Biogen (BIIB) 10-Q 1 Aug 24 2024 Q2 Quarterly report Financial data

Cover Page

Cover Page - shares	6 Months Ended
	Jun. 30, 2024	Jul. 31, 2024
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2024
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	2024
Document Fiscal Period Focus	Q2
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		145,661,642

Condensed Consolidated Statemen

Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Total revenue	$ 2,464.9	$ 2,456	$ 4,755.4	$ 4,919
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	546	592.7	1,088.2	1,255.5
Research and development	513.9	584.2	966.8	1,154.8
Selling, general and administrative	553.8	548	1,135.3	1,153
Amortization and impairment of acquired intangible assets	86.9	52.9	165.2	103.1
Collaboration profit sharing/(loss reimbursement)	62.4	56.9	128	114
Restructuring charges	6.6	34.4	18.1	44
Gain on sale of priority review voucher, net	88.6	0	88.6	0
Other (income) expense, net	85.2	(121.2)	178.9	(51.8)
Total cost and expense	1,766.2	1,747.9	3,591.9	3,772.6
Income before income tax (benefit) expense	698.7	708.1	1,163.5	1,146.4
Income tax (benefit) expense	115.1	114.8	186.5	165.5
Net income	583.6	593.3	977	980.9
Net income (loss) attributable to noncontrolling interests, net of tax	0	1.7	0	1.4
Net income attributable to Biogen Inc.	$ 583.6	$ 591.6	$ 977	$ 979.5
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 4.01	$ 4.09	$ 6.72	$ 6.78
Diluted earnings per share attributable to Biogen Inc.	$ 4	$ 4.07	$ 6.70	$ 6.74
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	145,600	144,700	145,400	144,600
Diluted earnings per share attributable to Biogen Inc.	145,900	145,500	145,900	145,400
Product, net
Total revenue	$ 1,899.6	$ 1,845.8	$ 3,611.5	$ 3,609.1
Revenue from anti-CD20 therapeutic programs
Total revenue	444.5	433.4	838.5	832.9
Contract manufacturing, royalty and other revenue
Total revenue	$ 120.8	$ 176.8	$ 305.4	$ 477

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to Biogen Inc.	$ 583.6	$ 591.6	$ 977	$ 979.5
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	0	(4.5)	0	1.2
Unrealized gains (losses) on cash flow hedges, net of tax	20.1	6.9	34.8	(28.5)
Unrealized gains (losses) on pension benefit obligation, net of tax	0	0.2	(0.1)	0.7
Currency translation adjustment	(5.6)	(3.4)	(27)	18.7
Total other comprehensive income (loss), net of tax	14.5	(0.8)	7.7	(7.9)
Comprehensive income (loss) attributable to Biogen Inc.	598.1	590.8	984.7	971.6
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	0	1.7	0	1.4
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	$ 598.1	$ 592.5	$ 984.7	$ 973

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Current assets:
Cash and cash equivalents	$ 1,908.9	$ 1,049.9
Inventory	2,506.1	2,527.4
Other current assets	615.3	1,182
Total current assets	7,108.5	6,859.3
Property, plant and equipment, net	3,249.3	3,309.7
Operating lease assets	389.4	420
Intangible assets, net	8,232.9	8,363
Goodwill	6,227.4	6,219.2
Deferred tax asset	915.1	928.6
Investments and other assets	681.5	745
Total assets	26,804.1	26,844.8
Current liabilities:
Current portion of term loan	0	150
Taxes payable	281.6	257.4
Accounts payable	354.5	403.3
Accrued expense and other	2,472.1	2,623.6
Total current liabilities	3,108.2	3,434.3
Notes payable and term loan	6,292	6,788.2
Deferred tax liability	590.6	641.8
Long-term operating lease liabilities	367.5	400
Other long-term liabilities	556.7	781.1
Total liabilities	10,915	12,045.4
Commitments, contingencies and guarantees
Biogen Inc. shareholders’ equity:
Preferred stock, par value $0.000 per share	0	0
Common stock, par value $— per share	0.1	0.1
Additional paid-in capital	407.5	302.5
Accumulated other comprehensive income (loss)	(146)	(153.7)
Retained earnings	18,604.6	17,627.6
Treasury stock, at cost	(2,977.1)	(2,977.1)
Total equity	15,889.1	14,799.4
Total liabilities and equity	26,804.1	26,844.8
Nonrelated Party
Current assets:
Accounts receivable, net of allowance for doubtful accounts of $2.5 and $2.4, respectively	1,627.1	1,664.1
Related Party
Current assets:
Accounts receivable, net of allowance for doubtful accounts of $2.5 and $2.4, respectively	$ 451.1	$ 435.9

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Statement of Financial Position [Abstract]
Accounts Receivable, Allowance for Credit Loss, Current	$ 2.4	$ 2.5
Preferred stock, par value (in dollars per share)	$ 0.001
Common stock, par value (in dollars per share)	$ 0.0005

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023
Cash flow from operating activities:
Net income	$ 977	$ 980.9
Adjustments to reconcile net income to net cash flow from operating activities:
Depreciation and amortization	306.4	229.7
Excess and obsolescence charges related to inventory	44.4	36.8
Amortization of acquired inventory step-up	134.2	0
Share-based compensation	143.4	147.1
Deferred income taxes	(47.1)	(170.9)
(Gain) loss on strategic investments	61.6	(26.8)
Gain on sale of priority review voucher, net	88.6	0
Other	65	60.5
Changes in operating assets and liabilities, net of effects of business acquired:
Inventory	(188.2)	(65.1)
Accrued expense and other current liabilities	(115.6)	(127.1)
Income tax assets and liabilities	(7.7)	(82.4)
Other changes in operating assets and liabilities, net	(103)	(55.5)
Net cash flow provided by (used in) operating activities	1,179	942.3
Cash flow from investing activities:
Purchases of property, plant and equipment	(79.4)	(137.6)
Proceeds from sales and maturities of marketable securities	0	1,682.2
Purchases of marketable securities	0	(4,120.3)
Proceeds from sale of equity interest in Samsung Bioepis	406.8	788.1
Proceeds from sale of priority review voucher	103	0
Acquisitions of intangible assets	(84.1)	(21)
Proceeds from sales of strategic investments	57.5	103.2
Other	(3.3)	(1.1)
Net cash flow provided by (used in) investing activities	400.5	(1,706.5)
Cash flow from financing activities:
Payments related to issuance of stock for share-based compensation arrangements, net	(40.8)	(54.6)
Repayment of borrowings	650	0
Net (distribution) contribution to noncontrolling interest	0	1.7
Other	6	(0.3)
Net cash flow provided by (used in) financing activities	(684.8)	(53.2)
Net increase (decrease) in cash and cash equivalents	894.7	(817.4)
Effect of exchange rate changes on cash and cash equivalents	(35.7)	15.9
Cash and cash equivalents, beginning of the period	1,049.9	3,419.3
Cash and cash equivalents, end of the period	1,908.9	2,617.8
Nonrelated Party
Changes in operating assets and liabilities, net of effects of business acquired:
Accounts receivable	12.3	21.8
Related Party
Changes in operating assets and liabilities, net of effects of business acquired:
Accounts receivable	$ (15.1)	$ (6.7)

