Biogen (BIIB) 10-Q 3 May 22 2022 Q1 Quarterly report Financial data

Cover Page

Cover Page - shares	3 Months Ended
	Mar. 31, 2022	May 02, 2022
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2022
Document Transition Report	false
Entity File Number	0-19311
Entity Registrant Name	BIOGEN INC.
Entity Central Index Key	0000875045
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	33-0112644
Entity Address, Address Line One	225 Binney Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02142
City Area Code	617
Local Phone Number	679-2000
Title of 12(b) Security	Common Stock, $0.0005 par value
Trading Symbol	BIIB
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		146,452,013

Condensed Consolidated Statemen

Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Total revenue	$ 2,531.8	$ 2,694
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	753.9	478.1
Research and development	551.7	514.2
Selling, general and administrative	634.9	595
Amortization and impairment of acquired intangible assets	66.9	98.1
Collaboration profit (loss) sharing	(117.3)	68.5
(Gain) loss on fair value remeasurement of contingent consideration	(7.1)	(33.8)
Restructuring charges	38.1	0
Total cost and expense	1,921.1	1,720.1
Income from operations	610.7	973.9
Other income (expense), net	(263.3)	(506.9)
Income before income tax expense and equity in loss of investee, net of tax	347.4	467
Income tax (benefit) expense	125.6	44.2
Equity in (income) loss of investee, net of tax	3.3	18.2
Net income	218.5	404.6
Net income (loss) attributable to noncontrolling interests, net of tax	(85.3)	(5.6)
Net income attributable to Biogen Inc.	$ 303.8	$ 410.2
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 2.06	$ 2.70
Diluted earnings per share attributable to Biogen Inc.	$ 2.06	$ 2.69
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	147,100	151,900
Diluted earnings per share attributable to Biogen Inc.	147,600	152,300
Product, net
Total revenue	$ 2,066.3	$ 2,211.7
Revenue from anti-CD20 therapeutic programs
Total revenue	399.4	389
Other
Total revenue	$ 66.1	$ 93.3

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 303.8	$ 410.2
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	(9.7)	(0.8)
Unrealized gains (losses) on cash flow hedges, net of tax	15.9	149.6
Gains (losses) on net investment hedges, net of tax	6.2	22.4
Unrealized gains (losses) on pension benefit obligation, net of tax	0.9	2
Currency translation adjustment	(21.8)	(48.5)
Total other comprehensive income (loss), net of tax	(8.5)	124.7
Comprehensive income (loss) attributable to Biogen Inc.	295.3	534.9
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	(85.3)	(4.9)
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	$ 210	$ 530

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Current assets:
Cash and cash equivalents	$ 1,749.3	$ 2,261.4
Marketable securities	2,002.4	1,541.1
Accounts receivable, net	1,632	1,549.4
Due from anti-CD20 therapeutic programs	389.4	412.3
Inventory	1,215.4	1,351.5
Other current assets	927.4	740.8
Total current assets	7,915.9	7,856.5
Marketable securities	1,001.6	892
Property, plant and equipment, net	3,372.8	3,416.4
Operating lease assets	359	375.4
Intangible assets, net	2,150.8	2,221.3
Goodwill	5,758	5,761.1
Deferred tax asset	1,288.8	1,415.1
Investments and other assets	1,767.5	1,939.5
Total assets	23,614.4	23,877.3
Current liabilities:
Current portion of notes payable	999.5	999.1
Taxes payable	317.3	174.7
Accounts payable	398.7	589.2
Accrued expense and other	2,231.1	2,535.2
Total current liabilities	3,946.6	4,298.2
Notes payable	6,275.7	6,274
Deferred tax liability	571.2	694.5
Long-term operating lease liabilities	312.3	330.4
Other long-term liabilities	1,287.9	1,320.5
Total liabilities	12,393.7	12,917.6
Commitments, contingencies and guarantees
Biogen Idec Inc. shareholders' equity
Preferred stock, par value $0.001 per share	0	0
Common stock, par value $0.0005 per share	0.1	0.1
Additional paid-in capital	119	68.2
Accumulated other comprehensive income (loss)	(115.2)	(106.7)
Retained earnings	14,215.5	13,911.7
Treasury stock, at cost	(2,977.1)	(2,977.1)
Total Biogen Inc. shareholders’ equity	11,242.3	10,896.2
Noncontrolling interests	(21.6)	63.5
Total equity	11,220.7	10,959.7
Total liabilities and equity	$ 23,614.4	$ 23,877.3

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2022	Dec. 31, 2021
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, par value (in dollars per share)	$ 0.0005	$ 0.0005

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Cash flow from operating activities:
Net income	$ 218.5	$ 404.6
Adjustments to reconcile net income to net cash flow from operating activities:
Depreciation and amortization	143.1	102.6
Impairment of intangible assets	0	44.3
Excess and obsolescence charges related to inventory	281.5	11.9
Share-based compensation	67.6	70
Contingent consideration	(7.1)	(33.8)
Deferred income taxes	1	(15)
(Gain) loss on strategic investments	191.1	437.6
(Gain) loss on equity method investments	3.3	18.2
Other	43.3	59.8
Changes in operating assets and liabilities, net:
Accounts receivable	(87.5)	37.2
Due from anti-CD20 therapeutic programs	22.9	43.8
Inventory	(142.6)	(112.5)
Accrued expense and other current liabilities	(461.6)	(283.6)
Income tax assets and liabilities	101.9	64.8
Other changes in operating assets and liabilities, net	(213.6)	(80.9)
Net cash flow provided by (used in) operating activities	161.8	769
Cash flows from investing activities:
Purchases of property, plant and equipment	(57.9)	(92.6)
Proceeds from sales and maturities of marketable securities	543.6	819.2
Purchases of marketable securities	(1,133.5)	(913.3)
Proceeds from divestiture of Hillerød, Denmark manufacturing operations	0	28.1
Proceeds from the sales of strategic investments	0	(91.2)
Other	(0.2)	2.7
Net cash flow provided by (used in) investing activities	(648)	(64.7)
Cash flows from financing activities:
Purchases of treasury stock	0	(600)
Payments related to issuance of stock for share-based compensation arrangements, net	(20.8)	(27.1)
Repayment of borrowings and premiums paid on debt exchange	0	(169.3)
Net (distribution) contribution to noncontrolling interest	0.2	0
Other	4.1	11.4
Net cash flow provided by (used in) financing activities	(16.5)	(785)
Net increase (decrease) in cash and cash equivalents	(502.7)	(80.7)
Effect of exchange rate changes on cash and cash equivalents	(9.4)	(33)
Cash and cash equivalents, beginning of the period	2,261.4	1,331.2
Cash and cash equivalents, end of the period	$ 1,749.3	$ 1,217.5

Condensed Consolidated Statem_4

Condensed Consolidated Statement of Equity Statement - USD ($) shares in Millions, $ in Millions	Total	Preferred stock	Common stock	Additional paid-in capital	Accumulated Other Comprehensive Income	Retained earnings	Treasury stock	Parent	Noncontrolling interest	2020 Share Repurchase Program	2020 Share Repurchase ProgramCommon stock	2020 Share Repurchase ProgramAdditional paid-in capital	2020 Share Repurchase ProgramRetained earnings	2020 Share Repurchase ProgramTreasury stock	2020 Share Repurchase ProgramParent
Beginning balance, shares at Dec. 31, 2020		0	176.2				23.8
Beginning balance at Dec. 31, 2020	$ 10,686.1	$ 0	$ 0.1	$ 0	$ (299)	$ 13,976.3	$ (2,977.1)	$ 10,700.3	$ (14.2)
Net income	404.6					410.2		410.2	(5.6)
Other comprehensive income (loss), net of tax	125.4				124.7			124.7	0.7
Capital contribution from noncontrolling interest	0.1								0.1
Repurchase of common stock, at cost, shares										(2.2)				(2.2)
Repurchase of common stock, at cost										$ (600)				$ (600)	$ (600)
Retirement of common stock pursuant to Share Repurchase Programs, at cost, shares											(2.2)			(2.2)
Retirement of common stock pursuant to Share Repurchase Progams, at cost												$ (93.8)	$ (506.2)	$ 600
Issuance of common stock under stock option and stock purchase plans, shares			0.1
Issuance of common stock under stock option and stock purchase plans	19.7			19.7				19.7
Issuance of common stock under award plan, shares			0.3
Issuance of common stock under stock award plan	(46.8)					(46.8)		(46.8)
Compensation related to share-based payments	72.6			72.6				72.6
Stockholders' Equity, Other	1.5			1.5				1.5
Ending balance at Mar. 31, 2021	10,663.2	$ 0	$ 0.1	0	(174.3)	13,833.5	$ (2,977.1)	10,682.2	(19)
Ending balance, shares at Mar. 31, 2021		0	174.4				23.8
Beginning balance, shares at Dec. 31, 2021		0	170.8				23.8
Beginning balance at Dec. 31, 2021	10,959.7	$ 0	$ 0.1	68.2	(106.7)	13,911.7	$ (2,977.1)	10,896.2	63.5
Net income	218.5					303.8		303.8	(85.3)
Other comprehensive income (loss), net of tax	(8.5)				(8.5)			(8.5)
Capital contribution from noncontrolling interest	0.2								0.2
Issuance of common stock under stock option and stock purchase plans, shares			0.1
Issuance of common stock under stock option and stock purchase plans	18.9			18.9				18.9
Issuance of common stock under award plan, shares			0.4
Issuance of common stock under stock award plan	(39.7)			(39.7)		0		(39.7)
Compensation related to share-based payments	70.4			70.4				70.4
Stockholders' Equity, Other	1.2			1.2				1.2
Ending balance at Mar. 31, 2022	$ 11,220.7	$ 0	$ 0.1	$ 119	$ (115.2)	$ 14,215.5	$ (2,977.1)	$ 11,242.3	$ (21.6)
Ending balance, shares at Mar. 31, 2022		0	171.3				23.8

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2022
Accounting Policies [Abstract]
Summary of significant accounting policies	References in these notes to "Biogen," the "company," "we," "us" and "our" refer to Biogen Inc. and its consolidated subsidiaries. Business Overview Biogen is a global biopharmaceutical company focused on discovering, developing and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases as well as related therapeutic adjacencies. We have a leading portfolio of medicines to treat multiple sclerosis (MS), have introduced the first approved treatment for spinal muscular atrophy (SMA) and are providing the first and only approved treatment to address a defining pathology of Alzheimer's disease. We also commercialize biosimilars of advanced biologics and focus on advancing our pipeline in neuroscience and specialized immunology. Lastly, we are focused on accelerating our efforts in digital health to support our commercial and pipeline programs while also creating opportunities for potential digital therapeutics. We support our drug discovery and development efforts through the commitment of significant resources to discovery, research and development programs and business development opportunities. Our marketed products include TECFIDERA, VUMERITY, AVONEX, PLEGRIDY, TYSABRI and FAMPYRA for the treatment of MS; SPINRAZA for the treatment of SMA; ADUHELM for the treatment of Alzheimer's disease; and FUMADERM for the treatment of severe plaque psoriasis. We have certain business and financial rights with respect to RITUXAN for the treatment of non-Hodgkin's lymphoma, chronic lymphocytic leukemia (CLL) and other conditions; RITUXAN HYCELA for the treatment of non-Hodgkin's lymphoma and CLL; GAZYVA for the treatment of CLL and follicular lymphoma; OCREVUS for the treatment of primary progressive MS (PPMS) and relapsing MS (RMS); and other potential anti-CD20 therapies, including mosunetuzumab, pursuant to our collaboration arrangements with Genentech, Inc. (Genentech), a wholly-owned member of the Roche Group. For additional information on our collaboration arrangements with Genentech, please read Note 18, Collaborative and Other Relationships, to our audited consolidated financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2021 (2021 Form 10-K). Our innovative drug development and commercialization activities are complemented by our biosimilar business that expands access to medicines and reduces the cost burden for healthcare systems. Through our collaboration with Samsung Bioepis Co., Ltd. (Samsung Bioepis) we market and sell BENEPALI, an etanercept biosimilar referencing ENBREL, IMRALDI, an adalimumab biosimilar referencing HUMIRA, and FLIXABI, an infliximab biosimilar referencing REMICADE, in certain countries in Europe. We have also secured the exclusive rights to commercialize BYOOVIZ, a ranibizumab biosimilar referencing LUCENTIS, which was approved in the U.S., the European Union (E.U.) and the United Kingdom (U.K.) during the third quarter of 2021. For additional information on our collaboration arrangements with Samsung Bioepis, please read Note 16, Collaborative and Other Relationships, to these unaudited condensed consolidated financial statements (condensed consolidated financial statements). Basis of Presentation In the opinion of management, our condensed consolidated financial statements include all adjustments, consisting of normal recurring accruals, necessary for a fair statement of our financial statements for interim periods in accordance with accounting principles generally accepted in the United States (U.S. GAAP). The information included in this quarterly report on Form 10-Q should be read in conjunction with our audited consolidated financial statements and the accompanying notes included in our 2021 Form 10-K. Our accounting policies are described in the Notes to Consolidated Financial Statements in our 2021 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, 2022, are not necessarily indicative of the operating results for the full year or for any other subsequent interim period. We operate as one operating segment, focused on discovering, developing and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases as well as related therapeutic adjacencies. Consolidation Our condensed consolidated financial statements reflect our financial statements, those of our wholly-owned subsidiaries and those of certain variable interest entities where we are the primary beneficiary. For consolidated entities where we own or are exposed to less than 100.0% of the economics, we record net income (loss) attributable to noncontrolling interests, net of tax in our condensed consolidated statements of income equal to the percentage of the economic or ownership interest retained in such entities by the respective noncontrolling parties. Intercompany balances and transactions are eliminated in consolidation. In determining whether we are the primary beneficiary of a variable interest entity, we apply a qualitative approach that determines whether we have both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. We continuously assess whether we are the primary beneficiary of a variable interest entity as changes to existing relationships or future transactions may result in us consolidating or deconsolidating one or more of our collaborators or partners. Use of Estimates The preparation of our condensed consolidated financial statements requires us to make estimates, judgments and assumptions that may affect the reported amounts of assets, liabilities, equity, revenue and expense and related disclosure of contingent assets and liabilities. On an ongoing basis we evaluate our estimates, judgments and assumptions. We base our estimates on historical experience and on various other assumptions that we believe are reasonable, the results of which form the basis for making judgments about the carrying values of assets, liabilities and equity and the amount of revenue and expense. Actual results may differ from these estimates. The length of time and full extent to which the COVID-19 pandemic directly or indirectly impacts our business, results of operations and financial condition, including sales, expense, reserves and allowances, the supply chain, manufacturing, clinical trials, research and development costs and employee-related costs, depends on future developments that are highly uncertain, subject to change and are difficult to predict, including as a result of new information that may emerge concerning COVID-19 and the actions taken to contain or treat COVID-19 as well as the economic impact on local, regional, national and international customers and markets. Additionally, the ongoing geopolitical tensions related to Russia's invasion of Ukraine, and the related sanctions and other penalties imposed, are creating substantial uncertainty in the global economy. The extent and duration of the conflict, sanctions and resulting market disruptions are highly unpredictable. We have made estimates of the impact of the COVID-19 pandemic and the ongoing geopolitical conflict within our condensed consolidated financial statements and there may be changes to those estimates in future periods. New Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board or other standard setting bodies that we adopt as of the specified effective date. We do not believe that the adoption of recently issued standards have had or may have a material impact on our condensed consolidated financial statements or disclosures.

Restructuring, Business Transfo

Restructuring, Business Transformation and Other Cost Saving Initiatives	3 Months Ended
	Mar. 31, 2022
Restructuring and Related Activities [Abstract]
Restructuring, Business Transformation and Other Cost Saving Initiatives	RESTRUCTURING, BUSINESS TRANSFORMATION AND OTHER COST SAVING INITIATIVES 2022 Cost Saving Initiatives In December 2021 and May 2022 we announced our plans to implement a series of cost-reduction measures during 2022. These savings are expected to be achieved through a number of initiatives, including reductions to our workforce, primarily within our global Alzheimer's infrastructure, the consolidation of certain real estate locations and operating efficiency gains across our selling, general and administrative and research and development functions. Under these initiatives, we expect to incur restructuring charges ranging from approximately $100.0 million to $150.0 million. These amounts are primarily related to severance and are expected to be substantially incurred and paid by the end of 2022. For the three months ended March 31, 2022, we recognized $27.7 million of employee related costs, primarily related to severance. These costs were recorded in restructuring charges in our condensed consolidated statements of income. Our restructuring reserve is included in accrued expense and other in our condensed consolidated balance sheets. Following an evaluation of our current capacity needs, in March 2022 we ceased using a patient services office space in Durham, North Carolina. We are marketing the space for sublease. Our decision to cease using the facility resulted in the immediate expense of certain leasehold improvements and other assets at this facility, which we do not believe can be adequately recovered in a sublease. As a result, for the three months ended March 31, 2022, we recognized approximately $10.4 million of accelerated depreciation expense, which was recorded in restructuring charges in our condensed consolidated statements of income. The following table summarizes the charges and spending related to our 2022 cost saving initiatives for the three months ended March 31, 2022: (In millions) Workforce Reduction Total Restructuring reserve as of December 31, 2021 $ — $ — Expense 27.7 27.7 Payment (6.2) (6.2) Restructuring reserve as of March 31, 2022 $ 21.5 $ 21.5

