assets and Goodwill (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Intangible assets
Total intangible assets, gross	$ 7,731.8		$ 7,727.3
Accumulated Amortization	(4,675.6)		(4,607.3)
Intangible assets, net	3,056.2		3,120.0
Amortization and impairment of acquired intangible assets	68.2	$ 103.9
Intangible Assets and Goodwill (Textual) [Abstract]
Accumulated impairment losses related to goodwill	0.0
Expected future amortization expense, 2019 (remaining nine months)	200.0
Expected future amortization expense, 2020	290.0
Expected future amortization expense, 2021	250.0
Expected future amortization expense, 2022	250.0
Expected future amortization expense, 2023	230.0
Expected future amortization expense, 2024	190.0
Out-licensed patents
Intangible assets
Cost	543.3		543.3
Net	0.9		1.0
Accumulated Amortization	$ (542.4)		(542.3)
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
Intangible assets
Cost	$ 3,005.3		3,005.3
Net	261.2		270.5
Accumulated Amortization	$ (2,744.1)		(2,734.8)
Developed technology \| Minimum
Intangible assets
Estimated life, (in years)	15 years
Developed technology \| Maximum
Intangible assets
Estimated life, (in years)	23 years
Developed technology \| AVONEX
Intangible assets
Net	$ 256.2
Acquired and in-licensed rights and patents
Intangible assets
Cost	3,638.7		3,638.7
Net	2,249.6		2,308.5
Accumulated Amortization	$ (1,389.1)		(1,330.2)
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
Acquired and in-licensed rights and patents \| TYSABRI
Intangible assets
Net	$ 1,980.8
Acquired and in-licensed rights and patents \| TECFIDERA
Intangible assets
Net	$ 63.1
In Process Research and Development
Intangible assets
Indefinite lived intangible assets useful life	Indefinite until commercialization
Cost and Net	$ 480.5		476.0
Accumulated Amortization	$ 0.0		0.0
Trademarks and Trade Names
Intangible assets
Indefinite lived intangible assets useful life	Indefinite
Cost and Net	$ 64.0		64.0
Accumulated Amortization	$ 0.0		$ 0.0

Intangible Assets and Goodwill (Details 1) $ in Millions	3 Months Ended
Intangible Assets and Goodwill (Details 1) $ in Millions	Mar. 31, 2019 USD ($)
Summary of roll forward of the changes in goodwill
Goodwill, beginning of period	$ 5,706.4
Other	2.8
Goodwill, end of period	5,639.7
Denmark Manufacturing Operations
Summary of roll forward of the changes in goodwill
Goodwill held for sale	$ (69.5)

Fair Value Measurements (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets:
Marketable debt securities	$ 3,038.5	$ 3,689.3
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	1,881.9	705.5
Marketable equity securities	904.0	615.4
Derivative contracts	104.4	66.9
Plan assets for deferred compensation	29.4	25.4
Total	5,958.2	5,102.5
Liabilities:
Derivative contracts	24.1	24.6
Contingent consideration obligations	421.3	409.8	$ 498.0	$ 523.6
Total	445.4	434.4
Fair Value, Measurements, Recurring \| Corporate debt securities
Assets:
Marketable debt securities	1,835.7	2,459.2
Fair Value, Measurements, Recurring \| Government securities
Assets:
Marketable debt securities	921.3	969.6
Fair Value, Measurements, Recurring \| Mortgage and other asset backed securities
Assets:
Marketable debt securities	281.5	260.5
Quoted Prices in Active Markets (Level 1) \| Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0.0	0.0
Marketable equity securities	19.5	51.7
Derivative contracts	0.0	0.0
Plan assets for deferred compensation	0.0	0.0
Total	19.5	51.7
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	1,881.9	705.5
Marketable equity securities	884.5	563.7
Derivative contracts	104.4	66.9
Plan assets for deferred compensation	29.4	25.4
Total	5,938.7	5,050.8
Liabilities:
Derivative contracts	24.1	24.6
Contingent consideration obligations	0.0	0.0
Total	24.1	24.6
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Corporate debt securities
Assets:
Marketable debt securities	1,835.7	2,459.2
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Government securities
Assets:
Marketable debt securities	921.3	969.6
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Mortgage and other asset backed securities
Assets:
Marketable debt securities	281.5	260.5
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	421.3	409.8
Total	421.3	409.8
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 1) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Notes payable, carrying value	$ 5,943.2	$ 5,936.5
Debt instruments, fair value	6,155.8	6,037.6
2.900% Senior Notes due 2020
Debt Instrument [Line Items]
Notes payable, fair value	1,500.7	1,489.5
Notes payable, carrying value	1,486.6	1,480.8
3.625% Senior Notes due 2022
Debt Instrument [Line Items]
Notes payable, fair value	1,022.0	1,000.4
Notes payable, carrying value	995.8	995.5
4.050% Senior Notes due 2025
Debt Instrument [Line Items]
Notes payable, fair value	1,795.1	1,745.1
Notes payable, carrying value	1,738.2	1,737.8
5.200% Senior Notes due 2045
Debt Instrument [Line Items]
Notes payable, fair value	1,838.0	1,802.6
Notes payable, carrying value	$ 1,722.6	$ 1,722.4