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
Beginning balance, shares at Dec. 31, 2022		0	167.9
Beginning balance at Dec. 31, 2022	$ 13,388.4	$ 0	$ 0.1	$ 73.3	$ (164.9)	$ 16,466.5	$ (2,977.1)	$ 13,397.9	$ (9.5)
Beginning balance, treasury stock, shares at Dec. 31, 2022							(23.8)
Net income	980.9					979.5		979.5	1.4
Other comprehensive income (loss), net of tax	(7.9)				(7.9)			(7.9)
Capital contribution from noncontrolling interest	1.7								1.7
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	29.4			29.4				29.4
Issuance of common stock under stock award plan, shares			0.6
Issuance of common stock under stock award plan	(84)			(84)				(84)
Compensation related to share-based payments	153			153				153
Other	(1)			(1)				(1)
Ending balance at Jun. 30, 2023	14,460.5	$ 0	$ 0.1	170.7	(172.8)	17,446	$ (2,977.1)	14,466.9	(6.4)
Ending balance, shares at Jun. 30, 2023		0	168.6
Ending balance, treasury stock, shares at Jun. 30, 2023							(23.8)
Beginning balance, shares at Mar. 31, 2023		0	168.6
Beginning balance at Mar. 31, 2023	13,787	$ 0	$ 0.1	91.2	(172)	16,854.4	$ (2,977.1)	13,796.6	(9.6)
Beginning balance, treasury stock, shares at Mar. 31, 2023							(23.8)
Net income	593.3					591.6		591.6	1.7
Other comprehensive income (loss), net of tax	(0.8)				(0.8)			(0.8)
Capital contribution from noncontrolling interest	1.5								1.5
Issuance of common stock under stock option and stock purchase plans, shares			0
Issuance of common stock under stock option and stock purchase plans	9.3			9.3				9.3
Issuance of common stock under stock award plan, shares			0
Issuance of common stock under stock award plan	(3.8)			(3.8)				(3.8)
Compensation related to share-based payments	74.1			74.1				74.1
Other	(0.1)			(0.1)				(0.1)
Ending balance at Jun. 30, 2023	14,460.5	$ 0	$ 0.1	170.7	(172.8)	17,446	$ (2,977.1)	$ 14,466.9	$ (6.4)
Ending balance, shares at Jun. 30, 2023		0	168.6
Ending balance, treasury stock, shares at Jun. 30, 2023							(23.8)
Beginning balance, shares at Dec. 31, 2023		0	168.7
Beginning balance at Dec. 31, 2023	14,799.4	$ 0	$ 0.1	302.5	(153.7)	17,627.6	$ (2,977.1)
Beginning balance, treasury stock, shares at Dec. 31, 2023							(23.8)
Net income	977					977
Other comprehensive income (loss), net of tax	7.7				7.7
Issuance of common stock under stock option and stock purchase plans, shares			0.2
Issuance of common stock under stock option and stock purchase plans	23.6			23.6
Issuance of common stock under stock award plan, shares			0.6
Issuance of common stock under stock award plan	(64.4)			(64.4)
Compensation related to share-based payments	149			149
Other	(3.2)			(3.2)
Ending balance at Jun. 30, 2024	15,889.1	$ 0	$ 0.1	407.5	(146)	18,604.6	$ (2,977.1)
Ending balance, shares at Jun. 30, 2024		0	169.5
Ending balance, treasury stock, shares at Jun. 30, 2024							(23.8)
Beginning balance, shares at Mar. 31, 2024		0	169.4
Beginning balance at Mar. 31, 2024	15,213	$ 0	$ 0.1	329.5	(160.5)	18,021	$ (2,977.1)
Beginning balance, treasury stock, shares at Mar. 31, 2024							(23.8)
Net income	583.6					583.6
Other comprehensive income (loss), net of tax	14.5				14.5
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	7.9			7.9
Issuance of common stock under stock award plan, shares			0
Issuance of common stock under stock award plan	(1.8)			(1.8)
Compensation related to share-based payments	72.9			72.9
Other	(1)			(1)
Ending balance at Jun. 30, 2024	$ 15,889.1	$ 0	$ 0.1	$ 407.5	$ (146)	$ 18,604.6	$ (2,977.1)
Ending balance, shares at Jun. 30, 2024		0	169.5
Ending balance, treasury stock, shares at Jun. 30, 2024							(23.8)

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2024
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 innovative therapies for people living with serious and complex diseases worldwide. We have a broad portfolio of medicines to treat MS, have introduced the first approved treatment for SMA, co-developed treatments to address a defining pathology of Alzheimer’s disease and launched the first approved treatment to target a genetic cause of ALS. Through our 2023 acquisition of Reata we market the first and only drug approved in the U.S. and the E.U. for the treatment of Friedreich's Ataxia in adults and adolescents aged 16 years and older. We are focused on advancing our pipeline in neurology, specialized immunology and rare diseases. We support our drug discovery and development efforts through internal research and development programs, external collaborations and acquisitions. Our marketed products include TECFIDERA, VUMERITY, AVONEX, PLEGRIDY, TYSABRI and FAMPYRA for the treatment of MS; SPINRAZA for the treatment of SMA; SKYCLARYS for the treatment of Friedreich's Ataxia; QALSODY for the treatment of ALS; and FUMADERM for the treatment of severe plaque psoriasis. We also have collaborations with Eisai on the commercialization of LEQEMBI for the treatment of Alzheimer's disease and Sage on the commercialization of ZURZUVAE for the treatment of PPD and we have certain business and financial rights with respect to RITUXAN for the treatment of non-Hodgkin's lymphoma, CLL and other conditions; RITUXAN HYCELA for the treatment of non-Hodgkin's lymphoma and CLL; GAZYVA for the treatment of CLL and follicular lymphoma; OCREVUS for the treatment of PPMS and RMS; LUNSUMIO for the treatment of relapsed or refractory follicular lymphoma; COLUMVI, a bispecific antibody for the treatment of non-Hodgkin's lymphoma; and have the option to add other potential anti-CD20 therapies, pursuant to our collaboration arrangements with Genentech, a wholly-owned member of the Roche Group. We commercialize a portfolio of biosimilars of advanced biologics including BENEPALI, an etanercept biosimilar referencing ENBREL, IMRALDI, an adalimumab biosimilar referencing HUMIRA, and FLIXABI, an infliximab biosimilar referencing REMICADE, in certain countries in Europe, as well as BYOOVIZ, a ranibizumab biosimilar referencing LUCENTIS, and TOFIDENCE, a tocilizumab biosimilar referencing ACTEMRA, in the U.S. and certain international markets. We also have commercialization rights related to OPUVIZ, an aflibercept biosimilar referencing EYLEA. For additional information on our collaboration arrangements, please read Note 19, 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 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 2023 Form 10-K. Our accounting policies are described in the Notes to Consolidated Financial Statements in our 2023 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 and six months ended June 30, 2024, 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 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. In November 2023 we terminated the Neurimmune Agreement, which resulted in the deconsolidation of our variable interest entity, Neurimmune. For additional information on the deconsolidation of Neurimmune, please read Note 20, Investments in Variable Interest Entities , to these condensed consolidated financial statements. 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. Significant Accounting Policies There have been no material changes to our significant accounting policies disclosed in Note 1, Summary of Significant Accounting Policies , to our audited consolidated financial statements included in our 2023 Form 10-K. New Accounting Pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies that we adopt as of the specified effective date. Unless otherwise discussed below, we do not believe that the adoption of recently issued standards have had or may have a material impact on our condensed consolidated financial statements or disclosures. Climate-Related Disclosures In March 2024 the SEC issued a final rule under SEC Release No. 33-11275, The Enhancement and Standardization of Climate-Related Disclosures for Investors . This new rule will require large accelerated filers to disclose material climate-related risks that are reasonably likely to have a material impact on their business, results of operations or financial condition. The required information about climate-related risks will also include disclosure of material direct greenhouse gas emissions from operations owned or controlled (Scope 1) and/or material indirect greenhouse gas emissions from purchased energy consumed in owned or controlled operations (Scope 2). Additionally, the new rules will require disclosure within the notes to the financial statements of the effects of severe weather events and other natural conditions and information on any climate-related targets or goals, subject to certain materiality thresholds. The final rule, if adopted, includes a phased-in compliance period which will begin phasing in with our annual report for the year ending December 31, 2025. In April 2024 the SEC voluntarily stayed implementation of the new climate-related disclosure requirements pending judicial review. Once the litigation is resolved, and if the rule remains in effect, the SEC will announce a new effective date. We are currently evaluating the potential impact that this new rule will have on our company's disclosures. Segment Reporting In November 2023 the FASB issued ASU No. 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosure . This standard requires disclosure of significant segment expenses that are regularly provided to the CODM and included within each reported measure of segment profit or loss, an amount and description of its composition for other segment items to reconcile to segment profit or loss and the title and position of the entity's CODM. The amendments in this update also expand the interim segment disclosure requirements. All disclosure requirements under this standard are also required for public entities with a single reportable segment. This standard is effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024. Early adoption is permitted and the amendments in this update are required to be applied on a retrospective basis. We are currently evaluating the potential impact that this new standard will have on our consolidated financial statements and related disclosures.