Revenues

Revenues	3 Months Ended
	Mar. 31, 2022
Revenue from Contract with Customer [Abstract]
Revenues	Product Revenue Revenue by product is summarized as follows: For the Three Months Ended March 31, 2022 2021 (In millions) United Rest of Total United Rest of Total Multiple Sclerosis (MS): Fumarate (1) $ 242.3 $ 295.6 $ 537.9 $ 236.0 $ 316.9 $ 552.9 Interferon (2) 182.3 127.3 309.6 241.8 158.7 400.5 TYSABRI 284.5 236.3 520.8 273.3 230.0 503.3 FAMPYRA — 26.2 26.2 — 26.6 26.6 Subtotal: MS 709.1 685.4 1,394.5 751.1 732.2 1,483.3 Spinal Muscular Atrophy: SPINRAZA 163.3 309.2 472.5 148.7 371.8 520.5 Alzheimer's disease: ADUHELM (3) 2.8 — 2.8 — — — Biosimilars: BENEPALI — 114.7 114.7 — 121.7 121.7 IMRALDI — 57.1 57.1 — 57.9 57.9 FLIXABI — 22.5 22.5 — 25.5 25.5 Subtotal: Biosimilars — 194.3 194.3 — 205.1 205.1 Other: FUMADERM — 2.2 2.2 — 2.8 2.8 Total product revenue $ 875.2 $ 1,191.1 $ 2,066.3 $ 899.8 $ 1,311.9 $ 2,211.7 (1) Fumarate includes TECFIDERA and VUMERITY. VUMERITY became commercially available in the E.U. during the fourth quarter of 2021. (2) Interferon includes AVONEX and PLEGRIDY. (3) In June 2021 the U.S. Food and Drug Administration (FDA) granted accelerated approval of ADUHELM, which became commercially available in the U.S. during the second quarter of 2021. For additional information, please read Note 16, Collaborative and Other Relationships - Eisai Co., Ltd. - ADUHELM Collaboration Agreement, to these condensed consolidated financial statements. We recognized revenue from two wholesalers accounting for 26.3% and 10.5% of gross product revenue for the three months ended March 31, 2022, and 30.0% and 9.3% of gross product revenue for the three months ended March 31, 2021. An analysis of the change in reserves for discounts and allowances is summarized as follows: (In millions) Discounts Contractual Returns Total Balance, December 31, 2021 $ 137.7 $ 759.6 $ 38.0 $ 935.3 Current provisions relating to sales in current year 169.0 659.5 4.0 832.5 Adjustments relating to prior years (3.9) (40.2) (2.5) (46.6) Payments/credits relating to sales in current year (101.2) (271.6) (0.5) (373.3) Payments/credits relating to sales in prior years (58.9) (314.5) (6.5) (379.9) Balance, March 31, 2022 $ 142.7 $ 792.8 $ 32.5 $ 968.0 The total reserves above, which are included in our condensed consolidated balance sheets, are summarized as follows: (In millions) As of March 31, 2022 As of December 31, 2021 Reduction of accounts receivable $ 136.9 $ 133.2 Component of accrued expense and other 831.1 802.1 Total revenue-related reserves $ 968.0 $ 935.3 Revenue from Anti-CD20 Therapeutic Programs Revenue from anti-CD20 therapeutic programs is summarized in the table below. For the purposes of this footnote, we refer to RITUXAN and RITUXAN HYCELA collectively as RITUXAN. For the Three Months Ended March 31, (In millions) 2022 2021 Biogen’s share of pre-tax profits in the U.S. for RITUXAN and GAZYVA $ 143.2 $ 174.1 OCREVUS and other revenue from anti-CD20 therapeutic programs 256.2 214.9 Total revenue from anti-CD20 therapeutic programs $ 399.4 $ 389.0 For additional information on our collaboration arrangements with Genentech, please read Note 18, Collaborative and Other Relationships, to our consolidated financial statements included in our 2021 Form 10-K. Other Revenue Other revenue is summarized as follows: For the Three Months Ended March 31, (In millions) 2022 2021 Revenue from collaborative and other relationships: Revenue earned under our technical development agreement, manufacturing services agreements and royalty revenue on biosimilar products with Samsung Bioepis $ 8.0 $ 3.9 Other royalty and corporate revenue: Royalty 10.6 6.2 Other corporate 47.5 83.2 Total other revenue $ 66.1 $ 93.3 We receive royalties from net sales on products related to patents that we have out-licensed and we record other corporate revenue primarily from amounts earned under contract manufacturing agreements.

Inventory

Inventory	3 Months Ended
	Mar. 31, 2022
Inventory Disclosure [Abstract]
Inventory	The components of inventory are summarized as follows: (In millions) As of March 31, 2022 As of December 31, 2021 Raw materials $ 354.5 $ 349.6 Work in process 655.6 814.0 Finished goods 205.3 187.9 Total inventory $ 1,215.4 $ 1,351.5 In April 2022 the Centers for Medicare and Medicaid Services (CMS) released the final National Coverage Decision (NCD) for the class of anti-amyloid treatments in Alzheimer’s disease, including ADUHELM. The final NCD confirmed coverage with evidence development, in which patients with Medicare can only access treatment if they are part of an approved clinical trial. We expect that this decision will reduce future demand for ADUHELM to a minimal level. During the first quarter of 2022 we wrote-off approximately $275.0 million of inventory related to ADUHELM, as a result of this CMS decision, which was recognized in cost of sales within our condensed consolidated statements of income. We have recognized approximately $136.0 million related to Eisai's 45.0% share of these charges in collaboration profit (loss) sharing within our condensed consolidated statements of income. During the fourth quarter of 2021 we wrote-off approximately $120.0 million of inventory in excess of forecasted demand related to ADUHELM, which was recognized in cost of sales within our condensed consolidated statements of income. We have recognized approximately $59.0 million related to Eisai's 45.0% share of these charges in collaboration profit (loss) sharing within our condensed consolidated statements of income during the fourth quarter of 2021. As of March 31, 2022, our total ADUHELM inventory was de minimis. As of December 31, 2021, we had approximately $223.0 million of ADUHELM inventory. For additional information please read Note 16, Collaborative and Other Relationships , to these condensed consolidated financial statements.

Intangible Assets and Goodwill

Intangible Assets and Goodwill	3 Months Ended
	Mar. 31, 2022
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, 2022 As of December 31, 2021 (In millions) Estimated Life Cost Accumulated Net Cost Accumulated Net Completed technology 4-28 years $ 7,413.1 $ (5,455.4) $ 1,957.7 $ 7,413.1 $ (5,388.5) $ 2,024.6 In-process research and development Indefinite until commercialization 129.1 — 129.1 132.7 — 132.7 Trademarks and trade names Indefinite 64.0 — 64.0 64.0 — 64.0 Total intangible assets $ 7,606.2 $ (5,455.4) $ 2,150.8 $ 7,609.8 $ (5,388.5) $ 2,221.3 Amortization and Impairments For the three months ended March 31, 2022, amortization and impairment of acquired intangible assets totaled $66.9 million, compared to $98.1 million in the prior year comparative period. For the three months ended March 31, 2022, we had no impairment charges. For the three months ended March 31, 2021 , amortization and impairment of acquired intangible assets reflects the impact of a $44.3 million impairment charge related to vixotrigine (BIIB074) for the potential treatment of trigeminal neuralgia (TGN). Completed Technology Completed technology primarily relates to our acquisition of all remaining rights to TYSABRI as well as other amounts related to our other marketed products and programs acquired through business combinations. IPR&D Related to Business Combinations In-process research and development (IPR&D) represents the fair value assigned to research and development assets that we acquired as part of a business combination and had not yet reached technological feasibility at the date of acquisition. Included in IPR&D balances are adjustments related to foreign currency exchange rate fluctuations. We review amounts capitalized as acquired IPR&D for impairment annually, as of October 31, and whenever events or changes in circumstances indicate to us that the carrying value of the assets might not be recoverable. The carrying value associated with our IPR&D assets as of March 31, 2022, relates to the IPR&D programs we acquired in connection with our acquisition of Convergence Pharmaceuticals Holdings Ltd. (Convergence). Vixotrigine In the periods since we acquired vixotrigine, there have been numerous delays in the initiation of Phase 3 studies for the potential treatment of TGN and for the potential treatment of diabetic painful neuropathy (DPN), another form of neuropathic pain. We have engaged with the FDA regarding the design of the Phase 3 studies of vixotrigine for the potential treatment of TGN and DPN and are now performing an additional clinical trial of vixotrigine, which is expected to be completed by the end of 2022. The performance of this additional clinical trial delayed the initiation of the Phase 3 studies of vixotrigine for the potential treatment of TGN, and, as a result, we recognized an impairment charge of $44.3 million related to vixotrigine for the potential treatment of TGN during the fi rst quarter of 2021. As of March 31, 2022, the carrying value associated with the remaining IPR&D asset for DPN was $129.1 million a nd the fair value of this asset was not significantly in excess of its carrying value. Upon the completion of the additional clinical trial we will reevaluate the carrying value of the program. Estimated Future Amortization of Intangible Assets The estimated future amortization of finite-lived intangible assets for the next five years is expected to be as follows: (In millions) As of March 31, 2022 2022 (remaining nine months) $ 200.0 2023 210.0 2024 195.0 2025 195.0 2026 180.0 2027 165.0 Goodwill The following table provides a roll forward of the changes in our goodwill balance: (In millions) As of March 31, 2022 Goodwill, December 31, 2021 $ 5,761.1 Other (3.1) Goodwill, March 31, 2022 $ 5,758.0 As of March 31, 2022, we had no ac

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2022
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, 2022 (In millions) Total Quoted Prices Significant Other Significant Assets: Cash equivalents $ 1,283.7 $ — $ 1,283.7 $ — Marketable debt securities: Corporate debt securities 1,579.4 — 1,579.4 — Government securities 1,252.7 — 1,252.7 — Mortgage and other asset backed securities 171.9 — 171.9 — Marketable equity securities 860.3 607.0 253.3 — Derivative contracts 97.2 — 97.2 — Plan assets for deferred compensation 35.8 — 35.8 — Total $ 5,281.0 $ 607.0 $ 4,674.0 $ — Liabilities: Derivative contracts $ 12.5 $ — $ 12.5 $ — Contingent consideration obligations 202.0 — — 202.0 Total $ 214.5 $ — $ 12.5 $ 202.0 As of December 31, 2021 (In millions) Total Quoted Prices Significant Other Significant Assets: Cash equivalents $ 1,632.2 $ — $ 1,632.2 $ — Marketable debt securities: Corporate debt securities 1,108.2 — 1,108.2 — Government securities 1,192.7 — 1,192.7 — Mortgage and other asset backed securities 132.2 — 132.2 — Marketable equity securities 1,048.5 181.7 866.8 — Derivative contracts 80.9 — 80.9 — Plan assets for deferred compensation 33.4 — 33.4 — Total $ 5,228.1 $ 181.7 $ 5,046.4 $ — Liabilities: Derivative contracts $ 10.8 $ — $ 10.8 $ — Contingent consideration obligations 209.1 — — 209.1 Total $ 219.9 $ — $ 10.8 $ 209.1 There have been no material impairments of our assets measured and carried at fair value as of March 31, 2022 and December 31, 2021. In addition, there have been no changes in valuation techniques as of March 31, 2022 and December 31, 2021. The fair value of Level 2 instruments classified as cash equivalents and marketable debt securities was determined through third-party pricing services. The fair value of Level 2 instruments classified as marketable equity securities represents our investments in the common stock of Sangamo Therapeutics, Inc. (Sangamo) and Sage Therapeutics, Inc. (Sage) and are valued using an option pricing valuation model as the investments are each subject to certain holding period restrictions. The holding period restrictions for our investment in the common stock of Denali Therapeutics Inc. (Denali) and a portion of our Sangamo investment expired during the first quarter of 2022 and the second quarter of 2021, respectively. The fair value of our Denali investment and this portion of our Sangamo investment were Level 1 measurements as of March 31, 2022. For additional information on our investments in Sangamo, Denali and Sage common stock, please read Note 7, Financial Instruments, to these condensed consolidated financial statements. For a description of our validation procedures related to prices provided by third-party pricing services and our option pricing valuation model, please read Note 1, Summary of Significant Accounting Policies - Fair Value Measurements, to our consolidated financial statements included in our 2021 Form 10-K. The following tables summarize the significant unobservable inputs in the fair value measurement of our contingent consideration obligations as of March 31, 2022 and December 31, 2021: As of March 31, 2022 (In millions) Fair Value Valuation Technique Unobservable Input(s) Range Weighted Average Liabilities: Contingent consideration obligations $ 202.0 Discounted cash flow Discount rate 2.89% 2.89% Expected timing of achievement of development milestones 2023 to 2027 — As of December 31, 2021 (In millions) Fair Value Valuation Technique Unobservable Input(s) Range Weighted Average Liabilities: Contingent consideration obligations $ 209.1 Discounted cash flow Discount rate 1.30% 1.30% Expected timing of achievement of development milestones 2023 to 2027 — The weighted average discount rate was calculated based on the relative fair value of our contingent consideration obligations. In addition, we apply various probabilities of technological and regulatory success, ranging from 10.9% to certain probability as of March 31, 2022, to the valuation models to estimate the fair values of our contingent consideration obligations. Debt Instruments The fair and carrying values of our debt instruments, which are Level 2 liabilities, are summarized as follows: As of March 31, 2022 As of December 31, 2021 (In millions) Fair Carrying Fair Carrying 3.625% Senior Notes due September 15, 2022 $ 1,008.7 $ 999.5 $ 1,020.0 $ 999.1 4.050% Senior Notes due September 15, 2025 1,797.6 1,743.4 1,895.2 1,742.9 2.250% Senior Notes due May 1, 2030 1,341.6 1,492.2 1,475.9 1,492.0 5.200% Senior Notes due September 15, 2045 1,240.0 1,100.0 1,463.0 1,099.9 3.150% Senior Notes due May 1, 2050 1,225.3 1,473.3 1,457.7 1,473.2 3.250% Senior Notes due February 15, 2051 581.8 466.8 692.9 466.0 Total $ 7,195.0 $ 7,275.2 $ 8,004.7 $ 7,273.1 The fair values of each of our series of Senior Notes were determined through market, observable and corroborated sources. For additional information related to our Senior Notes, please read Note 12, Indebtedness, to our consolidated financial statements included in our 2021 Form 10-K. Contingent Consideration Obligations In connection with our acquisitions of Convergence and Biogen International Neuroscience GmbH, we agreed to make additional payments based upon the achievement of certain milestone events. The following table provides a roll forward of the fair values of our contingent consideration obligations, which includes Level 3 measurements: For the Three Months Ended March 31, (In millions) 2022 2021 Fair value, beginning of period $ 209.1 $ 259.8 Changes in fair value (7.1) (33.8) Fair value, end of period $ 202.0 $ 226.0 As of March 31, 2022 and December 31, 2021, approximately $202.0 million and $209.1 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 any remaining balances reflected as a component of accrued expense and other. For the three months ended March 31, 2022, changes in the fair value of our contingent consideration obligations were primarily due to an increase in discount rates used to revalue these obligations and delays in the expected timing of the achievement of certain remaining developmental milestones related to our vixotrigine programs. For the three months ended March 31, 2021, changes in the fair value of our contingent consideration obligations were primarily due to delays in the expected timing of the achievement of certain remaining developmental milestones related to our vixotrigine programs.

Financial Instruments

Financial Instruments	3 Months Ended
	Mar. 31, 2022
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, 2022 As of December 31, 2021 Commercial paper $ 278.5 $ 247.6 Overnight reverse repurchase agreements 13.9 200.0 Money market funds 585.0 901.6 Short-term debt securities 406.3 283.0 Total $ 1,283.7 $ 1,632.2 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, classified as available-for-sale: As of March 31, 2022 (In millions) Amortized Gross Gross Fair Marketable debt securities Corporate debt securities: Current $ 1,126.7 $ 0.1 $ (2.2) $ 1,124.6 Non-current 460.5 0.1 (5.8) 454.8 Government securities: Current 878.7 1.0 (2.6) 877.1 Non-current 379.0 0.2 (3.6) 375.6 Mortgage and other asset backed securities: Current 0.7 — — 0.7 Non-current 173.4 — (2.2) 171.2 Total marketable debt securities $ 3,019.0 $ 1.4 $ (16.4) $ 3,004.0 Marketable equity securities Marketable equity securities, current $ 33.9 $ — $ — $ 33.9 Marketable equity securities, non-current 1,133.1 4.5 (311.2) 826.4 Total marketable equity securities $ 1,167.0 $ 4.5 $ (311.2) $ 860.3 As of December 31, 2021 (In millions) Amortized Gross Gross Fair Marketable debt securities Corporate debt securities: Current $ 723.6 $ 0.1 $ (0.3) $ 723.4 Non-current 385.4 0.2 (0.8) 384.8 Government securities: Current 817.0 — (0.4) 816.6 Non-current 377.0 0.1 (1.0) 376.1 Mortgage and other asset backed securities: Current 1.1 — — 1.1 Non-current 131.8 — (0.7) 131.1 Total marketable debt securities $ 2,435.9 $ 0.4 $ (3.2) $ 2,433.1 Marketable equity securities Marketable equity securities, current $ 33.9 $ 9.9 $ — $ 43.8 Marketable equity securities, non-current 1,133.1 151.0 (279.4) 1,004.7 Total marketable equity securities $ 1,167.0 $ 160.9 $ (279.4) $ 1,048.5 Summary of Contractual Maturities: Available-for-Sale Debt Securities The estimated fair value and amortized cost of our marketable debt securities available-for-sale by contractual maturity are summarized as follows: As of March 31, 2022 As of December 31, 2021 (In millions) Estimated Amortized Estimated Amortized Due in one year or less $ 2,002.4 $ 2,006.0 $ 1,541.1 $ 1,541.7 Due after one year through five years 982.9 993.7 868.2 870.2 Due after five years 18.7 19.3 23.8 24.0 Total marketable debt securities $ 3,004.0 $ 3,019.0 $ 2,433.1 $ 2,435.9 The average maturity of our marketable debt securities available-for-sale as of March 31, 2022 and December 31, 2021, was approximately 10 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) 2022 2021 Proceeds from maturities and sales $ 543.6 $ 819.2 Realized gains — 0.2 Realized losses 0.6 0.7 Realized losses for the three months ended March 31, 2022 and 2021, primarily relate to sales of corporate bonds, agency mortgage-backed securities and other asset-backed securities. Strategic Investments As of March 31, 2022 and December 31, 2021, our strategic investment portfolio was comprised of investments totaling $919.5 million and $1,110.3 million, respectively, which are included in other current assets and investments and other assets in our condensed consolidated balance sheets. Our strategic investment portfolio includes investments in equity securities of certain biotechnology companies, which are reflected within our disclosures included in Note 6, Fair Value Measurements, to these condensed consolidated financial statements, venture capital funds where the underlying investments are in equity securities of certain biotechnology companies and non-marketable equity securities. The decrease in our strategic investment portfolio for the three months ended March 31, 2022, was primarily due to decreases in the fair value of our investments in Denali, Sage and Sangamo common stock. For additional information on our investments in Denali, Ionis Pharmaceuticals, Inc. (Ionis), Sage and Sangamo common stock, please read Note 18, Collaborative and Other Relationships , to our consolidated financial statements included in our 2021 Form 10-K.