Fair Value Measurements (Details 2) - USD ($) $ in Millions	3 Months Ended
Fair Value Measurements (Details 2) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration	$ 11.5	$ (5.6)
Payments	0.0	(20.0)
Fair Value, Measurements, Recurring
Business Acquisition, Contingent Consideration [Line Items]
Fair value, beginning of period	409.8	523.6
Fair value, end of period	$ 421.3	$ 498.0

Fair Value Measurements (Details Textual) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Business Acquisition, Contingent Consideration [Line Items]
Asset impairment charges	$ 0.0
Fair value measurements, changes in valuation inputs	0
Contingent consideration obligations	$ 274.0	$ 265.0
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%

Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 1,881.9	$ 705.5
Commercial paper
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	446.0	231.2
Overnight reverse repurchase agreements
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	192.7	0.0
Money market funds
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	1,074.4	279.5
Short-term debt securities
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 168.8	$ 194.8

Financial Instruments (Details 1) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 3,035.1	$ 3,694.0
Gross unrealized gains	4.6	1.3
Gross unrealized losses	(1.2)	(6.0)
Fair value	3,038.5	3,689.3
Marketable equity securities, fair value	3,038.5	3,689.3
Corporate debt securities Current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	981.0	1,608.4
Gross unrealized gains	0.5	0.0
Gross unrealized losses	(0.1)	(0.9)
Fair value	981.4	1,607.5
Corporate debt securities Non-current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	852.0	854.9
Gross unrealized gains	2.8	0.7
Gross unrealized losses	(0.5)	(3.9)
Fair value	854.3	851.7
Government securities Current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	683.6	706.1
Gross unrealized gains	0.2	0.1
Gross unrealized losses	(0.1)	(0.4)
Fair value	683.7	705.8
Government securities Non-current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	237.5	264.0
Gross unrealized gains	0.3	0.1
Gross unrealized losses	(0.2)	(0.3)
Fair value	237.6	263.8
Mortgage and other asset backed securities Current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	0.7	0.1
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Fair value	0.7	0.1
Mortgage and other asset backed securities Non-current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	280.3	260.5
Gross unrealized gains	0.8	0.4
Gross unrealized losses	(0.3)	(0.5)
Fair value	280.8	260.4
Marketable equity securities
Debt Securities, Available-for-sale [Line Items]
Marketable equity securities, amortized cost	489.3	496.2
Marketable equity securities, gross unrealized gains	421.7	127.7
Marketable equity securities, gross unrealized losses	(7.0)	(8.5)
Marketable equity securities, fair value	$ 904.0	$ 615.4