Acquisitions

Acquisitions	6 Months Ended
	Jun. 30, 2024
Business Combinations [Abstract]
Acquisitions	Reata Pharmaceuticals, Inc. On September 26, 2023, we completed the acquisition of all of the issued and outstanding shares of Reata, a biopharmaceutical company focused on developing therapeutics that regulate cellular metabolism and inflammation in serious neurologic diseases. As a result of this transaction we acquired SKYCLARYS (omaveloxolone), the first and only drug approved in the U.S. and the E.U. for the treatment of Friedreich's Ataxia in adults and adolescents aged 16 years and older, as well as other clinical and preclinical pipeline programs. The acquisition of Reata is expected to complement our global portfolio of neuromuscular and rare disease therapies. The addition of SKYCLARYS is anticipated to provide potential operating synergies with SPINRAZA and QALSODY. Under the terms of this acquisition, we paid Reata shareholders $172.50 in cash for each issued and outstanding Reata share, which totaled approximately $6.6 billion. In addition, we agreed to pay approximately $983.9 million in cash for Reata's outstanding equity awards, inclusive of employer taxes, of which approximately $590.5 million was attributable to pre-acquisition services and is therefore reflected as a component of total purchase price paid. Of the $983.9 million paid to Reata's equity award holders, we recognized approximately $393.4 million as compensation attributable to the post-acquisition service period, of which $196.4 million was recognized as a charge to selling, general and administrative expense with the remaining $197.0 million as a charge to research and development expense within our condensed consolidated statements of income for the year ended December 31, 2023. These amounts were associated with the accelerated vesting of stock options and RSUs previously granted to Reata employees and required no future services to vest. We funded this acquisition through available cash, cash equivalents and marketable securities, supplemented by the issuance of a $1.0 billion term loan under our term loan credit agreement. For additional information on our term loan credit agreement, please read Note 13, Indebtedness , to these condensed consolidated financial statements. We accounted for this acquisition as a business combination using the acquisition method of accounting in accordance with ASC Topic 805, Business Combinations , and recorded assets acquired and liabilities assumed at their respective fair values as of the acquisition date. Purchase Price Consideration Total consideration transferred for the acquisition of Reata is summarized as follows: (In millions) As of September 26, 2023 Cash consideration paid to Reata shareholders (1) $ 6,602.9 Fair value of Reata equity compensation pre-acquisition services and related taxes (2) 590.5 Total consideration $ 7,193.4 (1) Represents cash consideration transferred of $172.50 per outstanding Reata common stock based on 38.3 million Reata shares outstanding at closing. (2) Represents the fair value of Reata stock options and stock units issued to Reata equity award holders and the related taxes attributable to pre-acquisition vesting services. Preliminary Purchase Price Allocation The following table summarizes the provisional amounts recognized for assets acquired and liabilities assumed as of the acquisition date, as well as measurement period adjustments made year-to-date to the amounts initially recorded as of the acquisition date on September 26, 2023. The measurement period adjustments summarized below resulted from updates to our valuation assumptions related to the estimated amounts and timing of future cash flows associated with certain intangible assets, updates of our assumptions related to the quantities, selling location and remaining manufacturing and selling costs of acquired inventory, and other assets and liabilities. The related impact to our condensed consolidated statements of income that would have been recognized in previous periods if the adjustments were recognized as of the acquisition date is immaterial. (In millions) Amounts Recognized as of Acquisition Date Cash and cash equivalents $ 267.3 Accounts receivable 15.9 Inventory 1,259.0 Other current assets 53.6 Intangible assets: Completed technology for SKYCLARYS (U.S.) 4,200.0 In-process research and development (omaveloxolone) 2,300.0 Priority review voucher 100.0 Other clinical programs 40.0 Operating lease assets 121.2 Accrued expense and other (1) (106.4) Debt payable (159.9) Contingent payable to Blackstone (300.0) Deferred tax liability (1) (916.5) Operating lease liabilities (151.8) Other assets and liabilities, net (2.5) Total identifiable net assets 6,719.9 Goodwill (1) 473.5 Total assets acquired and liabilities assumed $ 7,193.4 (1) Includes measurement period adjustments recorded in the first quarter of 2024 that increased accrued expense and other by $4.9 million, deferred tax liability by $4.1 million and goodwill by $9.0 million. Inventory: Total inventory acquired was approximately $1.3 billion, which reflects a step-up in the fair value of finished goods and work-in-process inventory for SKYCLARYS. The fair value was determined based on the estimated selling price of the inventory, less the remaining manufacturing and selling costs and a normal profit margin on those manufacturing and selling efforts. This fair value step-up adjustment is being amortized to cost of sales within our condensed consolidated statements of income as the inventory is sold, which is expected to be within approximately 3 years from the acquisition date. For the three and six months ended June 30, 2024, amortization from the fair value step-up adjustment was approximately $90.1 million and $134.2 million, respectively, which includes approximately $46.0 million of inventory used for clinical purposes rather than for commercial sale during the second quarter of 2024, and therefore reflected within research and development expense within our condensed consolidated statements of income. Intang ible assets: Intangible assets are comprised of $4.2 billion related to SKYCLARYS commercialization rights in the U.S., $2.3 billion of IPR&D related to the omaveloxolone program outside the U.S., which had not yet received regulatory approval in the E.U. as of the acquisition date, $100.0 million related to a rare pediatric disease priority voucher which may be used to obtain priority review by the FDA for a future regulatory submission or sold to a third party and $40.0 million related to other clinical programs. The estimated fair values of the program related intangible assets were determined using a multi-period excess earnings method, a form of the income approach, utilizing a discount rate of 14.3% and the estimated fair value of the priority review voucher was based on recent external purchase and sale transactions of similar vouchers. Our valuation of the SKYCLARYS commercialization rights reflects the assumption that, using an economic consumption model, the related $4.2 billion intangible asset will be amortized over its expected economic life. Upon SKYCLARYS receiving E.U. regulatory approval in February 2024, we began selling the product in certain countries in Europe, and began amortizing the $2.3 billion I PR&D asset related to the program outside the U.S. over its expected economic life using an economic consumption model. These fair value measurements were based on significant inputs not observable in the market and thus represent Level 3 fair value measurements. Leases: We assumed responsibility for a single-tenant, build-to-suit building of approximately 327,400 square feet of office and laboratory space located in Plano, Texas, with an initial lease term of 16 years. We recorded a lease liability of approximately $151.8 million, which represents the net present value of rental expense over the remaining lease term of approximately 15 years, with a corresponding right-of-use asset of approximately $121.2 million, which represents our estimate of the fair value for a market participant of the current rental market in the Dallas, Texas area. Included in our estimate of the market rental rate is the value of any leasehold improvements or tenant allowances related to the building. We do not intend to occupy this building and are evaluating opportunities to sublease the property. Goodwill: Goodwill was calculated as the excess of the consideration transferred over the net assets recognized and represents the future economic benefits arising from the other assets acquired that could not be individually identified and separately recognized. We recognized goodwill of approximately $473.5 million, which is not deductible for tax purposes. The goodwill recognized from our acquisition of Reata is primarily the result of the deferred tax consequences from the transaction recorded for financial statement purposes. Acquisition-related expenses: Acquisition-related expense, primarily comprised of regulatory, advisory and legal fees, and other transaction costs, totaled approximately $28.4 million and were recorded within selling, general and administrative expense within our condensed consolidated statements of income for the year ended December 31, 2023. Assumptions in the Allocations of Purchase Price T he results of operations of Reata, along with the estimated fair values of the assets acquired and liabilities assumed in the Reata acquisition, have been included in our condensed consolidated financial statements since the closing of the Reata acquisition on September 26, 2023. Our preliminary estimate of the fair value of the specifically identifiable assets acquired and liabilities assumed as of the date of acquisition is subject to the finalization of management's analysis related to certain matters, such as finalizing our assessment of income taxes. The final determination of these fair values will be completed as additional information becomes available but no later than one year from the acquisition date. The final determination may result in asset and liability fair values that are different than the preliminary estimates. Human Immunology Biosciences On July 2, 2024, we completed the acquisition of all of the issued and outstanding shares of HI-Bio, a privately-held clinical-stage biotechnology company focused on targeted therapies for patients with severe immune-mediated diseases. HI-Bio's lead asset, felzartamab, an anti-CD38 antibody currently being evaluated for three leading indications, aMR, PMN and IgAN. Felzartamab has received Breakthrough Therapy Designation and Orphan Drug Designation from the FDA for development in the treatment of PMN and has received ODD in the treatment of antibody-mediated rejection in kidney transplant recipients. The acquisition of HI-Bio is expected to augment our pipeline and build on our expertise in immunology. Under the terms of this acquisition, we made an upfront payment to HI-Bio of approximately $1.15 billion which was funded through available cash on hand. Additionally, we may pay up to $650.0 million in potential milestone payments to the former shareholders of HI-Bio. We expect this transaction to be accounted for as an acquisition of a business and recognized within our condensed consolidated financial statements during the third quarter of 2024. In addition to lead program felzartamab, the HI-Bio pipeline acquired includes izastobart/HIB210, an anti-C5aR1 antibody currently in a Phase 1 trial, and the potential for continued development in a range of complement-mediated diseases.