Derivative Instruments

Derivative Instruments	3 Months Ended
	Mar. 31, 2022
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments	Foreign Currency Forward Contracts - Hedging Instruments Due to the global nature of our operations, portions of our revenue and operating expense are recorded in currencies other than the U.S. dollar. The value of revenue and operating expense measured in U.S. dollars is therefore subject to changes in foreign currency exchange rates. In order to mitigate these changes, we use foreign currency forward contracts to lock in exchange rates associated with a portion of our forecasted international revenue and operating expense. Foreign currency forward contracts in effect as of March 31, 2022 and December 31, 2021, had durations of 1 to 18 months and 1 to 15 months, respectively. These contracts have been designated as cash flow hedges and unrealized gains or losses on the portion of these foreign currency forward contracts that are included in the effectiveness test are reported in accumulated other comprehensive income (loss) (referred to as AOCI in the table below). Realized gains and losses of such contracts are recognized in revenue when the sale of product in the currency being hedged is recognized and in operating expense when the expense in the currency being hedged is recorded. We recognize all cash flow hedge reclassifications from accumulated other comprehensive income (loss) and fair value changes of excluded portions in the same line item in our condensed consolidated statements of income that has been impacted by the hedged item. The notional amount of foreign currency forward contracts that were entered into to hedge forecasted revenue and operating expense is summarized as follows: Notional Amount (In millions) As of March 31, 2022 As of December 31, 2021 Euro $ 1,631.2 $ 1,828.0 British pound 126.6 166.2 Swiss franc 128.6 — Japanese yen 55.0 72.7 Canadian dollar 44.6 59.9 Total foreign currency forward contracts $ 1,986.0 $ 2,126.8 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 is summarized as follows: (In millions) As of March 31, 2022 As of December 31, 2021 Unrealized gains $ 76.4 $ 60.8 Unrealized (losses) (4.3) (7.0) Net unrealized gains (losses) $ 72.1 $ 53.8 We expect the net unrealized gains of $72.1 million to be settled over the next 18 months, of which $72.0 million of these net unrealized gains are expected to be settled over the next 12 months, with any amounts in accumulated other comprehensive income (loss) to be reported as an adjustment to revenue or operating expense. We consider the impact of our and our counterparties’ credit risk on the fair value of the contracts as well as the ability of each party to execute its contractual obligations. As of March 31, 2022 and December 31, 2021, credit risk did not materially change the fair value of our foreign currency forward contracts. The following table summarizes the effect of foreign currency forward contracts designated as hedging instruments in our condensed consolidated statements of income: For the Three Months Ended March 31, Net Gains/(Losses) Net Gains/(Losses) Location 2022 2021 Location 2022 2021 Revenue $ 20.9 $ (23.1) Revenue $ (6.5) $ (3.0) Operating expense (0.3) (0.4) Operating expense (0.1) (0.1) Net Investment Hedges - Hedging Instruments In February 2012 we entered into a joint venture agreement with Samsung BioLogics establishing an entity, Samsung Bioepis, to develop, manufacture and market biosimilar products. In June 2018 we exercised our option under our joint venture agreement to increase our ownership percentage in Samsung Bioepis from approximately 5.0% to approximately 49.9%. The share purchase transaction was completed in November 2018 and, upon closing, we paid 759.5 billion South Korean won ($676.6 million) to Samsung BioLogics. Our investment in the equity of Samsung Bioepis is exposed to the currency fluctuations in the South Korean won. In order to mitigate the currency fluctuations between the U.S. dollar and South Korean won, we have entered into foreign currency forward contracts. Foreign currency forward contracts in effect as of March 31, 2022, had a remaining duration of seven months. These contracts have been designated as net investment hedges. We recognize changes in the spot exchange rate in accumulated other comprehensive income (loss). The pre-tax portion of the fair value of these foreign currency forward contracts that were included in accumulated other comprehensive income (loss) in total equity reflected net gains of $20.6 million and $10.6 million as of March 31, 2022 and December 31, 2021, 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 net losses of $2.5 million and $3.6 million as of March 31, 2022 and December 31, 2021, respectively. In conjunction with the closing of our sale of equity in Samsung Bioepis to Samsung BioLogics, which occurred in April 2022, we concurrently closed our foreign currency forward contracts. 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) Net Gains/(Losses) Net Gains/(Losses) Location 2022 2021 Location 2022 2021 Location 2022 2021 Gains (losses) on net investment hedge $ 10.1 $ 23.8 Gains (losses) on net investment hedge $ (3.3) $ (1.4) Other income (expense) $ (1.1) $ 0.1 For additional information on our collaboration arrangements with Samsung Bioepis, please read Note 16, Collaborative and Other Relationships, to these condensed consolidated financial statements. Foreign Currency Forward Contracts - Other Derivative Instruments We also enter into other foreign currency forward contracts, usually with durations of one month or less, to mitigate the foreign currency risk related to certain balance sheet positions. We have not elected hedge accounting for these transactions. The aggregate notional amount of these outstanding foreign currency forward contracts was $1,333.4 million and $1,268.0 million as of March 31, 2022 and December 31, 2021, respectively. Net losses of $12.2 million and $17.4 million related to these contracts were recorded as a component of other income (expense), net for the three months ended March 31, 2022 and 2021, respectively. Summary of Derivative Instruments While certain of our derivative instruments are subject to netting arrangements with our counterparties, we do not offset derivative assets and liabilities in our condensed consolidated balance sheets. The amounts in the table below would not be substantially different if the derivative assets and liabilities were offset. The following table summarizes the fair value and presentation in our condensed consolidated balance sheets of our outstanding derivative instruments, including those designated as hedging instruments: (In millions) Balance Sheet Location As of March 31, 2022 As of December 31, 2021 Cash Flow Hedging Instruments: Asset derivative instruments Other current assets $ 79.8 $ 66.2 Investments and other assets 1.6 5.5 Liability derivative instruments Accrued expense and other 2.4 6.6 Other long-term liabilities 2.2 — Net Investment Hedging Instruments: Asset derivative instruments Other current assets 10.9 4.1 Other Derivative Instruments: Asset derivative instruments Other current assets 4.9 5.1 Liability derivative instruments Accrued expense and other 7.9 4.2

Property, Plant and Equipment

Property, Plant and Equipment	3 Months Ended
	Mar. 31, 2022
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, plant and equipment are recorded at historical cost, net of accumulated depreciation. Accumulated depreciation on property, plant and equipment was $2,082.0 million and $2,006.6 million as of March 31, 2022 and December 31, 2021, respectively. For the three months ended March 31, 2022, depreciation expense totaled $76.3 million compared to $48.8 million in the prior year comparative period. Solothurn, Switzerland Manufacturing Facility In order to support our future growth and drug development pipeline, we are building a large-scale biologics manufacturing facility in Solothurn, Switzerland. Upon completion, this facility will include 393,000 square feet related to a large-scale biologics manufacturing facility, 290,000 square feet of warehouse, utilities and support space and 51,000 square feet of administrative space. As of March 31, 2022 and December 31, 2021, we had approximately $682.6 million and $677.0 million, respectively, capitalized as construction in progress related to this facility. In the second quarter of 2021, a portion of the facility received a Good Manufacturing Practice multi-product license from the Swiss Agency for Therapeutic Products, resulting in approximately $1.2 billion of fixed assets being placed in service during the second quarter of 2021. In April 2022 the FDA approved the Prior Approval Supplement for the Solothurn facility for ADUHELM. We estimate the remainder of the facility will be operational during the first half of 2023.

Indebtedness

Indebtedness	3 Months Ended
	Mar. 31, 2022
Debt Disclosure [Abstract]
Indebtedness	Exchange Offer In February 2021 we completed our Exchange Offer of our tendered 2045 Senior Notes for our 2051 Senior Notes and cash, and an offer to purchase our tendered 2045 Senior Notes for cash. An aggregate principal amount of approximately $624.6 million of our 2045 Senior Notes was exchanged for an aggregate principal amount of approximately $700.7 million of our 2051 Senior Notes and aggregate cash payments of approximately $151.8 million. Our Exchange Offer has been accounted for as a debt modification; as such, the cash component has been reflected as additional debt discount and is amortized as an adjustment to interest expense over the term of our 2051 Senior Notes. In addition, we redeemed an aggregate principal amount of approximately $8.9 million of our 2045 Senior Notes for aggregate cash payments of approximately $12.1 million, excluding accrued and unpaid interest. The redemption has been accounted for as a debt extinguishment; as such, we recognized a pre-tax charge of $3.2 million upon the extinguishment of such 2045 Senior Notes. This charge, which was recognized in interest expense in other income (expense), net in our condensed consolidated statements of income for the three months ended March 31, 2021, reflects the payment of an early call premium and the write-off of the remaining unamortized original debt issuance costs and discount balances associated with such 2045 Senior Notes. Upon settlement, we also made aggregate cash payments of approximately $13.8 million to settle all accrued and unpaid interest from the last interest payment date on our 2045 Senior Notes that were exchanged or redeemed. We incurred approximately $6.1 million of costs associated with our Exchange Offer, which was recognized in interest expense in other income (expense), net in our condensed consolidated statements of income for the three months ended March 31, 2021.

Equity

Equity	3 Months Ended
	Mar. 31, 2022
Equity [Abstract]
Equity	Share Repurchases In October 2020 our Board of Directors authorized a program to repurchase up to $5.0 billion of our common stock (2020 Share Repurchase Program). Our 2020 Share Repurchase Program does not have an expiration date. All share repurchases under our 2020 Share Repurchase Program will be retired. Under our 2020 Share Repurchase Program, we repurchased and retired approximately 2.2 million shares of our common stock at a cost of approximately $600.0 million during the three months ended March 31, 2021. There were no share repurchases of our common stock during the three months ended March 31, 2022. Approximately $2.8 billion remained available under our 2020 Share Repurchase Program as of March 31, 2022. Accumulated Other Comprehensive Income (Loss) The following tables summarize the changes in accumulated other comprehensive income (loss), net of tax by component: (In millions) Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Gains (Losses) on Net Investment Hedge, Net of Tax Unfunded Status of Postretirement Benefit Plans, Net of Tax Currency Translation Adjustments Total Balance, December 31, 2021 $ (2.2) $ 53.8 $ 25.5 $ (44.8) $ (139.0) $ (106.7) Other comprehensive income (loss) before reclassifications (10.2) 34.4 5.1 0.9 (21.8) 8.4 Amounts reclassified from accumulated other comprehensive income (loss) 0.5 (18.5) 1.1 — — (16.9) Net current period other comprehensive income (loss) (9.7) 15.9 6.2 0.9 (21.8) (8.5) Balance, March 31, 2022 $ (11.9) $ 69.7 $ 31.7 $ (43.9) $ (160.8) $ (115.2) (In millions) Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Gains (Losses) on Net Investment Hedge, Net of Tax Unfunded Status of Postretirement Benefit Plans, Net of Tax Currency Translation Adjustments Total Balance, December 31, 2020 $ 1.4 $ (179.0) $ (8.5) $ (66.3) $ (46.6) $ (299.0) Other comprehensive income (loss) before reclassifications (1.2) 128.6 22.4 2.0 (48.5) 103.3 Amounts reclassified from accumulated other comprehensive income (loss) 0.4 21.0 — — — 21.4 Net current period other comprehensive income (loss) (0.8) 149.6 22.4 2.0 (48.5) 124.7 Balance, March 31, 2021 $ 0.6 $ (29.4) $ 13.9 $ (64.3) $ (95.1) $ (174.3) The following table summarizes the amounts reclassified from accumulated other comprehensive income (loss): (In millions) Income Statement Location Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) For the Three Months Ended March 31, 2022 2021 Gains (losses) on securities available for sale Other income (expense) $ (0.6) $ (0.5) Income tax benefit (expense) 0.1 0.1 Gains (losses) on cash flow hedges Revenue 20.9 (23.1) Operating expense (0.3) (0.4) Other income (expense) (0.1) 0.2 Income tax benefit (expense) (2.0) 2.3 Gains (losses) on net investment hedge Other income (expense) (1.1) — Total reclassifications, net of tax $ 16.9 $ (21.4)

Earnings per Share

Earnings per Share	3 Months Ended
	Mar. 31, 2022
Earnings Per Share [Abstract]
Earnings per Share	Basic and diluted shares outstanding used in our earnings per share calculation are calculated as follows: For the Three Months Ended March 31, (In millions) 2022 2021 Numerator: Net income attributable to Biogen Inc. $ 303.8 $ 410.2 Denominator: Weighted average number of common shares outstanding 147.1 151.9 Effect of dilutive securities: Time-vested restricted stock units 0.3 0.2 Market stock units 0.1 0.1 Performance stock units settled in stock 0.1 0.1 Dilutive potential common shares 0.5 0.4 Shares used in calculating diluted earnings per share 147.6 152.3 Amounts excluded from the calculation of net income per diluted share because their effects were anti-dilutive were insignificant.

Share-based Payments

Share-based Payments	3 Months Ended
	Mar. 31, 2022
Share-based Payment Arrangement [Abstract]
Share-based Payments	Share-based Compensation Expense The following table summarizes share-based compensation expense included in our condensed consolidated statements of income: For the Three Months Ended March 31, (In millions) 2022 2021 Research and development $ 25.7 $ 33.6 Selling, general and administrative 46.1 44.9 Subtotal 71.8 78.5 Capitalized share-based compensation costs (2.8) (2.6) Share-based compensation expense included in total cost and expense 69.0 75.9 Income tax effect (12.8) (14.0) Share-based compensation expense included in net income attributable to Biogen Inc. $ 56.2 $ 61.9 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) 2022 2021 Market stock units $ 5.9 $ 16.5 Time-vested restricted stock units 51.3 42.8 Performance stock units settled in stock 8.3 6.3 Performance stock units settled in cash 1.4 6.0 Employee stock purchase plan 4.9 6.9 Subtotal 71.8 78.5 Capitalized share-based compensation costs (2.8) (2.6) Share-based compensation expense included in total cost and expense $ 69.0 $ 75.9 We estimate the fair value of our obligations associated with our performance stock units settled in cash at the end of each reporting period through expected settlement. Cumulative adjustments to these obligations are recognized each quarter to reflect changes in the stock price and estimated outcome of the performance-related conditions.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2022
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, 2022 2021 Statutory rate 21.0 % 21.0 % State taxes (0.4) 0.8 Taxes on foreign earnings (10.8) (10.9) Tax credits (2.5) (3.7) Purchased intangible assets 0.6 0.8 GILTI 0.3 0.9 Neurimmune tax impacts 24.2 (0.6) Other 3.8 1.2 Effective tax rate 36.2 % 9.5 % Changes in Tax Rate During the second quarter of 2021 we recorded a net deferred tax asset in Switzerland of approximately $490.0 million on Neurimmune SubOne AG's (Neurimmune) tax basis in ADUHELM, the realization of which is dependent on future sales of ADUHELM. During the fourth quarter of 2021, due to reduced future expected revenue associated with ADUHELM, we recorded a valuation allowance of approximately $390.0 million. During the first quarter of 2022, upon issuance of the final NCD related to ADUHELM, we recorded an additional valuation allowance of approximately $85.0 million to reduce the net value of this deferred tax asset to zero. These adjustments to our deferred tax assets and their valuation allowances are each recorded with an equal and offsetting amount assigned to net income (loss) attributable to noncontrolling interests, net of tax in our condensed consolidated statements of income, resulting in a zero net impact to net income attributable to Biogen Inc. For the three months ended March 31, 2022, compared to the same period in 2021, the increase in our effective tax rate was primarily due to a deferred tax expense related to a valuation allowance, as discussed above, and the non-cash tax effects of changes in the value of our equity investments. The tax effects of this change in value of our equity investments were recorded discretely, since changes in value of equity investments cannot be forecasted. For additional information on our collaboration arrangement with Neurimmune, please read Note 17, Investments in Variable Interest Entities , to these condensed consolidated financial statements. Tax Basis in Samsung Bioepis As of March 31, 2022 and December 31, 2021, the carrying value of our investment in Samsung Bioepis totaled 709.4 billion South Korean won ($586.4 million) and 713.3 billion South Korean won ($599.9 million), respectively, which is classified as a component of investments and other assets in our condensed consolidated balance sheets. As of March 31, 2022, we have not recorded deferred taxes of approximately $70.0 million on our tax over book basis related to this joint venture. Accounting for Uncertainty in Income Taxes We and our subsidiaries are routinely examined by various taxing authorities. We file income tax returns in various U.S. states and in U.S. federal and other foreign jurisdictions. With few exceptions, we are no longer subject to U.S. federal tax examination for years before 2017 or state, local or non-U.S. income tax examinations for years before 2012. The U.S. Internal Revenue Service and other national tax authorities routinely examine our intercompany transfer pricing with respect to intellectual property related transactions and it is possible that they may disagree with one or more positions we have taken with respect to such valuations. It is reasonably possible that we will adjust the value of our uncertain tax positions related to certain transfer pricing, collaboration matters and other issues as we receive additional information from various taxing authorities, including reaching settlements with such authorities. We estimate that it is reasonably possible that our gross unrecognized tax benefits, exclusive of interest, could decrease by up to approximately $500.0 million, including approximately $455.0 million related to the unrecognized tax benefits related to Neurimmune's tax basis in ADUHELM in the next 12 months as a result of various audit closures, settlements and expiration of the statute of limitations.

Other Consolidated Financial St

Other Consolidated Financial Statement Detail	3 Months Ended
	Mar. 31, 2022
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) 2022 2021 Interest income $ 2.9 $ 2.9 Interest expense (66.1) (64.7) Gains (losses) on investments, net (191.1) (436.6) Foreign exchange gains (losses), net (8.3) (8.6) Other, net (0.7) 0.1 Total other income (expense), net $ (263.3) $ (506.9) Gains (losses) on investments, net, as reflected in the table above, relate to debt securities, equity securities of certain biotechnology companies, venture capital funds where the underlying investments are in equity securities of certain biotechnology companies and non-marketable equity securities. The following table summarizes our gains (losses) on investments, net that relates to our equity securities held as of March 31, 2022 and 2021: For the Three Months Ended March 31, (In millions) 2022 2021 Net gains (losses) recognized during the period on equity securities $ (190.7) $ (436.1) Less: Net gains (losses) realized during the period on equity securities 0.2 6.2 Unrealized gains (losses) recognized during the period on equity securities $ (190.9) $ (442.3) The net unrealized losses recognized during the three months ended March 31, 2022, primarily reflect decreases in the aggregate fair value of our investments in Denali, Sage and Sangamo common stock of approximately $205.5 million. Accrued Expense and Other Accrued expense and other consists of the following: (In millions) As of March 31, 2022 As of December 31, 2021 Revenue-related reserves for discounts and allowances $ 831.1 $ 802.1 Collaboration expense 266.2 324.7 Royalties and licensing fees 216.0 234.7 Employee compensation and benefits 206.6 345.1 Other 711.2 828.6 Total accrued expense and other $ 2,231.1 $ 2,535.2 Other Long-term Liabilities Other long-term liabilities were $1,287.9 million and $1,320.5 million as of March 31, 2022 and December 31, 2021, respectively, and included accrued income taxes totaling $640.5 million and $664.5 million, respectively.