Financial Instruments (Details 2) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, amortized cost	$ 1,665.3	$ 2,314.6
Due in one year or less, estimated fair value	1,665.8	2,313.4
Due after one year through five years, amortized cost	1,234.8	1,235.9
Due after one year through five years, estimated fair value	1,237.8	1,232.7
Due after five years, amortized cost	135.0	143.5
Due after five years, estimated fair value	134.9	143.2
Amortized cost	3,035.1	3,694.0
Available-for-sale Securities	$ 3,038.5	$ 3,689.3

Financial Instruments (Details 3) - USD ($) $ in Millions	3 Months Ended
Financial Instruments (Details 3) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Proceeds from maturities and sales	$ 1,489.2	$ 4,068.9
Realized gains	0.6	1.8
Realized losses	$ (0.3)	$ (9.4)

Financial Instruments (Details Textual)	3 Months Ended
Financial Instruments (Details Textual)	Mar. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Average maturity of marketable securities, months	12 months

Financial Instruments Financial Instruments (Details Textual 2) - USD ($) shares in Millions, $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Jun. 05, 2018
Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio	$ 1,049.7
Ionis Pharmaceuticals
Business Acquisition [Line Items]
Investment in common stock, shares purchased			11.5
Purchase of common stock			$ 625.0
Other current assets \| Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio	$ 90.1
Other noncurrent assets \| Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio		$ 676.3

Derivative Instruments (Details) $ in Millions, ₩ in Billions	3 Months Ended				12 Months Ended
	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 KRW (₩)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018
Derivatives [Line Items]
Gain/Loss on fair value of foreign currency forward contracts	$ 44.3	$ (27.3)			$ (27.3)
Gain (losses) on net investment hedge	14.0			$ 0.0
Net gains (losses) of other income (expense) related to foreign currency forward contracts	$ (4.8)			5.6
Minimum
Derivatives [Line Items]
Range of durations of foreign currency forward contracts	1 month
Maximum
Derivatives [Line Items]
Range of durations of foreign currency forward contracts	12 months
Not designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	$ 895.0	735.1			735.1
Other current assets \| Designated as hedging instrument
Derivatives [Line Items]
Derivative asset, fair value	101.4	65.8			65.8
Other current assets \| Not designated as hedging instrument
Derivatives [Line Items]
Derivative asset, fair value	3.0	1.1			1.1
Accrued expenses and other \| Designated as hedging instrument
Derivatives [Line Items]
Derivative liability, fair value	6.6	6.9			6.9
Accrued expenses and other \| Not designated as hedging instrument
Derivatives [Line Items]
Derivative liability, fair value	8.2	3.2			3.2
Foreign exchange contract \| Designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	2,144.0	2,239.1			$ 2,239.1
Interest rate swap \| Designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	675.0
Net Investment Hedging
Derivatives [Line Items]
Remaining duration of Net Investment Hedges					7 months
Unrealized gain (loss) on net investment hedges in AOCI	8.0				$ 3.8
Derivative qualifying as net investment hedge, excluded component	9.5	7.3			7.3
Gain (losses) on net investment hedge	11.8			0.0
Gains (losses) on net investment hedge, excluded component	4.4			0.0
Revenue \| Cash flows, revenue \| Foreign exchange contract
Derivatives [Line Items]
Gain (loss) on reclassification of cash flow hedges, effective portion	14.8			(32.9)
Gain (loss) recognized in net income, excluded component	3.7			(0.9)
Operating expense \| Cash flows, operating expenses \| Foreign exchange contract
Derivatives [Line Items]
Gain (loss) on reclassification of cash flow hedges, effective portion	(0.5)			1.3
Gain (loss) recognized in net income, excluded component	(0.9)			(0.3)
Other income (expense) \| Net Investment Hedging
Derivatives [Line Items]
Gain (loss) recognized in net income, excluded component	2.2			$ 0.0
Euro \| Foreign exchange contract \| Designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	1,722.2	1,701.4			1,701.4
British pound \| Foreign exchange contract \| Designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	170.9	215.3			215.3
Swiss franc \| Foreign exchange contract \| Designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	99.2	131.4			131.4
Japan, Yen \| Foreign exchange contract \| Designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	79.5	98.8			98.8
Canadian dollar \| Foreign exchange contract \| Designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	$ 72.2	92.2			92.2
2.900% Senior Notes due 2020
Derivatives [Line Items]
Senior Notes interest rate	2.90%
2.900% Senior Notes due 2020 \| Interest rate swap \| Other long-term liabilities \| Designated as hedging instrument
Derivatives [Line Items]
Derivative liability, fair value	$ 9.3	14.5			$ 14.5
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