Dispositions

Dispositions	6 Months Ended
	Jun. 30, 2024
Divestitures [Abstract]
Dispositions	Sale of Joint Venture Equity Interest in Samsung Bioepis In April 2022 we completed the sale of our 49.9% equity interest in Samsung Bioepis to Samsung BioLogics in exchange for total consideration of approximately $2.3 billion. Under the terms of this transaction, we received approximately $1.0 billion in cash at closing, with approximately $1.3 billion in cash to be deferred over two payments. The first deferred payment of $812.5 million was received in April 2023 and the second deferred payment of $437.5 million was received in April 2024. For the three and six months ended June 30, 2024, we recognized gains of approximately $1.4 million and $7.5 million, respectively, to reflect the changes in fair value associated with the passage of time related to the second deferred payment due to us, which was received in April 2024. For the three and six months ended June 30, 2023, we recognized gains of approximately $2.6 million and $13.7 million, respectively, to reflect the changes in fair value associated with changes in interest rates and the passage of time related to the first deferred payment due to us. Additionally, for the three and six months ended June 30, 2023, we recognized gains of approximately $3.6 million and $9.8 million, respectively, to reflect the changes in fair value associated with changes in interest rates and the passage of time related to the second deferred payment due to us. These changes were recorded in other (income) expense, net in our condensed consolidated statements of income. For additional information on the sale of our equity interest in Samsung Bioepis, please read Note 3, Dispositions , to our consolidated financial statements included in our 2023 Form 10-K. Sale of Priority Review Voucher In April 2024 we completed the sale of our rare pediatric disease PRV, generated by the development associated with SPINRAZA, to a third party. In consideration for the PRV we received a cash payment of $103.0 million upon the closing of the PRV purchase, of which approximately $14.4 million is payable to Ionis. Our net portion of approximately $88.6 million was recognized in gain on sale of priority review voucher, net within our condensed consolidated statements of income for the three and six months ended June 30, 2024.

Restructuring, Business Transfo

Restructuring, Business Transformation and Other Cost Saving Initiatives	6 Months Ended
	Jun. 30, 2024
Restructuring and Related Activities [Abstract]
Restructuring, Business Transformation and Other Cost Saving Initiatives	2023 Fit for Growth Restructuring Program In July 2023 we initiated additional cost saving measures as part of our Fit for Growth program to reduce operating costs, while improving operating efficiency and effectiveness. The Fit for Growth program is expected to generate approximately $1.0 billion in gross operating expense savings by the end of 2025, some of which will be reinvested in various initiatives. The Fit for Growth program is currently estimated to include net headcount reductions of approximately 1,000 employees and we expect to incur restructuring charges ranging from approximately $260.0 million to $280.0 million. Total charges incurred from our 2023 cost saving initiatives are summarized as follows: For the Three Months Ended June 30, 2024 2023 (In millions) Severance Accelerated Depreciation and Other Costs Total Severance Costs Accumulated Depreciation and Other Costs Total Selling, general and administrative $ — $ 2.0 $ 2.0 $ — $ 11.5 $ 11.5 Research and development — 2.2 2.2 — 0.5 0.5 Restructuring charges 6.3 — 6.3 17.8 16.5 34.3 Total charges $ 6.3 $ 4.2 $ 10.5 $ 17.8 $ 28.5 $ 46.3 For the Six Months Ended June 30, 2024 2023 (In millions) Severance Costs Accelerated Depreciation Total Severance Costs Accumulated Depreciation Total Selling, general and administrative $ — $ 3.4 $ 3.4 $ — $ 11.5 $ 11.5 Research and development — 7.1 7.1 — 0.5 0.5 Restructuring charges 15.6 — 15.6 24.9 16.5 41.4 Total charges $ 15.6 $ 10.5 $ 26.1 $ 24.9 $ 28.5 $ 53.4 Other Costs: includes costs associated with items such as asset abandonment and write-offs, facility closure costs, pretax gains and losses resulting from the termination of certain leases, employee non-severance expense, consulting fees and other costs. Reata Integration Following the close of our Reata acquisition in September 2023, we implemented an integration plan designed to realize operating synergies through cost savings and avoidance. Under this initiative, we estimate we will incur total integration charges ranging from approximately $35.0 million to $40.0 million, related to severance and employment costs, which are expected to be paid by the end of 2024. These amounts were substantially incurred during 2023. Total charges incurred from our Reata integration are summarized as follows: For the Three Months Ended June 30, 2024 For the Six Months Ended June 30, 2024 (In millions) Severance Accelerated Depreciation and Other Costs Total Severance Costs Accumulated Depreciation and Other Costs Total Selling, general and administrative $ — $ 1.5 $ 1.5 $ — $ 3.3 $ 3.3 Research and development — 3.3 3.3 — 6.0 6.0 Restructuring charges 0.3 — 0.3 2.5 — 2.5 Total charges $ 0.3 $ 4.8 $ 5.1 $ 2.5 $ 9.3 $ 11.8 In connection with our acquisition of Reata we assumed responsibility for a single-tenant, build-to-suit building of approximately 327,400 square feet of office and laboratory space located in Plano, Texas, with an initial lease term of 16 years. We do not intend to occupy this building and are evaluating opportunities to sublease the property. For additional information on our acquisition of Reata, please read Note 2, Acquisitions , to these condensed consolidated financial statements. Charges and spending related to workforce reductions from our 2023 Fit for Growth program and Reata Integration are summarized as follows: Workforce Reductions (In millions) 2024 2023 Restructuring reserve as of January 1 $ 75.4 $ 35.9 Expense 11.5 7.1 Payment (42.2) (15.6) Foreign currency and other adjustments 0.8 0.6 Restructuring reserve as of March 31 45.5 28.0 Expense 6.6 17.8 Payment (11.9) (13.4) Foreign currency and other adjustments — (0.1) Restructuring reserve as of June 30 $ 40.2 $ 32.3