Collaborative and Other Relatio

Collaborative and Other Relationships	3 Months Ended
	Mar. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborative and Other Relationships	Eisai Co., Ltd. Lecanemab Collaboration We have a collaboration agreement with Eisai Co., Ltd. (Eisai) to jointly develop and commercialize lecanemab (BAN2401), an anti-amyloid antibody, and elenbecestat, the oral BACE (base amyloid cleaving enzyme) inhibitor, two Eisai product candidates for the potential treatment of Alzheimer's disease (the Lecanemab Collaboration). In September 2019 we and Eisai discontinued the global Phase 3 studies of elenbecestat in early Alzheimer's disease. Eisai serves as the lead of lecanemab development and regulatory submissions globally with both companies co-commercializing and co-promoting the product, and Eisai having final decision-making authority. All costs, including research, development, sales and marketing expense, are shared equally between us and Eisai. If lecanemab receives marketing approval, we and Eisai will co-promote lecanemab and share profits equally. In March 2022 we extended our supply agreement related to lecanemab from five years to ten years, and we will manufacture the lecanemab drug substance in our Solothurn manufacturing facility. The Lecanemab Collaboration also provided Eisai with an option to jointly develop and commercialize ADUHELM (aducanumab) (ADUHELM Option). In October 2017 Eisai exercised its ADUHELM Option and we entered into a new collaboration agreement for the joint development and commercialization of ADUHELM (aducanumab) (the ADUHELM Collaboration Agreement). A summary of development and sales and marketing expense related to the Lecanemab Collaboration is as follows: For the Three Months Ended March 31, (In millions) 2022 2021 Total development expense incurred by the collaboration related to the advancement of lecanemab and elenbecestat $ 77.0 $ 55.5 Biogen's share of lecanemab and elenbecestat development expense reflected in research and development expense in our condensed consolidated statements of income 38.5 27.7 Total sales and marketing expense incurred by the Lecanemab Collaboration 15.9 5.7 Biogen's share of lecanemab and elenbecestat sales and marketing expense reflected in selling, general and administrative expense in our condensed consolidated statements of income 8.0 2.9 For additional information on our Lecanemab Collaboration, please read Note 18, Collaborative and Other Relationships , to our consolidated financial statements included in our 2021 Form 10-K. ADUHELM Collaboration Agreement Under our initial ADUHELM Collaboration Agreement, we would lead the ongoing development of ADUHELM, and we and Eisai would co-promote ADUHELM with a region-based profit split. Beginning January 1, 2019, Eisai was reimbursing us for 45.0% of development costs incurred by the collaboration for the advancement of ADUHELM (ADUHELM development expense). In June 2021 ADUHELM was granted accelerated approval by the FDA for the treatment of Alzheimer's disease and had its first commercial sale. As a result of the launch of ADUHELM in the U.S., we made a $100.0 million milestone payment to Neurimmune. During the second quarter of 2021 we recognized net profit-sharing income of $45.0 million to reflect Eisai's 45.0% share of the $100.0 million milestone payment, which was recorded in collaboration profit (loss) sharing in our condensed consolidated statements of income. In March 2022 we amended our ADUHELM Collaboration Agreement with Eisai. Effective March 2022 we have sole decision making and commercialization rights worldwide on ADUHELM and beginning January 1, 2023, Eisai will receive a tiered royalty based on net sales of ADUHELM, rather than sharing global profits and losses. Eisai's share of development, commercialization and manufacturing expense is limited to $335.0 million for the period from January 1, 2022 to December 31, 2022. Once the tiered royalty model commences on January 1, 2023, Eisai will not participate in ADUHELM’s economics beyond these royalties. A summary of development expense, sales and marketing expense and milestone payments related to the ADUHELM Collaboration Agreement is as follows: For the Three Months Ended March 31, (In millions) 2022 2021 Total ADUHELM development expense $ 44.2 $ 47.0 Biogen's share of ADUHELM development expense reflected in research and development expense in our condensed consolidated statements of income 24.3 25.8 Total ADUHELM sales and marketing expense incurred by the ADUHELM Collaboration Agreement 95.0 111.8 Biogen's share of ADUHELM sales and marketing expense reflected in selling, general and administrative expense and collaboration profit (loss) sharing in our condensed consolidated statements of income 50.9 60.3 C o-promotion Profits and Losses In the U.S. we recognize revenue on sales to third parties as a component of product revenue, net in our condensed consolidated statements of income. We also record the related cost of revenue and sales and marketing expense in our condensed consolidated statements of income as these costs are incurred. Payments made to and received from Eisai for its 45.0% share of the co-promotion profits or losses in the U.S. are recognized in collaboration profit (loss) sharing in our condensed consolidated statements of income. For the three months ended March 31, 2022, we recognized a net reduction to our operating expense of $181.7 million to reflect Eisai's 45.0% share of net collaboration losses in the U.S. 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. During the first quarter of 2022 we recorded approximately $275.0 million of gross charges associated with inventory and purchase commitments in excess of forecasted demand related to ADUHELM, as well as approximately $45.0 million of gross idle capacity charges, which were recognized in cost of sales within our condensed consolidated statements of income. We have recognized approximately $160.0 million related to Eisai's 45.0% share of these charges in collaboration profit (loss) sharing within our condensed consolidated statements of income. Amounts receivable from Eisai related to the agreements discussed above were $402.7 million and $285.4 million as of March 31, 2022 and December 31, 2021 , respectively. Amounts payable to Eisai related to the agreements discussed above were $36.5 million and $46.5 million as of March 31, 2022 and December 31, 2021, respectively . For additional information on the ADUHELM Collaboration Agreement, please read Note 18, Collaborative and Other Relationships, to our consolidated financial statements included in our 2021 Form 10-K. UCB We have a collaboration agreement with UCB to jointly develop and commercialize dapirolizumab pegol, an anti-CD40L pegylated Fab, for the potential treatment of systemic lupus erythematosus and other future agreed indications. Either we or UCB may propose development of dapirolizumab pegol in additional indications. If the parties do not agree to add an indication as an agreed indication to the collaboration, we or UCB may, at the sole expense of the applicable party, pursue development in such excluded indication(s), subject to an opt-in right of the non-pursuing party after proof of clinical activity. All costs incurred for agreed indications, including research, development, sales and marketing expense, are shared equally between us and UCB. Upon marketing approval, we and UCB will co-promote dapirolizumab pegol and share profits equally. A summary of development expense related to the UCB collaboration agreement is as follows: For the Three Months Ended March 31, (In millions) 2022 2021 Total UCB collaboration development expense $ 17.6 $ 16.9 Biogen's share of UCB development expense reflected in research and development expense in our condensed consolidated statements of income 8.8 8.4 Sage Therapeutics, Inc. In November 2020 we entered into a global collaboration and license agreement with Sage to jointly develop and commercialize BIIB125 (zuranolone) for the potential treatment of major depressive disorder and postpartum depression and BIIB124 (SAGE-324) for the potential treatment of essential tremor with potential in other neurological conditions such as epilepsy. Under this collaboration, both companies will share equal responsibility and costs for development as well as profits and losses for commercialization in the U.S. Outside of the U.S., we are responsible for development and commercialization, excluding Japan, Taiwan and South Korea, with respect to zuranolone and may pay Sage potential tiered royalties in the high teens to low twenties. A summary of development and sales and marketing expense related to this collaboration is as follows: For the Three Months Ended March 31, (In millions) 2022 2021 Total Sage collaboration development expense $ 38.7 $ 39.8 Biogen's share of Sage development expense reflected in research and development expense in our condensed consolidated statements of income 19.4 19.9 Total Sage sales and marketing expense incurred by the collaboration 18.4 5.3 Biogen's share of Sage sales and marketing expense reflected in selling, general and administrative expense in our condensed consolidated statements of income 9.2 2.7 Denali Therapeutics Inc. In August 2020 we entered into a collaboration and license agreement with Denali to co-develop and co-commercialize Denali's small molecule inhibitors of leucine-rich repeat kinase 2 (LRRK2) for Parkinson's disease. Under this collaboration, both companies share responsibility and costs for global development based on specified percentages as well as profits and losses for commercialization in the U.S. and China. Outside the U.S. and China we are responsible for commercialization and may pay Denali potential tiered royalties. In addition to the LRRK2 program, we also have an exclusive option to license two preclinical programs from Denali’s Transport Vehicle platform, including its Antibody Transport Vehicle (ATV): ATV enabled anti-amyloid beta program and a second program utilizing its Transport Vehicle technology. Further, we have the right of first negotiation on two additional ATV enabled therapeutics for indications within specific neurodegenerative diseases, should Denali decide to seek a collaboration for such programs. A summary of development expense related to this collaboration is as follows: For the Three Months Ended March 31, (In millions) 2022 2021 Total Denali collaboration development expense $ 14.9 $ 15.4 Biogen's share of Denali development expense reflected in research and development expense in our condensed consolidated statements of income 8.9 9.2 Sangamo Therapeutics, Inc. In February 2020 we entered into a collaboration and license agreement with Sangamo to develop and commercialize ST-501 for tauopathies, including Alzheimer's disease; ST-502 for synucleinopathies, including Parkinson’s disease; a third neuromuscular disease target; and up to nine additional neurological disease targets to be identified and selected within a five-year period. The companies are leveraging Sangamo’s proprietary zinc finger protein technology delivered via adeno-associated virus to modulate the expression of key genes involved in neurological diseases. Under this collaboration, we may pay Sangamo tiered royalties on potential net sales of any products developed under this collaboration in the high single digit to sub-teen percentages. A summary of development expense related to this collaboration is as follows: For the Three Months Ended March 31, (In millions) 2022 2021 Total Sangamo collaboration development expense $ 8.3 $ 4.8 Biogen's share of Sangamo development expense reflected in research and development expense in our condensed consolidated statements of income 5.5 4.2 InnoCare Pharma Limited In July 2021 we entered into a collaboration and license agreement with InnoCare Pharma Limited (InnoCare) for orelabrutinib, an oral small molecule Bruton’s tyrosine kinase inhibitor for the potential treatment of MS. Orelabrutinib is currently being studied in a multi-country, placebo-controlled Phase 2 trial in relapsing-remitting MS. Under the terms of the collaboration, we have exclusive rights to orelabrutinib in the field of MS worldwide and certain autoimmune diseases outside of China (including Hong Kong, Macau and Taiwan), while InnoCare retains ex clusive worldwide rights to orelabrutinib in the field of oncology and certain autoimmune diseases in China (including Hong Kong, Macau and Taiwan). In connection with the closing of this transaction in August 2021 we made an upfront payment of $125.0 million that was recorded as research and development expense in our condensed consolidated statements of income. We may also pay InnoCare up to approximately $812.5 million in potential development milestones and potential commercial payments should this collaboration achieve certain development, commercial milestones and sales thresholds. In addition, we may pay InnoCare tiered royalties on potential net sales of any products developed under this collaboration in the low to high teen percentages. Other Research and Discovery Arrangements These arrangements may include the potential for future milestone payments based on the achievement of certain clinical and commercial development payable over a period of several years. Other For the three months ended March 31, 2022, we recorded $19.5 million as research and development expense in our condensed consolidated statements of income related to other research and discovery related arrangements, compared to less than $1.0 million in the prior year comparative period. Samsung Bioepis Co., Ltd. Joint Venture Agreement In February 2012 we entered into a joint venture agreement with Samsung BioLogics establishing an entity, Samsung Bioepis, to develop, manufacture and market biosimilar products. As of March 31, 2022, our ownership percentage was 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. Upon investment, the equity method of accounting requires us to identify and allocate differences between the fair value of our investment and the carrying value of our interest in the underlying net assets of the investee. These basis differences are amortized over their economic life. The total basis difference was approximately $675.0 million and relates to inventory, developed technology, IPR&D and deferred tax balances. The basis differences related to inventory were amortized, net of tax, over their estimated useful lives of 1.5 years, and the basis differences related to developed technology and IPR&D for marketed products will be amortized, net of tax, over their estimated useful lives of 15 years. For the three months ended March 31, 2022, we recognized net losses on our investment of $3.3 million, reflecting our share of Samsung Bioepis' operating profits, net of tax totaling $4.0 million offset by amortization of basis differences totaling $7.3 million. For the three months ended March 31, 2021, we recognized net losses on our investment of $18.2 million, reflecting our share of Samsung Bioepis' operating losses, net of tax totaling $11.0 million and amortization of basis differences totaling $7.2 million. As of March 31, 2022 and December 31, 2021, the carrying value of our investment in Samsung Bioepis totaled 709.4 billion South Korean won ($586.4 million) and 713.3 billion South Korean won ($599.9 million), respectively, which is classified as a component of investments and other assets in our condensed consolidated balance sheets. In January 2022 we entered into an agreement to sell to Samsung BioLogics our equity in Samsung Bioepis, which was completed on April 20, 2022. Under the terms of the transaction, we received approximately $1.0 billion in cash at closing and will receive approximately $1.3 billion to be deferred over two payments of $812.5 million due at the first anniversary and $437.5 million due at the second anniversary of the closing of the transaction. We are also eligible to receive up to an additional $50.0 million upon the achievement of certain commercial milestones. 2019 Development and Commercialization Agreement In December 2019 we completed a transaction with Samsung Bioepis and secured the exclusive rights to commercialize two potential ophthalmology biosimilar products, BYOOVIZ (ranibizumab-nuna), a proposed ranibizumab biosimilar referencing LUCENTIS, and SB15, a proposed aflibercept biosimilar referencing EYLEA, in major markets worldwide, including the U.S., Canada, Europe, Japan and Australia. Samsung Bioepis will be responsible for development and will supply both products to us at a pre-specified gross margin. In connection with this transaction, we made an upfront payment of $100.0 million to Samsung Bioepis in January 2020, of which $63.0 million was recorded as research and development expense in our condensed consolidated statements of income in 2019 and $37.0 million was recorded as intangible assets, net in our condensed consolidated balance sheets in 2019. During the third quarter of 2021 we accrued $15.0 million in milestone payments related to the approval of BYOOVIZ in the U.S., the E.U. and the U.K., that were capitalized within intangible assets, net in our condensed consolidated balance sheets. We may also pay Samsung Bioepis up to approximately $180.0 million in additional development, regulatory and sales-based milestones. We also acquired an option to extend the term of our 2013 c ommercial agreement for BENEPALI, IMRALDI and FLIXABI by an additional five years, subject to payment of an option exercise fee of $60.0 million, and obtained an option to acquire exclusive rights to commercialize these products in China. 2013 Commercial Agreement We reflect revenue on sales of BENEPALI, IMRALDI and FLIXABI to third parties in product revenue, net in our condensed consolidated statements of income and record the related cost of revenue and sales and marketing expense in our condensed consolidated statements of income to their respective line items when these costs are incurred. We share 50.0% of the profit or loss related to our commercial agreement with Samsung Bioepis, which is recognized in collaboration profit (loss) sharing in our condensed consolidated statements of income. For the three months ended March 31, 2022, we recognized net profit-sharing expense of $64.4 million to reflect Samsung Bioepis' 50.0% sharing of the net collaboration profits, compared to a net profit-sharing expense of $68.5 million in the prior year comparative period. Other Services Simultaneous with the formation of Samsung Bioepis, we also entered into a technical development services agreement, a manufacturing agreement and a license agreement with Samsung Bioepis. Revenue related to these services is reflected in revenue from collaborative and other relationships as a component of other revenue in our condensed consolidated statements of income. Amounts receivable from Samsung Bioepis related to the agreements discussed above were $5.8 million and $4.1 million as of March 31, 2022 and December 31, 2021, respectively. Amounts payable to Samsung Bioepis related to the agreements discussed above were $102.5 million and $148.7 million as of March 31, 2022 and December 31, 2021, respectively. For additional information on our collaboration arrangements with Samsung Bioepis and our other significant collaboration arrangements, please read Note 18, Collaborative and Other Relationships,

Investments in Variable Interes

Investments in Variable Interest Entities	3 Months Ended
	Mar. 31, 2022
Investments in Variable Interest Entities [Abstract]
Investments in Variable Interest Entities	Consolidated Variable Interest Entities Our condensed consolidated financial statements include the financial results of variable interest entities in which we are the primary beneficiary. The following are our significant variable interest entities. Neurimmune SubOne AG We have a collaboration and license agreement with Neurimmune for the development and commercialization of antibodies for the potential treatment of Alzheimer's disease, including ADUHELM (as amended, the Neurimmune Agreement). We are responsible for the development, manufacturing and commercialization of all collaboration products. The Neurimmune Agreement is effective for the longer of the duration of certain patents relating to a licensed product or 12 years from the first commercial sale of a licensed product. We consolidate the results of Neurimmune as we determined that we are the primary beneficiary of Neurimmune because we have the power through the collaboration to direct the activities that most significantly impact the entity’s economic performance and we are required to fund 100.0% of the research and development costs incurred in support of the collaboration. In June 2021 ADUHELM was granted accelerated approval by the FDA. Under the terms of the Neurimmune Agreement, we were required to pay Neurimmune a milestone payment of $100.0 million related to the launch of ADUHELM in the U.S. During the second quarter of 2021 we made this $100.0 million payment, which was recognized as a charge to net income (loss) attributable to noncontrolling interests, net of tax in our condensed consolidated statements of income. In addition, during the second quarter of 2021 we recognized net profit-sharing income of $45.0 million to reflect Eisai's 45.0% share of the $100.0 million milestone payment, which was recognized in collaboration profit (loss) sharing in our condensed consolidated statements of income. Additionally, if aducanumab receives regulatory approval in the jurisdictions where we have submitted filings, we may pay additional milestones to Neurimmune, including $50.0 million if launched in Japan. Milestones payable to Neurimmune are shared expenses under the ADUHELM Collaboration Agreement. Research and development costs for which we reimburse Neurimmune are reflected in research and development expense in our condensed consolidated statements of income. During the three months ended March 31, 2022 and 2021, amounts reimbursed were immaterial. During the second quarter of 2021 we recorded a net deferred tax asset in Switzerland of approximately $490.0 million on Neurimmune's tax basis in ADUHELM, the realization of which is dependent on future sales of ADUHELM. During the fourth quarter of 2021, due to reduced future expected revenue associated with ADUHELM, we recorded a valuation allowance of approximately $390.0 million. During the first quarter of 2022, upon issuance of the final NCD related to ADUHELM, we recorded an additional valuation allowance of approximately $85.0 million to reduce the net value of this deferred tax asset to zero. These adjustments to our deferred tax assets and their valuation allowances are each recorded with an equal and offsetting amount assigned to net income (loss) attributable to noncontrolling interests, net of tax in our condensed consolidated statements of income, resulting in a zero net impact to net income attributable to Biogen Inc. Excluding the impact of the Neurimmune deferred tax asset, 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. For additional information on our collaboration arrangements with Eisai, please read Note 16, Collaborative and Other Relationships, to these condensed consolidated financial statements. Unconsolidated Variable Interest Entities We have relationships with various variable interest entities that we do not consolidate as we lack the power to direct the activities that significantly impact the economic success of these entities. These relationships include investments in certain biotechnology companies and research collaboration agreements. As of March 31, 2022 and December 31, 2021, the carrying value of our investments in certain biotechnology companies representing potential unconsolidated variable interest entities totaled $24.1 million and $24.6 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 previous contractually required amounts. For additional information on our investments in Neurimmune and other variable interest entities, please read Note 19, Investments in Variable Interest Entities, to our consolidated financial statements included in our 2021 Form 10-K.