Property, Plant and Equipment Property, Plant and Equipment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 1,841.0		$ 1,797.4
Depreciation	52.9	$ 65.0
Solothurn, Switzerland
Property, Plant and Equipment [Line Items]
Construction in progress	1,700.0		$ 1,600.0
Contractual commitments for the construction of the facility	92.0
Building
Property, Plant and Equipment [Line Items]
Property, plant and equipment held for sale	312.8
Machinery and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment held for sale	$ 286.5

Leases (Details) - USD ($) $ in Millions	3 Months Ended
Leases (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Operating lease assets	$ 447.8	$ 0.0
Long-term operating lease liabilities	436.1	0.0
Operating lease liabilities	508.6
Operating lease cost	15.7
Net lease cost	15.7
ASC 842 - Minimum lease payments 2019 (remaining nine months)	69.4
ASC 842 - Minimum lease payments, 2020	81.7
ASC 842 - Minimum lease payments, 2021	76.0
ASC 842 - Minimum lease payments, 2022	69.2
ASC 842 - Minimum lease payments, 2023	68.6
ASC 842 - Minimum lease payments, 2024	66.1
ASC 842 - Minimum lease payments, thereafter	146.2
ASC 842 - Minimum lease payments	577.2
Less: Interest	$ 68.6
ASC 840 - Minimum lease payments, 2019 (remaining 9 months)		87.0
ASC 840 - Minimum lease payments, 2020		80.7
ASC 840 - Minimum lease payments, 2021		75.9
ASC 840 - Minimum lease payments, 2022		71.7
ASC 940 - Minimum lease payments, 2023		71.0
ASC 840 - Minimum lease payments, thereafter		215.3
ASC 840 - Minimum lease payments total		601.6
ASC 840 - Sublease income, 2019 (remaining 9 months)		26.8
ASC 840 - Sublease income, 2020		25.6
ASC 840 - Sublease income, 2021		23.7
ASC 840 - Sublease income, 2022		24.0
ASC 840 - Sublease income, 2023		24.3
ASC 840 - Sublease income, thereafter		58.4
ASC 840 - Sublease income total		182.8
ASC 840 - Net minimum lease payments 2019 (remaining 9 months)		60.2
ASC 840 - Net minimum lease payments, 2020		55.1
ASC 840 - Net minimum lease payments, 2021		52.2
ASC 840 - Net minimum lease payments, 2022		47.7
ASC 840 - Net minimum lease payments, 2023		46.7
ASC 840 - Net minimum lease payments, thereafter		156.9
ASC 840 - Net minimum lease payments total		$ 418.8
Weighted average remaining lease term in years	7 years 10 months 18 days
Weighted average discount rate	3.33%
Cash paid for amounts included in measurement of lease liabilities	$ 19.4
Operating lease assets obtained in exchange for lease obligations	5.4
Research and development
Operating lease cost	0.3
Net lease cost	0.3
Selling, general and administrative
Operating lease cost	23.5
Sublease income	(7.1)
Net lease cost	23.5
Other income (expense)
Sublease income	(1.0)
Operating lease assets
Operating lease assets	447.8
Accrued expenses and other
Operating lease liability, current	72.5
Long-term operating lease liabilities
Long-term operating lease liabilities	$ 436.1

Leases Leases (Details Textual) $ in Millions	Mar. 31, 2019 USD ($)
Minimum
Operating Leased Assets [Line Items]
Lessee, Operating Lease, Term of Contract	1 year
Lessee, Operating Lease, Renewal Term	1 year
Maximum
Operating Leased Assets [Line Items]
Lessee, Operating Lease, Term of Contract	10 years
Lessee, Operating Lease, Renewal Term	6 years
Denmark Manufacturing Operations
Operating Leased Assets [Line Items]
Operating lease assets	$ 2.5
Operating lease liabilities	$ 2.5