Revenues

Revenues	6 Months Ended
	Jun. 30, 2024
Revenue from Contract with Customer [Abstract]
Revenues	Product Revenue Revenue by product is summarized as follows: For the Three Months Ended June 30, 2024 2023 (In millions) United Rest of Total United Rest of Total Multiple Sclerosis: TECFIDERA $ 44.1 $ 208.1 $ 252.2 $ 66.5 $ 187.7 $ 254.2 VUMERITY 144.2 21.6 165.8 130.3 15.9 146.2 Total Fumarate 188.3 229.7 418.0 196.8 203.6 400.4 AVONEX 117.2 65.6 182.8 145.9 74.4 220.3 PLEGRIDY 28.2 39.9 68.1 34.1 48.0 82.1 Total Interferon 145.4 105.5 250.9 180.0 122.4 302.4 TYSABRI 248.7 213.5 462.2 259.9 223.2 483.1 FAMPYRA — 18.7 18.7 — 23.4 23.4 Subtotal: Multiple Sclerosis 582.4 567.4 1,149.8 636.7 572.6 1,209.3 Rare Disease: SPINRAZA 157.3 271.8 429.1 155.8 281.3 437.1 SKYCLARYS (1) 75.6 24.4 100.0 — — — QALSODY (2) 4.6 0.4 5.0 0.9 — 0.9 Subtotal: Rare Disease 237.5 296.6 534.1 156.7 281.3 438.0 Biosimilars: BENEPALI — 117.3 117.3 — 109.2 109.2 IMRALDI — 53.2 53.2 — 58.8 58.8 FLIXABI — 13.1 13.1 — 20.1 20.1 BYOOVIZ (3) 10.3 3.4 13.7 7.0 — 7.0 TOFIDENCE (4) 0.8 — 0.8 — — — Subtotal: Biosimilars 11.1 187.0 198.1 7.0 188.1 195.1 Other: ZURZUVAE (5) 14.9 — 14.9 — — — Other (6) 0.8 1.9 2.7 0.6 2.8 3.4 Subtotal: Other 15.7 1.9 17.6 0.6 2.8 3.4 Total product revenue, net $ 846.7 $ 1,052.9 $ 1,899.6 $ 801.0 $ 1,044.8 $ 1,845.8 (1) SKYCLARYS was obtained as part of our acquisition of Reata in September 2023. SKYCLARYS became commercially available in the U.S. during the second quarter of 2023 and we began recognizing revenue from SKYCLARYS in the U.S. during the fourth quarter of 2023, subsequent to our acquisition. SKYCLARYS was approved and became commercially available in the E.U. during the first quarter of 2024. (2) QALSODY became commercially available in the U.S. during the second quarter of 2023 and commercially available in the E.U. during the second quarter of 2024. (3) BYOOVIZ became commercially available in certain international markets in 2023. (4) TOFIDENCE became commercially available in the U.S. during the second quarter of 2024. (5) ZURZUVAE became commercially available in the U.S. during the fourth quarter of 2023. (6) Other includes FUMADERM and ADUHELM. For the Six Months Ended June 30, 2024 2023 (In millions) United Rest of Total United Rest of Total Multiple Sclerosis: TECFIDERA $ 87.8 $ 418.7 $ 506.5 $ 141.2 $ 387.5 $ 528.7 VUMERITY 250.1 43.2 293.3 223.8 30.6 254.4 Total Fumarate 337.9 461.9 799.8 365.0 418.1 783.1 AVONEX 228.4 132.9 361.3 248.5 144.2 392.7 PLEGRIDY 56.8 76.4 133.2 64.0 91.3 155.3 Total Interferon 285.2 209.3 494.5 312.5 235.5 548.0 TYSABRI 462.5 431.0 893.5 505.3 450.6 955.9 FAMPYRA — 37.9 37.9 — 47.5 47.5 Subtotal: Multiple Sclerosis 1,085.6 1,140.1 2,225.7 1,182.8 1,151.7 2,334.5 Rare Disease: SPINRAZA 305.8 464.6 770.4 302.5 577.9 880.4 SKYCLARYS (1) 148.6 29.4 178.0 — — — QALSODY (2) 9.0 0.6 9.6 0.9 — 0.9 Subtotal: Rare Disease 463.4 494.6 958.0 303.4 577.9 881.3 Biosimilars: BENEPALI — 236.0 236.0 — 218.2 218.2 IMRALDI — 108.0 108.0 — 113.2 113.2 FLIXABI — 30.9 30.9 — 40.5 40.5 BYOOVIZ (3) 14.0 5.3 19.3 15.2 0.4 15.6 TOFIDENCE (4) 0.8 — 0.8 — — — Subtotal: Biosimilars 14.8 380.2 395.0 15.2 372.3 387.5 Other: ZURZUVAE (5) 27.3 — 27.3 — — — Other (6) 1.7 3.8 5.5 1.0 4.8 5.8 Subtotal: Other 29.0 3.8 32.8 1.0 4.8 5.8 Total product revenue, net $ 1,592.8 $ 2,018.7 $ 3,611.5 $ 1,502.4 $ 2,106.7 $ 3,609.1 (1) SKYCLARYS was obtained as part of our acquisition of Reata in September 2023. SKYCLARYS became commercially available in the U.S. during the second quarter of 2023 and we began recognizing revenue from SKYCLARYS in the U.S. during the fourth quarter of 2023, subsequent to our acquisition. SKYCLARYS was approved and became commercially available in the E.U. during the first quarter of 2024. (2) QALSODY became commercially available in the U.S. during the second quarter of 2023 and commercially available in the E.U. during the second quarter of 2024. (3) BYOOVIZ became commercially available in certain international markets in 2023. (4) TOFIDENCE became commercially available in the U.S. during the second quarter of 2024. (5) ZURZUVAE became commercially available in the U.S. during the fourth quarter of 2023. (6) Other includes FUMADERM and ADUHELM. We recognized revenue from two wholesalers accounting for 25.4% and 12.3% of gross product revenue for the three months ended June 30, 2024, and 25.5% and 12.0% of gross product revenue for the six months ended June 30, 2024. We recognized revenue from two wholesalers accounting for 27.0% and 9.3% of gross product revenue for the three months ended June 30, 2023, and 27.2% and 8.4% of gross product revenue for the six months ended June 30, 2023. An analysis of the change in reserves for discounts and allowances is summarized as follows: (In millions) Discounts Contractual Returns Total Balance, December 31, 2023 $ 173.3 $ 857.1 $ 31.6 $ 1,062.0 Current provisions relating to sales in current year 399.3 1,355.2 10.6 1,765.1 Adjustments relating to prior years 5.6 (27.3) 12.7 (9.0) Payments/credits relating to sales in current year (260.1) (797.8) (0.3) (1,058.2) Payments/credits relating to sales in prior years (155.8) (477.0) (11.1) (643.9) Balance, June 30, 2024 $ 162.3 $ 910.2 $ 43.5 $ 1,116.0 The total reserves above, which are included in our condensed consolidated balance sheets, are summarized as follows: (In millions) As of June 30, 2024 As of December 31, 2023 Reduction of accounts receivable $ 151.5 $ 135.5 Component of accrued expense and other 964.5 926.5 Total revenue-related reserves $ 1,116.0 $ 1,062.0 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 June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Royalty revenue on sales of OCREVUS $ 336.3 $ 325.5 $ 639.0 $ 609.1 Biogen’s share of pre-tax profits in the U.S. for RITUXAN, GAZYVA and LUNSUMIO 103.4 103.6 190.5 216.1 Other revenue from anti-CD20 therapeutic programs 4.8 4.3 9.0 7.7 Total revenue from anti-CD20 therapeutic programs $ 444.5 $ 433.4 $ 838.5 $ 832.9 For additional information on our collaboration arrangements with Genentech, please read Note 19, Collaborative and Other Relationships, to these condensed consolidated financial statements. Contract Manufacturing, Royalty and Other Revenue Contract manufacturing, royalty and other revenue is summarized in the table below. For the Three Months Ended June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Contract manufacturing revenue $ 100.0 $ 183.1 $ 252.2 $ 490.0 Royalty and other revenue 20.8 (6.3) 53.2 (13.0) Total contract manufacturing, royalty and other revenue $ 120.8 $ 176.8 $ 305.4 $ 477.0 Contract Manufacturing Revenue Contract manufacturing revenue primarily reflects amounts earned under contract manufacturing agreements with our strategic customers. During the first quarter of 2023 we began recognizing contract manufacturing revenue for LEQEMBI, upon accelerated approval of LEQEMBI in the U.S. Prior to accelerated approval, our share of contract manufacturing amounts related to LEQEMBI were recognized in research and development expense within our condensed consolidated statements of income. Royalty and Other Revenue Royalty and other revenue primarily reflects royalty revenue on biosimilar products from our license arrangements with Samsung Bioepis and our 50.0% share of LEQEMBI product revenue, net and cost of sales, including royalties, as we are not the principal, as well as royalties we receive from net sales on products related to patents that we have out-licensed. For additional information on our license arrangements with Samsung Bioepis and our collaboration arrangements with Eisai, please read Note 19, Collaborative and Other Relationships , to these condensed consolidated financial statements.