Litigation

Litigation	3 Months Ended
	Mar. 31, 2022
Loss Contingency, Information about Litigation Matters [Abstract]
Litigation	We are currently involved in various claims and legal proceedings, including the matters described below. For information as to our accounting policies relating to claims and legal proceedings, including use of estimates and contingencies, please read Note 1, Summary of Significant Accounting Policies, to our consolidated financial statements included in our 2021 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 ADUHELM Securities Litigation We and certain current and former officers are named as defendants in actions filed by shareholders on November 13, 2020 and February 7, 2022, and pending in the U.S. District Court for the District of Massachusetts. The actions allege violations of federal securities laws under 15 U.S.C §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5 and seek declarations of the actions as class actions and monetary relief. We have filed a motion to dismiss the action filed on November 13, 2020, which is pending. An estimate of the possible loss or range of loss in these actions cannot be made at this time. Derivative Action We and members of the Board of Directors are named as defendants in an action filed by a shareholder on February 9, 2022, in the U.S. District Court for the District of Massachusetts. The action alleges violations of federal securities laws under 15 U.S.C. §78n(a) and 17 C.F.R. §240 14.a-9, breaches of fiduciary duties and waste of corporate assets, and seeks declaratory and injunctive relief, monetary relief to Biogen, and attorneys’ fees and costs to the plaintiff. An estimate of the possible loss or range of loss cannot be made at this time. 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 (the French proceeding) and in November 2018 against Biogen GmbH in the Düsseldorf Regional Court (the German proceeding), alleging that IMRALDI, the adalimumab biosimilar product of Samsung Bioepis that Biogen has commercialized in Europe, infringes national counterparts of European Patent No. 3 148 510 (the EP '510 Patent, expiring in May 2035). In May 2020 the European Patent Office (EPO) held the EP '510 Patent invalid. Fresenius Kabi has appealed to the EPO's Technical Boards of Appeal, a hearing has been set for June 2022, and the German and French proceedings have been stayed pending the decision on appeal. In June 2020 Fresenius Kabi commenced proceedings in Denmark's Maritime and Commercial High Court alleging that IMRALDI infringes the Danish counterpart of European Patent No. 3 145 488 (the EP '488 Patent, expiring in May 2035) and a Danish utility model. In September 2021 the Court ruled that the patent and utility model are invalid and not infringed. Fresenius Kabi has appealed to the High Court of Eastern Denmark and the appeal is pending. The EPO has scheduled a hearing on the validity of the EP '488 Patent for October 2022. In July 2020 the Danish Patent Board of Appeal revoked the Danish utility models that Fresenius Kabi had asserted against Biogen and Fresenius Kabi has appealed to the Danish Maritime and Commercial High Court. No hearing has been scheduled. In July 2019 Gedeon Richter Nyrt commenced proceedings for damages and injunctive relief against Biogen GmbH in the Düsseldorf Regional Court alleging infringement of the German counterpart of European Patent No. 3 212 667, which expires in October 2035. The case has been stayed pending proceedings in the EPO seeking to invalidate the patent. In November 2020 Gedeon Richter Nyrt commenced additional proceedings against Biogen GmbH in the Düsseldorf Regional Court alleging infringement of a German utility model. In October 2021 Biogen filed cancellation proceedings in respect of the German utility model and the infringement proceedings have been stayed pending the outcome of the cancellation proceedings. An estimate of the possible loss or range of loss in the IMRALDI patent litigation described above cannot be made at this time. Qui Tam Litigation In July 2015 a qui tam action filed by Michael Bawduniak on behalf of the U.S. and certain states was unsealed by the U.S. District Court for the District of Massachusetts. The action alleges sales and promotional activities in violation of the federal False Claims Act and state law counterparts and seeks damages of $981.1 million plus statutory trebling of damages, civil penalties, attorneys’ fees and costs. The U.S. has not made an intervention decision. A trial is scheduled for the third quarter of 2022. An estimate of the possible loss cannot be made at this time. Dispute with Former Convergence Shareholders In November and December 2019 Shareholder Representative Services LLC, on behalf of the former shareholders of Convergence, sent us correspondence asserting claims of $200.0 million for alleged breach of the contract under which we acquired Convergence. We dispute the claims. ERISA Class Action Litigation In September 2020 the U.S. District Court for the District of Massachusetts consolidated two cases filed against us in July and August 2020 by participants in the Biogen 401(k) Savings Plan alleging breach of fiduciary duty under ERISA. Plaintiffs seek a declaration of the action as a class action and monetary and other relief. An estimate of the possible loss or range of loss cannot be made at this time. Humana Patient Assistance Litigation In September 2021 Humana Inc. (Humana) filed suit against us in the U.S. District Court for the District of Massachusetts alleging damages related to our providing MS patients with free medications and making charitable contributions to non-profit organizations that assist MS patients. Humana alleges violation of the federal RICO Act and state laws and seeks statutory treble damages, attorneys' fees and costs. We filed a motion to dismiss, which is pending. An estimate of the possible loss cannot be made at this time. Other Matters Government Investigations The U.S. House of Representatives Committees on Oversight and Reform and Energy and Commerce and the Office of Inspector General of the U.S. Department of Health and Human Services have announced investigations relating to ADUHELM. In addition, the Company has received a civil investigative demand from the Federal Trade Commission and a subpoena from the Securities and Exchange Commission seeking information relating to ADUHELM, including healthcare sites, ADUHELM’s approval and ADUHELM’s marketing. TECFIDERA Patent Matters In 2017 to 2020 we filed patent infringement proceedings relating to TECFIDERA Orange-Book listed patents pursuant to the Drug Price Competition and Patent Term Restoration Act of 1984, commonly known as the Hatch-Waxman Act (the Delaware Actions), against Accord Healthcare Inc., Alkem Laboratories Ltd., Amneal Pharmaceuticals LLC, Cipla Limited, Graviti Pharmaceuticals Pvt. Ltd., Hetero USA, Inc., Lupin Atlantis Holdings SA, Macleods Pharmaceuticals, Ltd., MSN Laboratories Pvt. Ltd., Pharmathen S.A., Prinston Pharmaceutical Inc., Sandoz Inc., Shilpa Medicare Limited, Slayback Pharma LLC, Sun Pharmaceutical Industries, Ltd., Sun Pharmaceutical Industries, Inc., Sun Pharma Global FZE, Torrent Pharmaceuticals Ltd., TWi Pharmaceuticals, Inc., Windlas Healthcare Pvt. Ltd. and Zydus Pharmaceuticals (USA) Inc. (collectively, the Delaware Defendants) in the U.S. District Court for the District of Delaware (the Delaware Court) and against Mylan (the West Virginia Action) in the U.S. District Court for the Northern District of West Virginia (the West Virginia Court). In November 2021 the Federal Circuit affirmed the West Virginia Court judgment that the asserted claims of our U.S. Patent No. 8,399,514 (the '514 Patent) are invalid for lack of written description. The Delaware Court entered judgment for the Delaware Defendants on the grounds that the judgment of the West Virginia Court applies to the Delaware Actions under principles of collateral estoppel. The appeals in the Delaware Actions are stayed. In July 2018 Mylan Pharmaceuticals, Inc. (Mylan) also filed a petition with the U.S. Patent Trial and Appeal Board (PTAB) for inter partes review of the '514 Patent. In November 2021 the Federal Circuit ruled that the PTAB decision upholding the patentability of the ‘514 patent was moot, but in April 2022 the Federal Circuit vacated that ruling and stayed the appeal pending any final action by the United States Supreme Court in the West Virginia Action. TYSABRI Patent Revocation Matters In November 2017 Swiss Pharma International AG, affiliated with the Polpharma Group, filed an action in the Commercial Court of Rome to invalidate the Italian counterpart of the European Patent No. 1 485 127 (the EP '127 Patent) which covers administration of natalizumab (TYSABRI) to treat MS and expires in February 2023. A hearing has been set for June 2022. In August 2020 Polpharma Biologics S.A., also affiliated with the Polpharma Group, brought an action in the Polish Patent Office to revoke our Polish Patent No. 215263, which corresponds to the EP '127 Patent and expires in February 2023. The action was suspended by the Polish Patent Office in April 2021 pending examination of our amended patent claims. In June 2021 Polpharma Biologics S.A., Sandoz B.V. and Sandoz AG filed an action in the District Court of the Hague, Netherlands to invalidate the Dutch counterpart of our European Patent 2 676 967 (the EP '967 Patent), which expires in 2027 and covers methods of treatment using natalizumab (TYSABRI) and pre-treatment testing of patients. A hearing has been set for September 2022. In July 2021 the EPO revoked the EP ‘967 Patent. We have appealed to the EPO’s Technical Boards of Appeal. A hearing date has not been set. In September 2021 Polpharma Biologics S.A., Sandoz AG, Sandoz Limited and Sandoz GmbH filed an action in the English High Court to revoke the U.K. counterpart of the EP ‘967 Patent and seeking a declaration that the patent would not be infringed by the marketing of Polpharma’s proposed natalizumab biosimilar. A hearing has been set for November 2022. Annulment Proceedings in General Court of the European Union relating to TECFIDERA Pharmaceutical Works Polpharma SA (Polpharma) and Mylan Ireland Ltd. (Mylan Ireland) each filed actions in the General Court of the European Union (Polpharma in October 2018 and Mylan Ireland in November 2020) to annul the European Medicines Agency's (EMA) decision not to validate their applications to market generic versions of TECFIDERA on the grounds that TECFIDERA benefits from regulatory data protection. On May 5, 2021, the European General Court annulled the EMA's non-validation decision with respect to Polpharma. We have appealed the decision to the European Court of Justice and the appeal is pending. The case brought by Mylan Ireland has been stayed. 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.

Subsequent Events

Subsequent Events	3 Months Ended
	Mar. 31, 2022
Subsequent Events [Abstract]
Subsequent Events	SUBSEQUENT EVENTSOn May 3, 2022, Biogen Inc. (the "Company") announced that it had begun a search for a new Chief Executive Officer ("CEO"). The Company has agreed with its current CEO, Michel Vounatsos, that he will continue to serve as CEO during the interim period. Mr. Vounatsos will remain on the Board of Directors of Biogen until he steps down from his CEO position.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2022
Accounting Policies [Abstract]
Overview	Biogen is a global biopharmaceutical company focused on discovering, developing and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases as well as related therapeutic adjacencies. We have a leading portfolio of medicines to treat multiple sclerosis (MS), have introduced the first approved treatment for spinal muscular atrophy (SMA) and are providing the first and only approved treatment to address a defining pathology of Alzheimer's disease. We also commercialize biosimilars of advanced biologics and focus on advancing our pipeline in neuroscience and specialized immunology. Lastly, we are focused on accelerating our efforts in digital health to support our commercial and pipeline programs while also creating opportunities for potential digital therapeutics. We support our drug discovery and development efforts through the commitment of significant resources to discovery, research and development programs and business development opportunities. Our marketed products include TECFIDERA, VUMERITY, AVONEX, PLEGRIDY, TYSABRI and FAMPYRA for the treatment of MS; SPINRAZA for the treatment of SMA; ADUHELM for the treatment of Alzheimer's disease; and FUMADERM for the treatment of severe plaque psoriasis. We have certain business and financial rights with respect to RITUXAN for the treatment of non-Hodgkin's lymphoma, chronic lymphocytic leukemia (CLL) and other conditions; RITUXAN HYCELA for the treatment of non-Hodgkin's lymphoma and CLL; GAZYVA for the treatment of CLL and follicular lymphoma; OCREVUS for the treatment of primary progressive MS (PPMS) and relapsing MS (RMS); and other potential anti-CD20 therapies, including mosunetuzumab, pursuant to our collaboration arrangements with Genentech, Inc. (Genentech), a wholly-owned member of the Roche Group. For additional information on our collaboration arrangements with Genentech, please read Note 18, Collaborative and Other Relationships, to our audited consolidated financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2021 (2021 Form 10-K). Our innovative drug development and commercialization activities are complemented by our biosimilar business that expands access to medicines and reduces the cost burden for healthcare systems. Through our collaboration with Samsung Bioepis Co., Ltd. (Samsung Bioepis) we market and sell BENEPALI, an etanercept biosimilar referencing ENBREL, IMRALDI, an adalimumab biosimilar referencing HUMIRA, and FLIXABI, an infliximab biosimilar referencing REMICADE, in certain countries in Europe. We have also secured the exclusive rights to commercialize BYOOVIZ, a ranibizumab biosimilar referencing LUCENTIS, which was approved in the U.S., the European Union (E.U.) and the United Kingdom (U.K.) during the third quarter of 2021. For additional information on our collaboration arrangements with Samsung Bioepis, please read Note 16, Collaborative and Other Relationships,
Basis of presentation	In the opinion of management, our condensed consolidated financial statements include all adjustments, consisting of normal recurring accruals, necessary for a fair statement of our financial statements for interim periods in accordance with accounting principles generally accepted in the United States (U.S. GAAP). The information included in this quarterly report on Form 10-Q should be read in conjunction with our audited consolidated financial statements and the accompanying notes included in our 2021 Form 10-K. Our accounting policies are described in the Notes to Consolidated Financial Statements in our 2021 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, 2022, are not necessarily indicative of the operating results for the full year or for any other subsequent interim period. We operate as one operating segment, focused on discovering, developing and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases as well as related therapeutic adjacencies.
Consolidation	Our condensed consolidated financial statements reflect our financial statements, those of our wholly-owned subsidiaries and those of certain variable interest entities where we are the primary beneficiary. For consolidated entities where we own or are exposed to less than 100.0% of the economics, we record net income (loss) attributable to noncontrolling interests, net of tax in our condensed consolidated statements of income equal to the percentage of the economic or ownership interest retained in such entities by the respective noncontrolling parties. Intercompany balances and transactions are eliminated in consolidation. In determining whether we are the primary beneficiary of a variable interest entity, we apply a qualitative approach that determines whether we have both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. We continuously assess whether we are the primary beneficiary of a variable interest entity as changes to existing relationships or future transactions may result in us consolidating or deconsolidating one or more of our collaborators or partners.
Use of estimates	The preparation of our condensed consolidated financial statements requires us to make estimates, judgments and assumptions that may affect the reported amounts of assets, liabilities, equity, revenue and expense and related disclosure of contingent assets and liabilities. On an ongoing basis we evaluate our estimates, judgments and assumptions. We base our estimates on historical experience and on various other assumptions that we believe are reasonable, the results of which form the basis for making judgments about the carrying values of assets, liabilities and equity and the amount of revenue and expense. Actual results may differ from these estimates. The length of time and full extent to which the COVID-19 pandemic directly or indirectly impacts our business, results of operations and financial condition, including sales, expense, reserves and allowances, the supply chain, manufacturing, clinical trials, research and development costs and employee-related costs, depends on future developments that are highly uncertain, subject to change and are difficult to predict, including as a result of new information that may emerge concerning COVID-19 and the actions taken to contain or treat COVID-19 as well as the economic impact on local, regional, national and international customers and markets. Additionally, the ongoing geopolitical tensions related to Russia's invasion of Ukraine, and the related sanctions and other penalties imposed, are creating substantial uncertainty in the global economy. The extent and duration of the conflict, sanctions and resulting market disruptions are highly unpredictable. We have made estimates of the impact of the COVID-19 pandemic and the ongoing geopolitical conflict within our condensed consolidated financial statements and there may be changes to those estimates in future periods.
New accounting pronouncements	From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board or other standard setting bodies that we adopt as of the specified effective date. We do not believe that the adoption of recently issued standards have had or may have a material impact on our condensed consolidated financial statements or disclosures.