Equity Share Repurchases (Details) - USD ($) shares in Millions, $ in Millions	1 Months Ended	3 Months Ended
	Apr. 24, 2019	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2016
Class of Stock [Line Items]
Payments for repurchase of common stock		$ 655.8	$ 250.0
2019 Share Repurchase Program
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount		$ 5,000.0
2018 Share Repurchase Program
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount				$ 3,500.0
Repurchase of common stock, at cost, shares		2.4
Payments for repurchase of common stock		$ 655.8
2016 Share Repurchase Program
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount					$ 5,000.0
Repurchase of common stock, at cost, shares			0.9
Payments for repurchase of common stock			$ 250.0
Subsequent Event \| 2018 Share Repurchase Program
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount	$ 1,000.0
Repurchase of common stock, at cost, shares	0.0
Payments for repurchase of common stock	$ 491.6

Equity Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	$ (240,400,000)	$ (318,400,000)
Balance, January 1, 2018	(240,400,000)	(318,400,000)	$ (316,900,000)
Other comprehensive income (loss), before reclassifications, net of tax	37,300,000	(24,400,000)
Amounts reclassified from accumulated other comprehensive income, net of tax	(16,700,000)	37,400,000
Other comprehensive income (loss), net of tax	20,600,000	13,200,000
Accumulated other comprehensive income (loss), net of tax ending balance	(219,800,000)	(303,900,000)
Other comprehensive income (loss), net of tax	20,600,000	13,000,000
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	(4,000,000)	(1,600,000)
Balance, January 1, 2018	(4,000,000)	(1,600,000)	(100,000)
Other comprehensive income (loss), before reclassifications, net of tax	7,100,000	(8,200,000)
Amounts reclassified from accumulated other comprehensive income, net of tax	(200,000)	6,000,000
Other comprehensive income (loss), net of tax	6,900,000	(2,200,000)
Accumulated other comprehensive income (loss), net of tax ending balance	2,900,000	(2,300,000)
Unrealized gains (losses) on cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	34,700,000	(104,500,000)
Balance, January 1, 2018	34,700,000	(104,500,000)	(104,500,000)
Other comprehensive income (loss), before reclassifications, net of tax	31,200,000	(60,400,000)
Amounts reclassified from accumulated other comprehensive income, net of tax	(14,300,000)	31,400,000
Other comprehensive income (loss), net of tax	16,900,000	(29,000,000)
Accumulated other comprehensive income (loss), net of tax ending balance	51,600,000	(133,500,000)
Gains (losses) on net investment hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	3,500,000	0
Balance, January 1, 2018	3,500,000	0	0
Other comprehensive income (loss), before reclassifications, net of tax	16,200,000	0
Amounts reclassified from accumulated other comprehensive income, net of tax	(2,200,000)	0
Other comprehensive income (loss), net of tax	14,000,000	0
Accumulated other comprehensive income (loss), net of tax ending balance	17,500,000	0
Unfunded status of postretirement benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	(31,300,000)	(36,800,000)
Balance, January 1, 2018	(31,300,000)	(36,800,000)	(36,800,000)
Other comprehensive income (loss), before reclassifications, net of tax	600,000	(500,000)
Amounts reclassified from accumulated other comprehensive income, net of tax	0	0
Other comprehensive income (loss), net of tax	600,000	(500,000)
Accumulated other comprehensive income (loss), net of tax ending balance	(30,700,000)	(37,300,000)
Currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	(243,300,000)	(175,500,000)
Balance, January 1, 2018	(243,300,000)	(175,500,000)	(175,500,000)
Other comprehensive income (loss), before reclassifications, net of tax	(17,800,000)	44,700,000
Amounts reclassified from accumulated other comprehensive income, net of tax	0	0
Other comprehensive income (loss), net of tax	(17,800,000)	44,700,000
Accumulated other comprehensive income (loss), net of tax ending balance	$ (261,100,000)	$ (130,800,000)
Accounting Standards Update 2016-01
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption			1,500,000
Accounting Standards Update 2016-01 \| Unrealized gains (losses) on securities available for sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption			1,500,000
Accounting Standards Update 2016-01 \| Unrealized gains (losses) on cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption			0
Accounting Standards Update 2016-01 \| Gains (losses) on net investment hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption			0
Accounting Standards Update 2016-01 \| Unfunded status of postretirement benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption			0
Accounting Standards Update 2016-01 \| Currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption			$ 0