Inventory

Inventory	6 Months Ended
	Jun. 30, 2024
Inventory Disclosure [Abstract]
Inventory	The components of inventory are summarized as follows: (In millions) As of June 30, 2024 As of December 31, 2023 Raw materials $ 401.2 $ 426.9 Work in process 1,821.2 1,926.8 Finished goods 395.0 255.4 Total inventory $ 2,617.4 $ 2,609.1 Balance Sheet Classification: Inventory $ 2,506.1 $ 2,527.4 Investments and other assets 111.3 81.7 Total inventory $ 2,617.4 $ 2,609.1 We recorded approximately $1.3 billion of acquired inventory, which includes measurement period adjustments, related to SKYCLARYS as a result of our acquisition of Reata in September 2023. The fair value was determined based on the estimated selling price of the inventory, less the remaining manufacturing and selling costs and a normal profit margin on those manufacturing and selling efforts. This fair value step-up adjustment is being amortized to cost of sales within our condensed consolidated statements of income when the inventory is sold, which is expected to be within approximately 3 years from the acquisition date. For the three and six months ended June 30, 2024, amortization from the fair value step-up adjustment was approximately $90.1 million and $134.2 million, respectively, which includes approximately $46.0 million of inventory used for clinical purposes rather than for commercial sale during the second quarter of 2024, and therefore reflected within research and development expense within our condensed consolidated statements of income. For additional information on our acquisition of Reata, please read Note 2, Acquisitions , to these condensed consolidated financial statements.

Intangible Assets and Goodwill

Intangible Assets and Goodwill	6 Months Ended
	Jun. 30, 2024
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 June 30, 2024 As of December 31, 2023 (In millions) Estimated Life Cost Accumulated Net Cost Accumulated Net Completed technology: Acquired and in-licensed rights and patents 2-22 years $ 10,515.3 $ (2,595.9) $ 7,919.4 $ 8,180.2 $ (2,440.7) $ 5,739.5 Developed technology and other 13-31 years 3,548.6 (3,439.1) 109.5 3,548.6 (3,429.1) 119.5 Total completed technology 14,063.9 (6,035.0) 8,028.9 11,728.8 (5,869.8) 5,859.0 In-process research and development Indefinite until commercialization 40.0 — 40.0 2,340.0 — 2,340.0 Priority review voucher Indefinite 100.0 — 100.0 100.0 — 100.0 Trademarks and trade names Indefinite 64.0 — 64.0 64.0 — 64.0 Total intangible assets $ 14,267.9 $ (6,035.0) $ 8,232.9 $ 14,232.8 $ (5,869.8) $ 8,363.0 Amortization and Impairments For the three and six months ended June 30, 2024, amortization and impairment of acquired intangible assets totale d $86.9 million and $165.2 million, respectively, compared to $52.9 million and $103.1 million, respectively, in the prior year comparative periods. The increases were primarily due to amortization for the Reata acquisition acquired intangible assets associated with SKYCLARYS. For the three and six months ended June 30, 2024 and 2023, we had no impairment charges. Completed Technology Completed technology primarily relates to our other marketed products and programs acquired through asset acquisitions, licenses and business combinations. Completed technology intangible assets are amortized over their estimated useful lives, which range between 2 to 31 years, with a remaining weighted average useful life of 12 years for acquired and in-licensed rights and patents and 10 years for developed technology and other. In connection with our acquisition of Reata in September 2023 we acquired SKYCLARYS, a commercially-approved product in the U.S., with an estimated fair value of approximately $4.2 billion, which includes measurement period adjustments. During the first quarter of 2024 SKYCLARYS was approved in the E.U. and became commercially available, which resulted in the reclassification of the related intangible asset, with an estimated fair value of approximately $2.3 billion, from IPR&D to completed technology. IPR&D Related to Business Combinations IPR&D represents the fair value assigned to research and development assets that we acquired as part of a business combination and had not yet reached technological feasibility at the date of acquisition. Included in IPR&D balances are adjustments related to foreign currency exchange rate fluctuations. The carrying value associated with our IPR&D assets as of December 31, 2023, related to the IPR&D programs we acquired in connection with our acquisition of Reata in September 2023, with an estimated fair value of approximately $2.3 billion, which includes measurement period adjustments. During the first quarter of 2024 SKYCLARYS was approved in the E.U. and became commercially available, which resulted in the reclassification of the related intangible asset from IPR&D to completed technology. Priority Review Voucher In connection with our acquisition of Reata in September 2023 we acquired a rare pediatric disease priority review voucher that may be used to obtain priority review by the FDA for a future regulatory submission or sold to a third party. We recorded the priority review voucher based on its estimated fair value of $100.0 million as an intangible asset. The estimated fair value was based on recent external purchase and sale transactions of similar vouchers. For additional information on our acquisition of Reata, please read Note 2, Acquisitions , to these condensed consolidated financial statements. 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 June 30, 2024 2024 (remaining six months) $ 185.0 2025 565.0 2026 660.0 2027 680.0 2028 710.0 2029 720.0 Goodwill The following table provides a roll forward of the changes in our goodwill balance: (In millions) As of June 30, 2024 Goodwill, December 31, 2023 $ 6,219.2 Goodwill resulting from Reata acquisition 9.0 Other (0.8) Goodwill, June 30, 2024 $ 6,227.4 For additional information on our acquisition of Reata, please read Note 2, Acquisitions, to these condensed consolidated financial statements. As of June 30, 2024, we had no ac cumulated impairment losses related to goodwill. Other includes adjustments related to foreign currency exchange rate fluctuations.