Restructuring, Business Trans_2

Restructuring, Business Transformation and Other Cost Saving Initiatives (Tables)	3 Months Ended
	Mar. 31, 2022
Restructuring and Related Activities [Abstract]
Schedule of Restructuring Reserve by Type of Cost	The following table summarizes the charges and spending related to our 2022 cost saving initiatives for the three months ended March 31, 2022: (In millions) Workforce Reduction Total Restructuring reserve as of December 31, 2021 $ — $ — Expense 27.7 27.7 Payment (6.2) (6.2) Restructuring reserve as of March 31, 2022 $ 21.5 $ 21.5

Revenues (Tables)

Revenues (Tables)	3 Months Ended
	Mar. 31, 2022
Revenue from Contract with Customer [Abstract]
Revenues by product	Revenue by product is summarized as follows: For the Three Months Ended March 31, 2022 2021 (In millions) United Rest of Total United Rest of Total Multiple Sclerosis (MS): Fumarate (1) $ 242.3 $ 295.6 $ 537.9 $ 236.0 $ 316.9 $ 552.9 Interferon (2) 182.3 127.3 309.6 241.8 158.7 400.5 TYSABRI 284.5 236.3 520.8 273.3 230.0 503.3 FAMPYRA — 26.2 26.2 — 26.6 26.6 Subtotal: MS 709.1 685.4 1,394.5 751.1 732.2 1,483.3 Spinal Muscular Atrophy: SPINRAZA 163.3 309.2 472.5 148.7 371.8 520.5 Alzheimer's disease: ADUHELM (3) 2.8 — 2.8 — — — Biosimilars: BENEPALI — 114.7 114.7 — 121.7 121.7 IMRALDI — 57.1 57.1 — 57.9 57.9 FLIXABI — 22.5 22.5 — 25.5 25.5 Subtotal: Biosimilars — 194.3 194.3 — 205.1 205.1 Other: FUMADERM — 2.2 2.2 — 2.8 2.8 Total product revenue $ 875.2 $ 1,191.1 $ 2,066.3 $ 899.8 $ 1,311.9 $ 2,211.7 (1) Fumarate includes TECFIDERA and VUMERITY. VUMERITY became commercially available in the E.U. during the fourth quarter of 2021. (2) Interferon includes AVONEX and PLEGRIDY. (3) In June 2021 the U.S. Food and Drug Administration (FDA) granted accelerated approval of ADUHELM, which became commercially available in the U.S. during the second quarter of 2021. For additional information, please read Note 16, Collaborative and Other Relationships - Eisai Co., Ltd. - ADUHELM Collaboration Agreement, to these condensed consolidated financial statements.
Analysis of change In reserves	An analysis of the change in reserves for discounts and allowances is summarized as follows: (In millions) Discounts Contractual Returns Total Balance, December 31, 2021 $ 137.7 $ 759.6 $ 38.0 $ 935.3 Current provisions relating to sales in current year 169.0 659.5 4.0 832.5 Adjustments relating to prior years (3.9) (40.2) (2.5) (46.6) Payments/credits relating to sales in current year (101.2) (271.6) (0.5) (373.3) Payments/credits relating to sales in prior years (58.9) (314.5) (6.5) (379.9) Balance, March 31, 2022 $ 142.7 $ 792.8 $ 32.5 $ 968.0
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, 2022 As of December 31, 2021 Reduction of accounts receivable $ 136.9 $ 133.2 Component of accrued expense and other 831.1 802.1 Total revenue-related reserves $ 968.0 $ 935.3
Revenues from anti-CD20 therapeutic programs	Revenue from anti-CD20 therapeutic programs is summarized in the table below. For the purposes of this footnote, we refer to RITUXAN and RITUXAN HYCELA collectively as RITUXAN. For the Three Months Ended March 31, (In millions) 2022 2021 Biogen’s share of pre-tax profits in the U.S. for RITUXAN and GAZYVA $ 143.2 $ 174.1 OCREVUS and other revenue from anti-CD20 therapeutic programs 256.2 214.9 Total revenue from anti-CD20 therapeutic programs $ 399.4 $ 389.0
Other revenues	Other revenue is summarized as follows: For the Three Months Ended March 31, (In millions) 2022 2021 Revenue from collaborative and other relationships: Revenue earned under our technical development agreement, manufacturing services agreements and royalty revenue on biosimilar products with Samsung Bioepis $ 8.0 $ 3.9 Other royalty and corporate revenue: Royalty 10.6 6.2 Other corporate 47.5 83.2 Total other revenue $ 66.1 $ 93.3

Inventory (Tables)

Inventory (Tables)	3 Months Ended
	Mar. 31, 2022
Inventory Disclosure [Abstract]
Components of inventory	The components of inventory are summarized as follows: (In millions) As of March 31, 2022 As of December 31, 2021 Raw materials $ 354.5 $ 349.6 Work in process 655.6 814.0 Finished goods 205.3 187.9 Total inventory $ 1,215.4 $ 1,351.5

Intangible Assets and Goodwill

Intangible Assets and Goodwill (Tables)	3 Months Ended
	Mar. 31, 2022
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, 2022 As of December 31, 2021 (In millions) Estimated Life Cost Accumulated Net Cost Accumulated Net Completed technology 4-28 years $ 7,413.1 $ (5,455.4) $ 1,957.7 $ 7,413.1 $ (5,388.5) $ 2,024.6 In-process research and development Indefinite until commercialization 129.1 — 129.1 132.7 — 132.7 Trademarks and trade names Indefinite 64.0 — 64.0 64.0 — 64.0 Total intangible assets $ 7,606.2 $ (5,455.4) $ 2,150.8 $ 7,609.8 $ (5,388.5) $ 2,221.3
Estimated future amortization for acquired intangible assets	The estimated future amortization of finite-lived intangible assets for the next five years is expected to be as follows: (In millions) As of March 31, 2022 2022 (remaining nine months) $ 200.0 2023 210.0 2024 195.0 2025 195.0 2026 180.0 2027 165.0
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, 2022 Goodwill, December 31, 2021 $ 5,761.1 Other (3.1) Goodwill, March 31, 2022 $ 5,758.0

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2022
Fair Value Disclosures [Abstract]
Summary of assets and liabilities recorded at fair value	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, 2022 (In millions) Total Quoted Prices Significant Other Significant Assets: Cash equivalents $ 1,283.7 $ — $ 1,283.7 $ — Marketable debt securities: Corporate debt securities 1,579.4 — 1,579.4 — Government securities 1,252.7 — 1,252.7 — Mortgage and other asset backed securities 171.9 — 171.9 — Marketable equity securities 860.3 607.0 253.3 — Derivative contracts 97.2 — 97.2 — Plan assets for deferred compensation 35.8 — 35.8 — Total $ 5,281.0 $ 607.0 $ 4,674.0 $ — Liabilities: Derivative contracts $ 12.5 $ — $ 12.5 $ — Contingent consideration obligations 202.0 — — 202.0 Total $ 214.5 $ — $ 12.5 $ 202.0 As of December 31, 2021 (In millions) Total Quoted Prices Significant Other Significant Assets: Cash equivalents $ 1,632.2 $ — $ 1,632.2 $ — Marketable debt securities: Corporate debt securities 1,108.2 — 1,108.2 — Government securities 1,192.7 — 1,192.7 — Mortgage and other asset backed securities 132.2 — 132.2 — Marketable equity securities 1,048.5 181.7 866.8 — Derivative contracts 80.9 — 80.9 — Plan assets for deferred compensation 33.4 — 33.4 — Total $ 5,228.1 $ 181.7 $ 5,046.4 $ — Liabilities: Derivative contracts $ 10.8 $ — $ 10.8 $ — Contingent consideration obligations 209.1 — — 209.1 Total $ 219.9 $ — $ 10.8 $ 209.1
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, 2022 As of December 31, 2021 (In millions) Fair Carrying Fair Carrying 3.625% Senior Notes due September 15, 2022 $ 1,008.7 $ 999.5 $ 1,020.0 $ 999.1 4.050% Senior Notes due September 15, 2025 1,797.6 1,743.4 1,895.2 1,742.9 2.250% Senior Notes due May 1, 2030 1,341.6 1,492.2 1,475.9 1,492.0 5.200% Senior Notes due September 15, 2045 1,240.0 1,100.0 1,463.0 1,099.9 3.150% Senior Notes due May 1, 2050 1,225.3 1,473.3 1,457.7 1,473.2 3.250% Senior Notes due February 15, 2051 581.8 466.8 692.9 466.0 Total $ 7,195.0 $ 7,275.2 $ 8,004.7 $ 7,273.1
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) 2022 2021 Fair value, beginning of period $ 209.1 $ 259.8 Changes in fair value (7.1) (33.8) Fair value, end of period $ 202.0 $ 226.0

Financial Instruments (Tables)

Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2022
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, 2022 As of December 31, 2021 Commercial paper $ 278.5 $ 247.6 Overnight reverse repurchase agreements 13.9 200.0 Money market funds 585.0 901.6 Short-term debt securities 406.3 283.0 Total $ 1,283.7 $ 1,632.2
Marketable debt and equity securities	The following tables summarize our marketable debt and equity securities, classified as available-for-sale: As of March 31, 2022 (In millions) Amortized Gross Gross Fair Marketable debt securities Corporate debt securities: Current $ 1,126.7 $ 0.1 $ (2.2) $ 1,124.6 Non-current 460.5 0.1 (5.8) 454.8 Government securities: Current 878.7 1.0 (2.6) 877.1 Non-current 379.0 0.2 (3.6) 375.6 Mortgage and other asset backed securities: Current 0.7 — — 0.7 Non-current 173.4 — (2.2) 171.2 Total marketable debt securities $ 3,019.0 $ 1.4 $ (16.4) $ 3,004.0 Marketable equity securities Marketable equity securities, current $ 33.9 $ — $ — $ 33.9 Marketable equity securities, non-current 1,133.1 4.5 (311.2) 826.4 Total marketable equity securities $ 1,167.0 $ 4.5 $ (311.2) $ 860.3 As of December 31, 2021 (In millions) Amortized Gross Gross Fair Marketable debt securities Corporate debt securities: Current $ 723.6 $ 0.1 $ (0.3) $ 723.4 Non-current 385.4 0.2 (0.8) 384.8 Government securities: Current 817.0 — (0.4) 816.6 Non-current 377.0 0.1 (1.0) 376.1 Mortgage and other asset backed securities: Current 1.1 — — 1.1 Non-current 131.8 — (0.7) 131.1 Total marketable debt securities $ 2,435.9 $ 0.4 $ (3.2) $ 2,433.1 Marketable equity securities Marketable equity securities, current $ 33.9 $ 9.9 $ — $ 43.8 Marketable equity securities, non-current 1,133.1 151.0 (279.4) 1,004.7 Total marketable equity securities $ 1,167.0 $ 160.9 $ (279.4) $ 1,048.5
Summary of contractual maturities: available-for-sale securities	The estimated fair value and amortized cost of our marketable debt securities available-for-sale by contractual maturity are summarized as follows: As of March 31, 2022 As of December 31, 2021 (In millions) Estimated Amortized Estimated Amortized Due in one year or less $ 2,002.4 $ 2,006.0 $ 1,541.1 $ 1,541.7 Due after one year through five years 982.9 993.7 868.2 870.2 Due after five years 18.7 19.3 23.8 24.0 Total marketable debt securities $ 3,004.0 $ 3,019.0 $ 2,433.1 $ 2,435.9
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) 2022 2021 Proceeds from maturities and sales $ 543.6 $ 819.2 Realized gains — 0.2 Realized losses 0.6 0.7

Derivative Instruments (Tables)

Derivative Instruments (Tables)	3 Months Ended
	Mar. 31, 2022
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Foreign currency forward contracts that were entered into to hedge forecasted revenue	The notional amount of foreign currency forward contracts that were entered into to hedge forecasted revenue and operating expense is summarized as follows: Notional Amount (In millions) As of March 31, 2022 As of December 31, 2021 Euro $ 1,631.2 $ 1,828.0 British pound 126.6 166.2 Swiss franc 128.6 — Japanese yen 55.0 72.7 Canadian dollar 44.6 59.9 Total foreign currency forward contracts $ 1,986.0 $ 2,126.8
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) Net Gains/(Losses) Location 2022 2021 Location 2022 2021 Revenue $ 20.9 $ (23.1) Revenue $ (6.5) $ (3.0) Operating expense (0.3) (0.4) Operating expense (0.1) (0.1)
Summary of the effect of derivatives designated as net investment hedging instruments on our consolidated statement of income	The following table summarizes the effect of our net investment hedge in our condensed consolidated financial statements: For the Three Months Ended March 31, Net Gains/(Losses) Net Gains/(Losses) Net Gains/(Losses) Location 2022 2021 Location 2022 2021 Location 2022 2021 Gains (losses) on net investment hedge $ 10.1 $ 23.8 Gains (losses) on net investment hedge $ (3.3) $ (1.4) Other income (expense) $ (1.1) $ 0.1
Summary of fair value and presentation of derivatives	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, 2022 As of December 31, 2021 Cash Flow Hedging Instruments: Asset derivative instruments Other current assets $ 79.8 $ 66.2 Investments and other assets 1.6 5.5 Liability derivative instruments Accrued expense and other 2.4 6.6 Other long-term liabilities 2.2 — Net Investment Hedging Instruments: Asset derivative instruments Other current assets 10.9 4.1 Other Derivative Instruments: Asset derivative instruments Other current assets 4.9 5.1 Liability derivative instruments Accrued expense and other 7.9 4.2
Derivative Instruments, Unrealized Gain (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 is summarized as follows: (In millions) As of March 31, 2022 As of December 31, 2021 Unrealized gains $ 76.4 $ 60.8 Unrealized (losses) (4.3) (7.0) Net unrealized gains (losses) $ 72.1 $ 53.8

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss) (Tables)	3 Months Ended
	Mar. 31, 2022
Equity [Abstract]
Schedule of Accumulated Other Comprehensive Income (Loss)	The following tables summarize the changes in accumulated other comprehensive income (loss), net of tax by component: (In millions) Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Gains (Losses) on Net Investment Hedge, Net of Tax Unfunded Status of Postretirement Benefit Plans, Net of Tax Currency Translation Adjustments Total Balance, December 31, 2021 $ (2.2) $ 53.8 $ 25.5 $ (44.8) $ (139.0) $ (106.7) Other comprehensive income (loss) before reclassifications (10.2) 34.4 5.1 0.9 (21.8) 8.4 Amounts reclassified from accumulated other comprehensive income (loss) 0.5 (18.5) 1.1 — — (16.9) Net current period other comprehensive income (loss) (9.7) 15.9 6.2 0.9 (21.8) (8.5) Balance, March 31, 2022 $ (11.9) $ 69.7 $ 31.7 $ (43.9) $ (160.8) $ (115.2) (In millions) Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Gains (Losses) on Net Investment Hedge, Net of Tax Unfunded Status of Postretirement Benefit Plans, Net of Tax Currency Translation Adjustments Total Balance, December 31, 2020 $ 1.4 $ (179.0) $ (8.5) $ (66.3) $ (46.6) $ (299.0) Other comprehensive income (loss) before reclassifications (1.2) 128.6 22.4 2.0 (48.5) 103.3 Amounts reclassified from accumulated other comprehensive income (loss) 0.4 21.0 — — — 21.4 Net current period other comprehensive income (loss) (0.8) 149.6 22.4 2.0 (48.5) 124.7 Balance, March 31, 2021 $ 0.6 $ (29.4) $ 13.9 $ (64.3) $ (95.1) $ (174.3)
Reclassification out of Accumulated Other Comprehensive Income	The following table summarizes the amounts reclassified from accumulated other comprehensive income (loss): (In millions) Income Statement Location Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) For the Three Months Ended March 31, 2022 2021 Gains (losses) on securities available for sale Other income (expense) $ (0.6) $ (0.5) Income tax benefit (expense) 0.1 0.1 Gains (losses) on cash flow hedges Revenue 20.9 (23.1) Operating expense (0.3) (0.4) Other income (expense) (0.1) 0.2 Income tax benefit (expense) (2.0) 2.3 Gains (losses) on net investment hedge Other income (expense) (1.1) — Total reclassifications, net of tax $ 16.9 $ (21.4)

Earnings per Share (Tables)

Earnings per Share (Tables)	3 Months Ended
	Mar. 31, 2022
Earnings Per Share [Abstract]
Basic and diluted earnings per share	Basic and diluted shares outstanding used in our earnings per share calculation are calculated as follows: For the Three Months Ended March 31, (In millions) 2022 2021 Numerator: Net income attributable to Biogen Inc. $ 303.8 $ 410.2 Denominator: Weighted average number of common shares outstanding 147.1 151.9 Effect of dilutive securities: Time-vested restricted stock units 0.3 0.2 Market stock units 0.1 0.1 Performance stock units settled in stock 0.1 0.1 Dilutive potential common shares 0.5 0.4 Shares used in calculating diluted earnings per share 147.6 152.3

Share-Based Payments (Tables)

Share-Based Payments (Tables)	3 Months Ended
	Mar. 31, 2022
Share-based Payment Arrangement [Abstract]
Share-based compensation expense included in condensed consolidated statements of income	The following table summarizes share-based compensation expense included in our condensed consolidated statements of income: For the Three Months Ended March 31, (In millions) 2022 2021 Research and development $ 25.7 $ 33.6 Selling, general and administrative 46.1 44.9 Subtotal 71.8 78.5 Capitalized share-based compensation costs (2.8) (2.6) Share-based compensation expense included in total cost and expense 69.0 75.9 Income tax effect (12.8) (14.0) Share-based compensation expense included in net income attributable to Biogen Inc. $ 56.2 $ 61.9
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) 2022 2021 Market stock units $ 5.9 $ 16.5 Time-vested restricted stock units 51.3 42.8 Performance stock units settled in stock 8.3 6.3 Performance stock units settled in cash 1.4 6.0 Employee stock purchase plan 4.9 6.9 Subtotal 71.8 78.5 Capitalized share-based compensation costs (2.8) (2.6) Share-based compensation expense included in total cost and expense $ 69.0 $ 75.9

Income Taxes (Tables)

Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2022
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, 2022 2021 Statutory rate 21.0 % 21.0 % State taxes (0.4) 0.8 Taxes on foreign earnings (10.8) (10.9) Tax credits (2.5) (3.7) Purchased intangible assets 0.6 0.8 GILTI 0.3 0.9 Neurimmune tax impacts 24.2 (0.6) Other 3.8 1.2 Effective tax rate 36.2 % 9.5 %

Other Consolidated Financial _2

Other Consolidated Financial Statement Detail (Tables)	3 Months Ended
	Mar. 31, 2022
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) 2022 2021 Interest income $ 2.9 $ 2.9 Interest expense (66.1) (64.7) Gains (losses) on investments, net (191.1) (436.6) Foreign exchange gains (losses), net (8.3) (8.6) Other, net (0.7) 0.1 Total other income (expense), net $ (263.3) $ (506.9)
Gain (loss) on investments in equity securities	The following table summarizes our gains (losses) on investments, net that relates to our equity securities held as of March 31, 2022 and 2021: For the Three Months Ended March 31, (In millions) 2022 2021 Net gains (losses) recognized during the period on equity securities $ (190.7) $ (436.1) Less: Net gains (losses) realized during the period on equity securities 0.2 6.2 Unrealized gains (losses) recognized during the period on equity securities $ (190.9) $ (442.3)
Accrued expenses and other	Accrued expense and other consists of the following: (In millions) As of March 31, 2022 As of December 31, 2021 Revenue-related reserves for discounts and allowances $ 831.1 $ 802.1 Collaboration expense 266.2 324.7 Royalties and licensing fees 216.0 234.7 Employee compensation and benefits 206.6 345.1 Other 711.2 828.6 Total accrued expense and other $ 2,231.1 $ 2,535.2

Collaborative and Other Relat_2

Collaborative and Other Relationships (Tables)	3 Months Ended
	Mar. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Activity Related to BAN2401 and Elenbecestat Collaboration	For the Three Months Ended March 31, (In millions) 2022 2021 Total development expense incurred by the collaboration related to the advancement of lecanemab and elenbecestat $ 77.0 $ 55.5 Biogen's share of lecanemab and elenbecestat development expense reflected in research and development expense in our condensed consolidated statements of income 38.5 27.7 Total sales and marketing expense incurred by the Lecanemab Collaboration 15.9 5.7 Biogen's share of lecanemab and elenbecestat sales and marketing expense reflected in selling, general and administrative expense in our condensed consolidated statements of income 8.0 2.9
Summary of Activity Related to Aducanumab Collaboration	A summary of development expense, sales and marketing expense and milestone payments related to the ADUHELM Collaboration Agreement is as follows: For the Three Months Ended March 31, (In millions) 2022 2021 Total ADUHELM development expense $ 44.2 $ 47.0 Biogen's share of ADUHELM development expense reflected in research and development expense in our condensed consolidated statements of income 24.3 25.8 Total ADUHELM sales and marketing expense incurred by the ADUHELM Collaboration Agreement 95.0 111.8 Biogen's share of ADUHELM sales and marketing expense reflected in selling, general and administrative expense and collaboration profit (loss) sharing in our condensed consolidated statements of income 50.9 60.3
Summary of Activity Related to Denali Therapeutics Collaboration	A summary of development expense related to this collaboration is as follows: For the Three Months Ended March 31, (In millions) 2022 2021 Total Denali collaboration development expense $ 14.9 $ 15.4 Biogen's share of Denali development expense reflected in research and development expense in our condensed consolidated statements of income 8.9 9.2