Equity Reclassification out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Nonoperating Income (Expense)	$ 357.3	$ (41.0)
Income Tax Expense (Benefit)	422.5	322.5
Product revenues	3,489.8	3,131.1
Net income attributable to Biogen Inc.	1,408.8	1,172.9
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.	16.7	(37.4)
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)	0.3	(7.6)
Income Tax Expense (Benefit)	(0.1)	1.6
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)	0.1	0.1
Income Tax Expense (Benefit)	(0.1)	0.1
Product revenues	14.8	32.9
Operating Expenses	0.5	1.3
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)	2.2	0.0
Income Tax Expense (Benefit)	$ 0.0	$ 0.0

Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net income attributable to Biogen Inc.	$ 1,408.8	$ 1,172.9
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	196.6	211.4
Effect of dilutive securities:
Dilutive potential common shares	0.4	0.3
Shares used in calculating diluted earnings per share	197.0	211.7
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock units	0.0	0.0
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.3	0.2
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	0.0	0.0

Share-Based Payments (Details) - USD ($) $ in Millions	3 Months Ended
Share-Based Payments (Details) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	$ 49.0	$ 46.7
Subtotal	49.5	50.4
Capitalized share-based compensation costs	(3.3)	(3.4)
Share-based compensation expense included in total costs and expenses	46.2	47.0
Income tax effect	(7.4)	(7.6)
Research and development
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	21.7	21.9
Selling, general and administrative
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	27.8	28.5
Total share-based compensation expense, net of tax
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	$ 38.8	$ 39.4

Share-Based Payments (Details 1) - USD ($) $ in Millions	3 Months Ended
Share-Based Payments (Details 1) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	$ 49.0	$ 46.7
Subtotal	49.5	50.4
Capitalized share-based compensation costs	(3.3)	(3.4)
Share-based compensation expense included in total costs and expenses	46.2	47.0
Market stock units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	7.7	6.1
Time-vested restricted stock units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	34.7	36.1
Cash settled performance shares
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	(1.0)	4.3
Performance units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	0.5	(0.8)
Performance stock units settled in stock
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	2.0	0.7
Performance stock units settled in cash
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	1.0	0.1
Employee stock purchase plan
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	$ 4.6	$ 3.9

Income Taxes (Details)	3 Months Ended
Income Taxes (Details)	Mar. 31, 2019	Mar. 31, 2018
Reconciliation between the U.S. federal statutory tax rate and effective tax rate
Statutory rate	21.00%	21.00%
State taxes	0.40%	0.90%
Taxes on foreign earnings	(4.60%)	(2.00%)
Credits and net operating loss utilization	(0.80%)	(0.80%)
Purchased intangible assets	0.30%	0.60%
Denmark assets held for sale	4.30%	0.00%
Effective Tax Rate Reconciliation, GILTI tax	1.90%	1.30%
Permanent items	0.40%	0.40%
Other	(0.20%)	0.20%
Effective tax rate	22.70%	21.60%

Income Taxes (Details Textual) $ in Millions	3 Months Ended
Income Taxes (Details Textual) $ in Millions	Mar. 31, 2019 USD ($)
Foreign tax authority
Income Tax Contingency [Line Items]
Income tax payment, including penalties and interest	$ 60.0
Denmark Manufacturing Operations
Income Tax Contingency [Line Items]
Tax expense on disposal group	59.1
Deferred tax liability held for sale	$ 46.5