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2024
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: Fair Value Measurements on a Recurring Basis As of June 30, 2024 (In millions) Total Quoted Prices Significant Other Significant Assets: Cash equivalents $ 1,449.7 $ — $ 1,449.7 $ — Marketable equity securities 302.7 302.7 — — Other current assets: Derivative contracts 23.4 — 23.4 — Other non-current assets: Plan assets for deferred compensation 38.5 — 38.5 — Derivative contracts 1.8 — 1.8 — Total $ 1,816.1 $ 302.7 $ 1,513.4 $ — Liabilities: Derivative contracts $ 18.6 $ — $ 18.6 $ — Total $ 18.6 $ — $ 18.6 $ — Fair Value Measurements on a Recurring Basis As of December 31, 2023 (In millions) Total Quoted Prices Significant Other Significant Assets: Cash equivalents $ 610.7 $ — $ 610.7 $ — Marketable equity securities 416.8 416.8 — — Other current assets: Receivable from Samsung BioLogics (1) 430.0 — — 430.0 Derivative contracts 11.9 — 11.9 — Other non-current assets: Plan assets for deferred compensation 37.5 — 37.5 — Total $ 1,506.9 $ 416.8 $ 660.1 $ 430.0 Liabilities: Derivative contracts $ 31.6 $ — $ 31.6 $ — Total $ 31.6 $ — $ 31.6 $ — (1) Represents the fair value of the second deferred payment due from Samsung BioLogics as a result of the sale of our 49.9% equity interest in Samsung Bioepis to Samsung BioLogics during the second quarter of 2022, for which we elected the fair value option. For additional information on the sale of our equity interest in Samsung Bioepis, please read Note 3, Dispositions, to these condensed consolidated financial statements. Our marketable equity securities represent investments in publicly traded equity securities. Our ability to liquidate our investments in Denali, Sage and Sangamo may be limited by the size of our interest, the volume of market related activity, our concentrated level of ownership and potential restrictions resulting from our status as a collaborator. Therefore, we may realize significantly less than the current value of such investments. For additional information on our investments in Denali, Sangamo and Sage common stock, please read Note 19, Collaborative and Other Relationships , to our consolidated financial statements included in our 2023 Form 10-K. There have been no material impairments of our assets measured and carried at fair value as of June 30, 2024 and December 31, 2023. In addition, there have been no changes to our valuation techniques as of June 30, 2024 and December 31, 2023. 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 2023 Form 10-K. Level 3 Assets and Liabilities Held at Fair Value There were no transfers of assets or liabilities into or out of Level 3 as of June 30, 2024 and December 31, 2023. Financial Instruments Not Carried at Fair Value Other Financial Instruments Due to the short-term nature of certain financial instruments, the carrying value reflected in our condensed consolidated balance sheets for current accounts receivable, due from anti-CD20 therapeutic programs, other current assets, accounts payable and accrued expense and other, approximates fair value. Debt Instruments The fair and carrying values of our debt instruments, which are Level 2 liabilities, are summarized as follows: As of June 30, 2024 As of December 31, 2023 (In millions) Fair Carrying Fair Carrying Current portion: 2023 Term Loan 364-day tranche $ — $ — $ 150.0 $ 150.0 Current portion of notes payable and term loan — — 150.0 150.0 Non-current portion: 2023 Term Loan three-year tranche — — 500.0 500.0 4.050% Senior Notes due September 15, 2025 1,718.7 1,747.6 1,721.5 1,746.6 2.250% Senior Notes due May 1, 2030 1,275.9 1,494.3 1,279.3 1,493.8 5.200% Senior Notes due September 15, 2045 1,018.8 1,100.8 1,089.7 1,100.7 3.150% Senior Notes due May 1, 2050 969.3 1,474.7 1,049.0 1,474.3 3.250% Senior Notes due February 15, 2051 466.2 474.6 498.2 472.8 Non-current portion of notes payable and term loan 5,448.9 6,292.0 6,137.7 6,788.2 Total notes payable and term loan $ 5,448.9 $ 6,292.0 $ 6,287.7 $ 6,938.2 In connection with our acquisition of Reata we drew $1.0 billion from our 2023 Term Loan, comprised of a $500.0 million floating rate 364-day tranche and a $500.0 million floating rate three-year tranche. As of June 30, 2024, our 2023 Term Loan was repaid in full. For additional information on our 2023 Term Loan, please read Note 13, Indebtedness , to these condensed consolidated financial statements. The fair values of each of our series of Senior Notes were determined through market, observable and corroborated sources. The changes in the fair values of our Senior Notes as of June 30, 2024, compared to December 31, 2023, are primarily related to increases in U.S. treasury yields partially offset by a decrease in credit spreads used to value our Senior Notes since December 31, 2023. For additional information related to our Senior Notes, please read Note 13, Indebtedness, to our consolidated financial statements included in our 2023 Form 10-K.

Financial Instruments

Financial Instruments	6 Months Ended
	Jun. 30, 2024
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 June 30, 2024 As of December 31, 2023 Money market funds $ 1,449.7 $ 610.7 Total $ 1,449.7 $ 610.7 The carrying value of our money market funds approximates 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 equity securities, classified as available-for-sale: As of June 30, 2024 (In millions) Amortized Gross Gross Fair Marketable equity securities Marketable equity securities, non-current $ 800.2 $ — $ (497.5) $ 302.7 Total marketable equity securities $ 800.2 $ — $ (497.5) $ 302.7 As of December 31, 2023 (In millions) Amortized Gross Gross Fair Marketable equity securities Marketable equity securities, current $ 31.6 $ — $ (21.0) $ 10.6 Marketable equity securities, non-current 948.3 — (542.1) 406.2 Total marketable equity securities $ 979.9 $ — $ (563.1) $ 416.8 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: (In millions) For the Three Months Ended June 30, 2023 For the Six Months Ended June 30, 2023 Proceeds from maturities and sales $ 1,275.5 $ 1,682.2 Realized gains 0.3 0.4 Realized losses 1.4 2.1 We partially funded our Reata acquisition through available cash, cash equivalents and marketable securities. As of December 31, 2023, we have sold all of our marketable debt securities. For additional information on our acquisition of Reata, please read Note 2, Acquisitions , to these condensed consolidated financial statements. Realized losses for the three and six months ended June 30, 2023, primarily relate to sales of U.S. treasuries and corporate bonds. Strategic Investments Our strategic investment portfolio includes investments in equity securities of certain biotechnology companies, which are reflected within our disclosures included in Note 8, Fair Value Measurements, to these condensed consolidated financial statements, as well as venture capital funds where the underlying investments are in equity securities of certain biotechnology companies and non-marketable equity securities. As of June 30, 2024, our strategic investment portfolio was comprised of investments totaling $352.3 million which are included in investments and other assets in our condensed consolidated balance sheets. As of December 31, 2023, our strategic investment portfolio was comprised of investments totaling $460.7 million which are included in other current assets and investments and other assets within our condensed consolidated balance sheets. The decrease in our strategic investment portfolio as of June 30, 2024, was primarily due to the decrease in the fair value of our investment in Sage common stock as well as the sale of a portion of our Denali and Sangamo common stock during 2024. For additional information on our strategic investments in Denali, Sangamo and Sage common stock, please read Note 19, Collaborative and Other Relationships , to our consolidated financial statements included in our 2023 Form 10-K.