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Details)	3 Months Ended
	Mar. 31, 2022segment
Accounting Policies [Abstract]
Number of reportable segments	1
Interest in subsidiary (less than given percentage)	100.00%

Restructuring, Business Trans_3

Restructuring, Business Transformation and Other Cost Saving Initiatives - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2022
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 38.1	$ 0
Employee related costs	27.7
Accelerated depreciation expense	$ 10.4
Minimum | Forecast
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 100
Maximum | Forecast
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 150

Restructuring, Business Trans_4

Restructuring, Business Transformation and Other Cost Saving Initiatives - Restructuring Reserve Roll Forward (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Restructuring Cost and Reserve [Line Items]
Expense	$ 38.1	$ 0
Workforce Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring reserve, beginning	0
Expense	27.7
Payment	(6.2)
Restructuring reserve, ending	$ 21.5

Revenues - Revenues by product

Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Revenue [Line Items]
Product revenues	$ 2,531.8	$ 2,694
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	537.9	552.9
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	242.3	236
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	295.6	316.9
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	309.6	400.5
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	182.3	241.8
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	127.3	158.7
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	520.8	503.3
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	284.5	273.3
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	236.3	230
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	26.2	26.6
FAMPYRA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0
FAMPYRA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	26.2	26.6
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,394.5	1,483.3
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	709.1	751.1
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	685.4	732.2
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	472.5	520.5
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	163.3	148.7
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	309.2	371.8
ADUHELM
Disaggregation of Revenue [Line Items]
Product revenues	2.8	0
ADUHELM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	2.8	0
ADUHELM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0	0
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	114.7	121.7
BENEPALI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0
BENEPALI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	114.7	121.7
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	57.1	57.9
IMRALDI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0
IMRALDI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	57.1	57.9
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	22.5	25.5
FLIXABI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0
FLIXABI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	22.5	25.5
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	194.3	205.1
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	194.3	205.1
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	2.2	2.8
FUMADERM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0
FUMADERM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	2.2	2.8
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,066.3	2,211.7
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	875.2	899.8
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,191.1	$ 1,311.9

Revenues Reserves for Discounts

Revenues Reserves for Discounts and Allowances (Details 1) $ in Millions	3 Months Ended
	Mar. 31, 2022USD ($)
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	$ 935.3
Current Provisions Relating To Sales In Current Year	832.5
Adjustments Relating To Prior Years	(46.6)
Payments/Returns Relating To Sales in Current Year	(373.3)
Payments/Returns Relating To Sales in Prior Year	(379.9)
Ending Balance	968
Discounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	137.7
Current Provisions Relating To Sales In Current Year	169
Adjustments Relating To Prior Years	(3.9)
Payments/Returns Relating To Sales in Current Year	(101.2)
Payments/Returns Relating To Sales in Prior Year	(58.9)
Ending Balance	142.7
Contractual adjustments
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	759.6
Current Provisions Relating To Sales In Current Year	659.5
Adjustments Relating To Prior Years	(40.2)
Payments/Returns Relating To Sales in Current Year	(271.6)
Payments/Returns Relating To Sales in Prior Year	(314.5)
Ending Balance	792.8
Returns
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	38
Current Provisions Relating To Sales In Current Year	4
Adjustments Relating To Prior Years	(2.5)
Payments/Returns Relating To Sales in Current Year	(0.5)
Payments/Returns Relating To Sales in Prior Year	(6.5)
Ending Balance	$ 32.5

Revenues- Reserves for Discount

Revenues- Reserves for Discounts and Allowances (Details 2) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Total Reserves	$ 968	$ 935.3
Reduction of accounts receivable
Total Reserves	136.9	133.2
Component of accrued expenses and other
Total Reserves	$ 831.1	$ 802.1

Revenues - Revenues from Anti-C

Revenues - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 2,531.8	$ 2,694
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	143.2	174.1
Other revenues from anti-CD20 therapeutic programs	256.2	214.9
Revenue from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 399.4	$ 389

Revenues - Other Revenues (Deta

Revenues - Other Revenues (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,531.8	$ 2,694
Royalty
Disaggregation of Revenue [Line Items]
Total revenue	10.6	6.2
Other corporate revenues
Disaggregation of Revenue [Line Items]
Total revenue	47.5	83.2
Other
Disaggregation of Revenue [Line Items]
Total revenue	66.1	93.3
Collaborative arrangement | ZINBRYTA
Disaggregation of Revenue [Line Items]
Total revenue	$ 8	$ 3.9

Revenues - Narrative (Details)

Revenues - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2022USD ($)wholesaler	Mar. 31, 2021USD ($)
Disaggregation of Revenue [Line Items]
Other Revenues	$ 2,531.8	$ 2,694
Number of wholesalers | wholesaler	2
Distributor One
Disaggregation of Revenue [Line Items]
Percentage of revenues from major distributors	26.30%	30.00%
Distributor Two
Disaggregation of Revenue [Line Items]
Percentage of revenues from major distributors	10.50%	9.30%
Other corporate revenues
Disaggregation of Revenue [Line Items]
Other Revenues	$ 47.5	$ 83.2
Other
Disaggregation of Revenue [Line Items]
Other Revenues	$ 66.1	$ 93.3

Inventory - Components of Inven

Inventory - Components of Inventory (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Components of inventories
Raw materials	$ 354.5	$ 349.6
Work in process	655.6	814
Finished goods	205.3	187.9
Total inventory	$ 1,215.4	$ 1,351.5

Inventory - Narrative (Details)

Inventory - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021
Inventory [Line Items]
Excess and obsolescence charges related to inventory	$ 281.5			$ 11.9
(Gain) loss on equity method investments	(3.3)			$ (18.2)
Eisai
Inventory [Line Items]
(Gain) loss on equity method investments	$ 181.7
Percentage of future development costs related to Eisai	45.00%			45.00%
ADUHELM
Inventory [Line Items]
Excess and obsolescence charges related to inventory	$ 275	$ 120
Inventory		223
ADUHELM | Eisai
Inventory [Line Items]
(Gain) loss on equity method investments	160	$ 59	$ 45	$ 45
ADUHELM | Eisai | Centers for Medicare and Medicaid Service
Inventory [Line Items]
(Gain) loss on equity method investments	$ 136

Intangible Assets and Goodwil_2

Intangible Assets and Goodwill (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Intangible assets
Total intangible assets, gross	$ 7,606.2		$ 7,609.8
Accumulated Amortization	(5,455.4)		(5,388.5)
Intangible assets, net	2,150.8		2,221.3
Amortization and impairment of acquired intangible assets	66.9	$ 98.1
Expected future amortization expense, 2019 (remaining three months)	200
Expected future amortization expense, 2020	210
Expected future amortization expense, 2021	195
Expected future amortization expense, 2022	195
Expected future amortization expense, 2023	180
Expected future amortization expense, 2024	165
Completed technology
Intangible assets
Cost	7,413.1		7,413.1
Net	1,957.7		2,024.6
Accumulated Amortization	$ (5,455.4)		(5,388.5)
Completed technology | Minimum
Intangible assets
Estimated life, (in years)	4 years
Completed technology | Maximum
Intangible assets
Estimated life, (in years)	28 years
In-process research and development
Intangible assets
Cost and Net	$ 129.1		132.7
Accumulated Amortization	0		0
Trademarks and Trade Names
Intangible assets
Cost and Net	64		64
Accumulated Amortization	0		$ 0
Vixotrigine
Intangible assets
Amortization and impairment of acquired intangible assets	44.3
TGN | In-process research and development
Intangible assets
Cost and Net	$ 129.1

Intangible Assets and Goodwil_3

Intangible Assets and Goodwill (Details 1) - USD ($)	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Summary of roll forward of the changes in goodwill
Goodwill, beginning of period	$ 5,761,100,000
Other	(3,100,000)
Goodwill, end of period	5,758,000,000
Accumulated impairment losses related to goodwill	0
Amortization and impairment of acquired intangible assets	$ 66,900,000	$ 98,100,000

Fair Value Measurements (Detail

Fair Value Measurements (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Assets:
Marketable debt securities	$ 3,004	$ 2,433.1
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	1,283.7	1,632.2
Marketable equity securities	860.3	1,048.5
Derivative contracts	97.2	80.9
Plan assets for deferred compensation	35.8	33.4
Total	5,281	5,228.1
Liabilities:
Derivative contracts	12.5	10.8
Contingent consideration obligations	202	209.1	$ 226	$ 259.8
Total	214.5	219.9
Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	1,579.4	1,108.2
Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	1,252.7	1,192.7
Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	171.9	132.2
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0	0
Marketable equity securities	607	181.7
Derivative contracts	0	0
Plan assets for deferred compensation	0	0
Total	607	181.7
Liabilities:
Derivative contracts	0	0
Contingent consideration obligations	0	0
Total	0	0
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	0	0
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	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
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	1,283.7	1,632.2
Marketable equity securities	253.3	866.8
Derivative contracts	97.2	80.9
Plan assets for deferred compensation	35.8	33.4
Total	4,674	5,046.4
Liabilities:
Derivative contracts	12.5	10.8
Contingent consideration obligations	0	0
Total	12.5	10.8
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	1,579.4	1,108.2
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	1,252.7	1,192.7
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	171.9	132.2
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0	0
Marketable equity securities	0	0
Derivative contracts	0	0
Plan assets for deferred compensation	0	0
Total	0	0
Liabilities:
Derivative contracts	0	0
Contingent consideration obligations	202	209.1
Total	202	209.1
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	0	0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	0	0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	$ 0	$ 0

Fair Value Measurements (Deta_2

Fair Value Measurements (Details Textual) - USD ($)	3 Months Ended
	Mar. 31, 2022	Dec. 31, 2021
Business Acquisition, Contingent Consideration [Line Items]
Asset impairment charges	$ 0
Contingent consideration obligations	$ 202,000,000	$ 209,100,000
Technological and Regulatory Success, Probability	10.90%
3.625% Senior Notes due 2022
Business Acquisition, Contingent Consideration [Line Items]
Senior notes interest rate	3.625%
4.050% Senior Notes due 2025
Business Acquisition, Contingent Consideration [Line Items]
Senior notes interest rate	4.05%
5.200% Senior Notes due 2045
Business Acquisition, Contingent Consideration [Line Items]
Senior notes interest rate	5.20%

Fair Value Measurements Fair Va

Fair Value Measurements Fair Value Measurements - Contingent Consideration (Details) - Discount rate	Mar. 31, 2022	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate	0.0289	0.0130
Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate	0.0289	0.0130

Fair Value Measurements - Nonre

Fair Value Measurements - Nonrecurring Assets (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Impairment of intangible assets	$ 0	$ 44.3

Fair Value Measurements (Deta_3

Fair Value Measurements (Details 1) - USD ($) $ in Millions	1 Months Ended
	Feb. 28, 2021	Mar. 31, 2022	Dec. 31, 2021
Debt Instrument [Line Items]
Notes payable, carrying value		$ 7,275.2	$ 7,273.1
Debt instruments, fair value		7,195	8,004.7
3.625% Senior Notes due 2022
Debt Instrument [Line Items]
Notes payable, fair value		1,008.7	1,020
Notes payable, carrying value		$ 999.5	999.1
Senior notes interest rate		3.625%
4.050% Senior Notes due 2025
Debt Instrument [Line Items]
Notes payable, fair value		$ 1,797.6	1,895.2
Notes payable, carrying value		$ 1,743.4	1,742.9
Senior notes interest rate		4.05%
2.250% Senior Notes due May 1, 2030
Debt Instrument [Line Items]
Notes payable, fair value		$ 1,240	1,475.9
Notes payable, carrying value		$ 1,100	1,492
Senior notes interest rate		2.25%
5.200% Senior Notes due 2045
Debt Instrument [Line Items]
Notes payable, fair value		$ 1,341.6	1,463
Notes payable, carrying value		$ 1,492.2	1,099.9
Senior notes interest rate		5.20%
Long Term Debt, Exchanged, Amount	$ 624.6
3.150% Senior Notes due May 1, 2050
Debt Instrument [Line Items]
Notes payable, fair value		$ 1,225.3	1,457.7
Notes payable, carrying value		$ 1,473.3	1,473.2
Senior notes interest rate		3.15%
3.250% Senior Notes, Due February 15, 2051
Debt Instrument [Line Items]
Notes payable, fair value		$ 581.8	692.9
Notes payable, carrying value		$ 466.8	$ 466
Senior notes interest rate		3.25%
Long Term Debt, Exchanged, Amount	$ 700.7

Fair Value Measurements (Deta_4

Fair Value Measurements (Details 2) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration impairment	$ 7.1	$ 33.8
Fair Value, Measurements, Recurring
Business Acquisition, Contingent Consideration [Line Items]
Fair value, beginning of period	209.1	259.8
Fair value, end of period	$ 202	$ 226

Financial Instruments (Details)

Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 1,283.7	$ 1,632.2
Commercial paper
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	278.5	247.6
Overnight reverse repurchase agreements
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	13.9	200
Money market funds
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	585	901.6
Short-term debt securities
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 406.3	$ 283

Financial Instruments (Details

Financial Instruments (Details 1) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Marketable debt securities
Amortized Cost	$ 3,019	$ 2,435.9
Gross unrealized gains	1.4	0.4
Gross unrealized losses	(16.4)	(3.2)
Fair value	3,004	2,433.1
Marketable equity securities
Amortized Cost	1,167	1,167
Gross Unrealized Gains	4.5	160.9
Gross Unrealized Losses	(311.2)	(279.4)
Fair Value	860.3	1,048.5
Corporate debt securities Current
Marketable debt securities
Amortized Cost	1,126.7	723.6
Gross unrealized gains	0.1	0.1
Gross unrealized losses	(2.2)	(0.3)
Fair value	1,124.6	723.4
Corporate debt securities Non-current
Marketable debt securities
Amortized Cost	460.5	385.4
Gross unrealized gains	0.1	0.2
Gross unrealized losses	(5.8)	(0.8)
Fair value	454.8	384.8
Government securities Current
Marketable debt securities
Amortized Cost	878.7	817
Gross unrealized gains	1	0
Gross unrealized losses	(2.6)	(0.4)
Fair value	877.1	816.6
Government securities Non-current
Marketable debt securities
Amortized Cost	379	377
Gross unrealized gains	0.2	0.1
Gross unrealized losses	(3.6)	(1)
Fair value	375.6	376.1
Mortgage and other asset backed securities Current
Marketable debt securities
Amortized Cost	0.7	1.1
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair value	0.7	1.1
Mortgage and other asset backed securities Non-current
Marketable debt securities
Amortized Cost	173.4	131.8
Gross unrealized gains	0	0
Gross unrealized losses	(2.2)	(0.7)
Fair value	171.2	131.1
Equity Securities, Current
Marketable equity securities
Amortized Cost	33.9	33.9
Gross Unrealized Gains	0	9.9
Gross Unrealized Losses	0	0
Fair Value	33.9	43.8
Equity Securities, Non-Current
Marketable equity securities
Amortized Cost	1,133.1	1,133.1
Gross Unrealized Gains	4.5	151
Gross Unrealized Losses	(311.2)	(279.4)
Fair Value	$ 826.4	$ 1,004.7

Financial Instruments (Detail_2

Financial Instruments (Details 2) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, amortized cost	$ 2,002.4	$ 1,541.1
Due in one year or less, estimated fair value	2,006	1,541.7
Due after one year through five years, amortized cost	982.9	868.2
Due after one year through five years, estimated fair value	993.7	870.2
Due after five years, amortized cost	18.7	23.8
Due after five years, estimated fair value	19.3	24
Estimated Fair Value	3,004	2,433.1
Amortized Cost	$ 3,019	$ 2,435.9

Financial Instruments (Detail_3

Financial Instruments (Details 3) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Proceeds from maturities and sales	$ 543.6	$ 819.2
Realized gains	0	0.2
Realized losses	$ 0.6	$ 0.7

Financial Instruments (Detail_4

Financial Instruments (Details Textual)	3 Months Ended	12 Months Ended
	Mar. 31, 2022	Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Average maturity of marketable securities, months	10 months	10 months

Financial Instruments Financial

Financial Instruments Financial Instruments (Details Textual 2) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Other noncurrent assets | Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio	$ 919.5	$ 1,110.3