- Definition

+ References + Details

Other Consolidated Financial Statement Detail (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Other Income (Expense), Net
Interest income	$ 31.2	$ 26.7
Interest expense	(47.9)	(50.5)
Gain (loss) on investments, net	376.4	(14.4)
Foreign exchange gains (losses), net	(2.2)	(1.0)
Other, net	(0.2)	(1.8)
Total other income (expense), net	357.3	$ (41.0)
Accrued Expenses and Other
Revenue-related reserves for discounts and allowances	1,083.4		$ 1,051.3
Collaboration expenses	216.6		261.6
Royalties and licensing fees	195.0		224.7
Employee compensation and benefits	160.8		320.9
Current portion of contingent consideration obligations	147.3		444.8
Construction in progress	99.7		125.2
Other	708.0		609.3
Total accrued expenses and other	2,435.0		2,861.2
Component of accrued expenses and other
Accrued Expenses and Other
Revenue-related reserves for discounts and allowances	$ 907.6		$ 874.7

Other Consolidated Financial Statement Detail Other Consolidated Financial Statement (Details Textual) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Other long-term liabilities	$ 1,353.8	$ 1,389.4
Accrued income taxes	$ 796.1	$ 791.4

Collaborative and Other Relationships Collaborative and Other Relationships - Collaborations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of future development costs related to Eisai	45.00%		15.00%
Loss on research and development contracts terminated with Eisai	$ 45.0
Eisai \| BAN2401 and Elenbecestat
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Expense incurred by the collaboration	68.0	$ 52.2
Expense reflected within statements of income	34.0	26.1
Skyhawk Therapeutics
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payment to Skyhawk	2,000.0
Upfront and milestone payments made to collaborative partner	74.0
Research and development expense	38.5
Prepaid research and development expenses	35.5
Research and development \| Eisai \| Aducanumab
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Expense incurred by the collaboration	162.5	63.6
Expense reflected within statements of income	89.4	63.6
Selling, general and administrative \| Eisai \| Aducanumab
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Expense incurred by the collaboration	20.9	7.1
Expense reflected within statements of income	$ 11.6	$ 3.9

Collaborative and Other Relationships Collaborative and Other Relationships - Equity Method Investments (Details) $ in Millions, ₩ in Billions	3 Months Ended
	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 KRW (₩)	Mar. 31, 2018 USD ($)	Mar. 31, 2019 KRW (₩)	Dec. 31, 2018 KRW (₩)	Nov. 07, 2018 USD ($)	Jun. 30, 2018
Schedule of Equity Method Investments [Line Items]
Our share of Samsung Bioepis gains (losses)	$ (28.7)			$ 0.0
Collaboration profit (loss) sharing	58.1			42.5
Total revenues	$ 3,489.8			3,131.1
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
Amortization of basis differences							$ 675.0
Loss recorded on Samsung Bioepis joint venture	$ 28.7
Our share of Samsung Bioepis gains (losses)	14.0
Amortization of basis differences	14.7
Investment in Samsung Bioepis	$ 642.1	$ 680.6			₩ 729.9	₩ 759.5
Biogen share of co-promotion profits or losses	50.00%
Collaboration profit (loss) sharing	$ 58.1			43.8
Collaborative arrangement \| Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Total revenues	$ 24.8			$ 17.9
Inventory \| Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Amortization of basis differences							115.0
Equity method investment basis difference amortization period	1 year 6 months
Developed technology \| Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Amortization of basis differences							615.0
Equity method investment basis difference amortization period	15 years
In Process Research and Development \| Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Amortization of basis differences							170.0
Deferred tax liability \| Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Amortization of basis differences							$ 225.0

Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Investment in Variable Interest Entities (Textual)
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities	$ 112.2	$ 28.7

Litigation Litigation (Details)	3 Months Ended
Litigation Litigation (Details)	Mar. 31, 2019
Brazil
Income Tax Contingency [Line Items]
Brazil tax assessment, including interest and penalties	70.0