Derivative Instruments (Details

Derivative Instruments (Details) $ in Millions, ₩ in Billions	1 Months Ended		3 Months Ended		6 Months Ended	9 Months Ended
	Nov. 30, 2018USD ($)	Nov. 30, 2018KRW (₩)	Mar. 31, 2022USD ($)	Mar. 31, 2021USD ($)	Jun. 30, 2021	Sep. 30, 2021USD ($)	Dec. 31, 2021USD ($)	May 31, 2018
Derivatives [Line Items]
Gain (loss) on foreign currency derivative instruments not designated as hedging instruments			$ (12.2)	$ 17.4
Gains (losses) on net investment hedges, net of tax			$ 6.2	22.4
Minimum
Derivatives [Line Items]
Range of durations of foreign currency forward contracts			1 month		1 month
Maximum
Derivatives [Line Items]
Range of durations of foreign currency forward contracts			18 months		15 months
Not designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount			$ 1,333.4				$ 1,268
Other current assets | Designated as hedging instrument
Derivatives [Line Items]
Derivative asset, fair value			79.8				66.2
Other current assets | Not designated as hedging instrument
Derivatives [Line Items]
Derivative asset, fair value			4.9				5.1
Other noncurrent assets | Designated as hedging instrument
Derivatives [Line Items]
Derivative asset, fair value			1.6				5.5
Accrued expenses and other | Designated as hedging instrument
Derivatives [Line Items]
Derivative liability, fair value			2.4				6.6
Accrued expenses and other | Not designated as hedging instrument
Derivatives [Line Items]
Derivative liability, fair value			7.9				4.2
Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount			1,986				2,126.8
Foreign Exchange Forward
Derivatives [Line Items]
Unrealized Gain on Derivatives			76.4	60.8
Unrealized Loss on Derivatives			(4.3)	(7)
Unrealized Gain (Loss) on Derivatives			72.1	53.8
Cash Flow Hedging [Member] | Foreign exchange contract | Other long-term liabilities
Derivatives [Line Items]
Derivative liability, fair value			$ 2.2				0
Net Investment Hedging
Derivatives [Line Items]
Remaining duration of Net Investment Hedges			7 months
Unrealized gain (loss) on net investment hedges in AOCI			$ 20.6			$ 10.6
Derivative qualifying as net investment hedge, excluded component			2.5				3.6
Gains (losses) on net investment hedges, net of tax			10.1	23.8
Gains (losses) on net investment hedge, excluded component			(3.3)	(1.4)
Net Investment Hedging | Foreign exchange contract | Other current assets
Derivatives [Line Items]
Derivative asset, fair value			10.9				4.1
Revenue | Foreign exchange contract
Derivatives [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax			20.9	(23.1)
Revenue | Cash flows, revenue | Foreign exchange contract
Derivatives [Line Items]
Gain on interest rate swap			(6.5)	(3)
Operating expense | Foreign exchange contract
Derivatives [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax			(0.3)	(0.4)
Operating expense | Cash flows, operating expenses | Foreign exchange contract
Derivatives [Line Items]
Gain on interest rate swap			(0.1)	(0.1)
Other income (expense) | Net Investment Hedging
Derivatives [Line Items]
Gain (loss) recognized in net income, excluded component			(1.1)	$ 0.1
Euro | Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount			1,631.2				1,828
British pound | Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount			55				72.7
Swiss franc | Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount			126.6				166.2
Japan, Yen | Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount			44.6				59.9
Canadian dollar | Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount			$ 128.6				$ 0
Samsung Bioepis
Derivatives [Line Items]
Equity Method Investment, Ownership Percentage			49.90%
Percentage of stake in entity								5.00%
Payments to acquire additional investment in equity method investment	$ 676.6	₩ 759.5
Short-term derivative
Derivatives [Line Items]
Range of durations of foreign currency forward contracts			12 months
Gain/Loss on fair value of foreign currency forward contracts			$ 72

Property, Plant and Equipment (

Property, Plant and Equipment (Details) $ in Millions	3 Months Ended
	Mar. 31, 2022USD ($)ft²	Mar. 31, 2021USD ($)	Dec. 31, 2021USD ($)
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 2,082		$ 2,006.6
Depreciation	$ 76.3	$ 48.8
Biologics Manufacturing
Property, Plant and Equipment [Line Items]
Number of square feet | ft²	393,000
Warehouse, Utilities and Support Space
Property, Plant and Equipment [Line Items]
Number of square feet | ft²	290,000
Administrative Space
Property, Plant and Equipment [Line Items]
Number of square feet | ft²	51,000
Solothurn, Switzerland
Property, Plant and Equipment [Line Items]
Construction in progress	$ 682.6		$ 677
Fixed assets placed into service	$ 1,200

Indebtedness (Details)

Indebtedness (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Feb. 28, 2021	Mar. 31, 2022	Mar. 31, 2021
Debt Instrument [Line Items]
Repayments of Senior Debt	$ 151.8
Interest Expense		$ 66.1	$ 64.7
5.200% Senior Notes due 2045
Debt Instrument [Line Items]
Repayments of Senior Debt	12.1
Debt Instrument, Redemption, Amount	8.9
Long Term Debt, Exchanged, Amount	624.6
Gain (Loss) on Extinguishment of Debt		3.2
Repayments of borrowings and premiums paid on debt exchange		13.8
Interest Expense		$ 6.1
3.250% Senior Notes, Due February 15, 2051
Debt Instrument [Line Items]
Long Term Debt, Exchanged, Amount	$ 700.7

Share Repurchases (Details)

Share Repurchases (Details) - USD ($) shares in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Oct. 31, 2020
Class of Stock [Line Items]
Payments for repurchase of common stock	$ 0	$ 600,000,000
2020 Share Repurchase Program
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount			$ 5,000,000,000
Repurchase of common stock, at cost, shares		2.2
Payments for repurchase of common stock		$ 600,000,000
Amount remaining under 2019 Share Repurchase Program	$ 2,800,000,000

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	$ (106.7)	$ (299)
Balance, January 1, 2018	(115.2)	(174.3)
Other comprehensive income (loss), before reclassifications, net of tax	8.4	103.3
Amounts reclassified from accumulated other comprehensive income, net of tax	(16.9)	21.4
Other comprehensive income (loss), net of tax	(8.5)	125.4
Accumulated other comprehensive income (loss), net of tax ending balance	(115.2)	(174.3)
Other comprehensive income (loss), net of tax	(8.5)	124.7
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	(2.2)	1.4
Balance, January 1, 2018	(11.9)	0.6
Other comprehensive income (loss), before reclassifications, net of tax	(10.2)	(1.2)
Amounts reclassified from accumulated other comprehensive income, net of tax	0.5	0.4
Other comprehensive income (loss), net of tax	(9.7)	(0.8)
Accumulated other comprehensive income (loss), net of tax ending balance	(11.9)	0.6
Unrealized gains (losses) on cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	53.8	(179)
Balance, January 1, 2018	69.7	(29.4)
Other comprehensive income (loss), before reclassifications, net of tax	34.4	128.6
Amounts reclassified from accumulated other comprehensive income, net of tax	(18.5)	21
Other comprehensive income (loss), net of tax	15.9	149.6
Accumulated other comprehensive income (loss), net of tax ending balance	69.7	(29.4)
Gains (losses) on net investment hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	25.5	(8.5)
Balance, January 1, 2018	31.7	13.9
Other comprehensive income (loss), before reclassifications, net of tax	5.1	22.4
Amounts reclassified from accumulated other comprehensive income, net of tax	1.1	0
Other comprehensive income (loss), net of tax	6.2	22.4
Accumulated other comprehensive income (loss), net of tax ending balance	31.7	13.9
Unfunded status of postretirement benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	(44.8)	(66.3)
Balance, January 1, 2018	(43.9)	(64.3)
Other comprehensive income (loss), before reclassifications, net of tax	0.9	2
Amounts reclassified from accumulated other comprehensive income, net of tax	0	0
Other comprehensive income (loss), net of tax	0.9	2
Accumulated other comprehensive income (loss), net of tax ending balance	(43.9)	(64.3)
Currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	(139)	(46.6)
Balance, January 1, 2018	(160.8)	(95.1)
Other comprehensive income (loss), before reclassifications, net of tax	(21.8)	(48.5)
Amounts reclassified from accumulated other comprehensive income, net of tax	0	0
Other comprehensive income (loss), net of tax	(21.8)	(48.5)
Accumulated other comprehensive income (loss), net of tax ending balance	$ (160.8)	$ (95.1)

Reclassification out of Accumul

Reclassification out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Nonoperating Income (Expense)	$ (263.3)	$ (506.9)
Income Tax Expense (Benefit)	125.6	44.2
Product revenues	2,531.8	2,694
Net income attributable to Biogen Inc.	303.8	410.2
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.	16.9	(21.4)
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)	(0.6)	(0.5)
Income Tax Expense (Benefit)	0.1	0.1
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)	0.2
Income Tax Expense (Benefit)	(2)	2.3
Product revenues	20.9	(23.1)
Operating Expenses	(0.3)	(0.4)
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)	$ (1.1)	$ 0

Earnings per Share (Details)

Earnings per Share (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Numerator:
Net income attributable to Biogen Inc.	$ 303.8	$ 410.2
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	147,100	151,900
Effect of dilutive securities:
Dilutive potential common shares	500	400
Shares used in calculating diluted earnings per share	147,600	152,300
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	300	200
Market stock units
Effect of dilutive securities:
Stock units	100	100
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	100	100

Share-Based Payments (Details)

Share-Based Payments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	$ (70.4)	$ (72.6)
Subtotal	71.8	78.5
Capitalized share-based compensation costs	(2.8)	(2.6)
Share-based compensation expense included in total costs and expenses	69	75.9
Income tax effect	(12.8)	(14)
Research and development
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	25.7	33.6
Selling, general and administrative
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	46.1	44.9
Total share-based compensation expense, net of tax
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	$ 56.2	$ 61.9

Share-Based Payments (Details 1

Share-Based Payments (Details 1) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	$ (70.4)	$ (72.6)
Subtotal	71.8	78.5
Capitalized share-based compensation costs	(2.8)	(2.6)
Share-based compensation expense included in total costs and expenses	69	75.9
Market stock units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	5.9	16.5
Time-vested restricted stock units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	51.3	42.8
Performance stock units settled in stock
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	8.3	6.3
Performance stock units settled in cash
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	1.4	6
Employee stock purchase plan
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	$ 4.9	$ 6.9

Income Taxes (Details Textual)

Income Taxes (Details Textual) $ in Millions, ₩ in Billions	Mar. 31, 2023USD ($)	Mar. 31, 2022USD ($)	Mar. 31, 2022KRW (₩)	Dec. 31, 2021USD ($)	Dec. 31, 2021KRW (₩)	Jun. 30, 2021USD ($)
Income Tax Contingency [Line Items]
Deferred tax assets, unrecognized tax benefit		$ 85		$ 390
Deferred tax assets, net						$ 490
Samsung Bioepis
Income Tax Contingency [Line Items]
Investment in Samsung Bioepis		586.4	₩ 709.4	$ 599.9	₩ 713.3
Deferred tax assets, net		$ 70
Forecast
Income Tax Contingency [Line Items]
Decrease in unrecognized tax benefits is reasonably possible	$ 500
Deferred tax assets, unrecognized tax benefit	$ 455

Income Taxes (Details)

Income Taxes (Details)	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Reconciliation between the U.S. federal statutory tax rate and effective tax rate
Statutory rate	21.00%	21.00%
State taxes	(0.40%)	0.80%
Taxes on foreign earnings	(10.80%)	(10.90%)
Credits and net operating loss utilization	(2.50%)	(3.70%)
Purchased intangible assets	0.60%	0.80%
GILTI	0.30%	0.90%
Neurimmune tax impacts	0.242	(0.006)
Other	3.80%	1.20%
Effective tax rate	36.20%	9.50%

Other Consolidated Financial _3

Other Consolidated Financial Statement Detail (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Other Income (Expense), Net
Interest income	$ 2.9	$ 2.9
Interest expense	(66.1)	(64.7)
Gains (losses) on investments, net	(191.1)	(436.6)
Foreign exchange gains (losses), net	(8.3)	(8.6)
Other, net	(0.7)	0.1
Total other income (expense), net	(263.3)	(506.9)
Equity Securities, FV-NI, Gain (Loss), Alternative [Abstract]
Net gains (losses) recognized during the period on equity securities	(190.7)	(436.1)
Less: Net gains (losses) realized during the period on equity securities	0.2	6.2
Unrealized gains (losses) recognized during the period on equity securities	(190.9)	$ (442.3)
Accrued Expenses and Other
Revenue-related reserves for discounts and allowances	968		$ 935.3
Collaboration expense	206.6		345.1
Royalties and licensing fees	216		234.7
Collaboration expense	266.2		324.7
Other	711.2		828.6
Total accrued expense and other	2,231.1		2,535.2
Ionis, Sangamo, Denali and Sage
Equity Securities, FV-NI, Gain (Loss), Alternative [Abstract]
Unrealized gains (losses) recognized during the period on equity securities	205.5
Component of accrued expenses and other
Accrued Expenses and Other
Revenue-related reserves for discounts and allowances	$ 831.1		$ 802.1

Other Consolidated Financial _4

Other Consolidated Financial Statement (Details Textual) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Net gains recognized on the increase in fair value of equity securities	$ (190.9)	$ (442.3)
Net gains (losses) realized during the period on equity securities	0.2	$ 6.2
Other long-term liabilities	1,287.9		$ 1,320.5
Accrued income taxes	640.5		$ 664.5
Ionis, Sangamo, Denali and Sage
Net gains recognized on the increase in fair value of equity securities	$ 205.5

Collaborative and Other Relat_3

Collaborative and Other Relationships - Collaborations (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended					12 Months Ended
	Aug. 31, 2021	Jun. 30, 2021	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2022	Dec. 31, 2021
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and Development Expense (Excluding Acquired in Process Cost)			$ 551.7			$ 514.2
Cost of sales, excluding amortization and impairment of acquired intangible assets			753.9			478.1
(Gain) loss on equity method investments			(3.3)			(18.2)
Maximum | Forecast
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and development expense								$ 335
Samsung Bioepis
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
(Gain) loss on equity method investments			4			11
Loss recorded on Samsung Bioepis joint venture			3.3			18.2
Amortization			7.3			$ 7.2
ADUHELM
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Cost of sales, excluding amortization and impairment of acquired intangible assets			$ 275
Eisai
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of future development costs related to Eisai			45.00%			45.00%
Collaboration agreement term			10 years						5 years
Cost of Goods Sold, Capacity Charges			$ 45
(Gain) loss on equity method investments			181.7
Due from Related Parties			402.7	$ 285.4					$ 285.4
Due to Related Parties			36.5	46.5					46.5
Eisai | BAN2401 and Elenbecestat
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense Incurred By Collaboration			77			$ 55.5
Expense reflected within statements of income			38.5			27.7
Eisai | ADUHELM
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Additional milestone payment		$ 100
(Gain) loss on equity method investments			$ 160	59	$ 45	45
Samsung Bioepis
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront and milestone payments made to collaborative partner							$ 100
Collaboration agreement term			5 years
Research and development expense							$ 63
Due from Related Parties			$ 5.8	4.1					4.1
Due to Related Parties			$ 102.5	$ 148.7					148.7
Sangamo Therapeutics, Inc. Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Term of collaboration agreement			5 years
Other research and discovery
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and Development Expense (Excluding Acquired in Process Cost)			$ 19.5			1
InnoCare Pharma Limited (InnoCare) Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront and milestone payments made to collaborative partner	$ 125
Contingent additional milestone payment	$ 812.5
Research and development | Eisai | ADUHELM
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			44.2			47
Expense reflected within statements of income			24.3			25.8
Research and development | Denali Therapeutics
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			14.9			15.4
Expense reflected within statements of income			8.9			9.2
Research and development | Samsung Bioepis
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			8.3						4.8
Expense reflected within statements of income			5.5						$ 4.2
Research and development | Sage Therapeutics Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			38.7			39.8
Expense reflected within statements of income			19.4			19.9
Research and development | UCB Pharma S.A. [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			17.6			16.9
Expense reflected within statements of income			8.8			8.4
Selling, general and administrative | Eisai | BAN2401 and Elenbecestat
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			15.9			5.7
Expense reflected within statements of income			8			2.9
Selling, general and administrative | Eisai | ADUHELM
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			95			111.8
Expense reflected within statements of income			50.9			60.3
Selling, general and administrative | Sage Therapeutics Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration			18.4			5.3
Expense reflected within statements of income			$ 9.2			$ 2.7

Collaborative and Other Relat_4

Collaborative and Other Relationships - Equity Method Investments (Details) $ in Millions, ₩ in Billions	1 Months Ended	3 Months Ended
	Jan. 31, 2022USD ($)	Mar. 31, 2022USD ($)	Dec. 31, 2021USD ($)	Jun. 30, 2021USD ($)	Mar. 31, 2021USD ($)	Mar. 31, 2020USD ($)	Mar. 31, 2022KRW (₩)	Dec. 31, 2021KRW (₩)	Jan. 31, 2020USD ($)	Nov. 07, 2018USD ($)
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments		$ (3.3)			$ (18.2)
Collaboration profit (loss) sharing		$ (117.3)			68.5
Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Percentage of stake in entity maximum		49.90%					49.90%
Amortization of basis differences										$ 675
Loss recorded on Samsung Bioepis joint venture		$ 3.3			18.2
Income (loss) from equity method investments		4			11
Investment in Samsung Bioepis		586.4	$ 599.9				₩ 709.4	₩ 713.3
Collaboration profit (loss) sharing		64.4			68.5
Eisai
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments		181.7
Due to Related Parties		36.5	46.5
Eisai | ADUHELM
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments		$ 160	59	$ 45	$ 45
Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Proceeds from Divestiture of Interest in Joint Venture	$ 1,000
Payables To Divestiture Of Interest In Joint Venture	1,300
Equity Method Investments, Expected Profit Share		50.00%
Upfront and milestone payments made to collaborative partner						$ 100
Research and development expense						$ 63
Prepaid research and development expenses									$ 37
Accrued milestone payments		$ 15
Estimated additional payments upon achievement of development and commercial milestones	50	180
Contract Option Exercise Fee		60
Due to Related Parties		$ 102.5	$ 148.7
Samsung Bioepis | Payment Due At First Anniversary
Schedule of Equity Method Investments [Line Items]
Payables To Divestiture Of Interest In Joint Venture	812.5
Samsung Bioepis | Payment Due At Second Anniversary
Schedule of Equity Method Investments [Line Items]
Payables To Divestiture Of Interest In Joint Venture	$ 437.5
Inventory | Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Equity method investment basis difference amortization period		1 year 6 months
Developed technology | Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Equity method investment basis difference amortization period		15 years

Investments in Variable Inter_2

Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended				12 Months Ended
	Jun. 30, 2021	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2021
Variable Interest Entity [Line Items]
(Gain) loss on equity method investments		$ (3.3)			$ (18.2)
Deferred tax assets, net	$ 490			$ 490
Deferred tax assets, unrecognized tax benefit		85	$ 390			$ 390
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities		$ 24.1	24.6			$ 24.6
Neurimmune
Variable Interest Entity [Line Items]
Collaboration agreement term		12 years
Research and development costs, percentage		100.00%
Eisai
Variable Interest Entity [Line Items]
Collaboration agreement term		10 years				5 years
(Gain) loss on equity method investments		$ 181.7
Percentage of future development costs related to Eisai		45.00%			45.00%
Eisai | ADUHELM
Variable Interest Entity [Line Items]
(Gain) loss on equity method investments		$ 160	$ 59	45	$ 45
Additional milestone payment	100
Eisai | ADUHELM | JAPAN
Variable Interest Entity [Line Items]
Additional milestone payment	50
Eisai | Regulatory Milestones | Neurimmune | ADUHELM
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party	$ 100			$ 100

Litigation (Details)

Litigation (Details) - USD ($) $ in Millions	Jul. 15, 2021	Dec. 31, 2020
Loss Contingency, Information about Litigation Matters [Abstract]
Loss Contingency, Estimate of Possible Loss	$ 981.1	$ 200