Derivative Instruments

Derivative Instruments	6 Months Ended
	Jun. 30, 2024
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. We enter into foreign currency forward contracts and foreign currency options with financial institutions with the primary objective to mitigate the impact of foreign currency exchange rate fluctuations on our international revenue and operating expense. Foreign currency forward contracts and foreign currency options in effect as of June 30, 2024 and December 31, 2023, had durations of 1 to 15 months and 1 to 12 months, respectively. These contracts have been designated as cash flow hedges and unrealized gains and losses on the portion of these foreign currency forward contracts and foreign currency options that are included in the effectiveness test are reported in AOCI. Realized gains and losses of such contracts and options 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 AOCI and fair value changes of excluded portions in the same line item in our condensed consolidated statements of income that have been impacted by the hedged item. The notional amount of foreign currency forward contracts and foreign currency options that were entered into to hedge forecasted revenue and operating expense is summarized as follows: Notional Amount (In millions) As of June 30, 2024 As of December 31, 2023 Euro $ 1,420.3 $ 1,169.0 British pound 69.6 — Swiss franc 142.4 — Canadian dollar 26.2 — Total foreign currency forward contracts and options $ 1,658.5 $ 1,169.0 The pre-tax portion of the fair value of these foreign currency forward contracts and foreign currency options that were included in AOCI in total equity is summarized as follows: (In millions) As of June 30, 2024 As of December 31, 2023 Unrealized gains $ 12.9 $ — Unrealized (losses) (9.5) (34.8) Net unrealized gains (losses) $ 3.4 $ (34.8) We expect the net unrealized gains of approximately $3.4 million to be settled over the next 15 months, of which approximately $1.9 million of these net unrealized gains are expected to be settled over the next 12 months, with any amounts in AOCI 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 June 30, 2024 and December 31, 2023, credit risk did not materially change the fair value of our foreign currency forward contracts and forward currency options. The following tables summarize the effect of foreign currency forward contracts and forward currency options designated as hedging instruments in our condensed consolidated statements of income: For the Three Months Ended June 30, Net Gains/(Losses) Net Gains/(Losses) Excluded from Effectiveness Testing and Location 2024 2023 Location 2024 2023 Revenue $ 7.6 $ 2.4 Revenue $ 0.9 $ (1.0) Operating expense (4.9) 1.4 Operating expense (0.7) (0.8) For the Six Months Ended June 30, Net Gains/(Losses) Net Gains/(Losses) Excluded from Effectiveness Testing and Location 2024 2023 Location 2024 2023 Revenue $ 10.5 $ 20.0 Revenue $ 1.1 $ 0.6 Operating expense (7.0) 0.9 Operating expense (1.9) (2.9) 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 $1,199.3 million and $1,301.5 million as of June 30, 2024 and December 31, 2023, respectively. Net losses of $5.7 million and $30.0 million related to these contracts were recorded as a component of other (income) expense, net for the three and six months ended June 30, 2024, respectively, compared to net losses of $7.2 million and $5.4 million, respectively, in the prior year comparative periods. 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 June 30, 2024 As of December 31, 2023 Cash Flow Hedging Instruments: Asset derivative instruments Other current assets $ 17.6 $ 0.3 Investments and other assets 1.8 — Liability derivative instruments Accrued expense and other 8.3 26.5 Other Derivative Instruments: Asset derivative instruments Other current assets 5.8 11.6 Liability derivative instruments Accrued expense and other 10.3 5.1

Property, Plant and Equipment

Property, Plant and Equipment	6 Months Ended
	Jun. 30, 2024
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 $2,537.4 million and $2,402.5 million as of June 30, 2024 and December 31, 2023, respectively. For the three and six months ended June 30, 2024, depreciation expense totaled $71.9 million and $141.2 million, respectively, compared to $64.5 million and $126.6 million, respectively, in the prior year comparative periods. Solothurn, Switzerland Manufacturing Facility In order to support our future growth and drug development pipeline, we built a large-scale biologics manufacturing facility in Solothurn, Switzerland. This facility includes 393,000 square feet related to a large-scale biologics manufacturing facility, 290,000 square feet of warehouse, utilities and support space and 51,000 square feet of administrative space. As of December 31, 2023, we had approximately $728.8 million capitalized as construction in progress related to this facility. In the second quarter of 2021 a portion of this facility (the first manufacturing suite) received a GMP multi-product license from the SWISSMEDIC and was placed into service. The second manufacturing suite, which was also licensed to operate by the SWISSMEDIC, became operational in the first quarter of 2024, resulting in approximately $717.3 million of fixed assets being placed into service. Solothurn has been approved for the manufacture of LEQEMBI by the FDA.

Leases

Leases	6 Months Ended
	Jun. 30, 2024
Leases [Abstract]
Leases	6100 Legacy Drive Lease In connection with our acquisition of Reata in September 2023 we assumed responsibility for a single-tenant, build-to-suit building of approximately 327,400 square feet of office and laboratory space located in Plano, Texas, with an initial lease term of 16 years. We recorded a lease liability of approximately $151.8 million, which represents the net present value of rental expense over the remaining lease term of approximately 15 years, with a corresponding right-of-use asset of approximately $121.2 million, which represents our estimate of the fair value for a market participant of the current rental market in the Dallas, Texas area. Included in our estimate of the market rental rate is the value of any leasehold improvements or tenant allowances related to the building. We do not intend to occupy this building and are evaluating opportunities to sublease the property. For additional information on our acquisition of Reata, please read Note 2, Acquisitions , to these condensed consolidated financial statements.

Indebtedness

Indebtedness	6 Months Ended
	Jun. 30, 2024
Debt Disclosure [Abstract]
Indebtedness	Credit Agreement In connection with our acquisition of Reata in September 2023 we entered into a $1.5 billion term loan credit agreement (2023 Term Loan). On the closing date of the Reata acquisition we drew $1.0 billion from the 2023 Term Loan, comprised of a $500.0 million floating rate 364-day tranche and a $500.0 million floating rate three-year tranche. The remaining unused commitment of $500.0 million was terminated. As of December 31, 2023, we repaid $350.0 million of the 364--day tranche. The remaining $150.0 million portion of the 364-day tranche was repaid during the first quarter of 2024. Additionally, during the first quarter of 2024 we repaid $250.0 million of the three-year tranche, with the remaining $250.0 million portion of the three-year tranche being subsequently repaid during the second quarter of 2024. As of June 30, 2024, the 2023 Term Loan was paid in full. For additional information on our acquisition of Reata, please read Note 2, Acquisitions , to these condensed consolidated financial statements.

Equity

Equity	6 Months Ended
	Jun. 30, 2024
Equity [Abstract]
Equity	Share Repurchases In October 2020 our Board of Directors authorized our 2020 Share Repurchase Program, which is a program to repurchase up to $5.0 billion of our common stock. Our 2020 Share Repurchase Program does not have an expiration date. All shares repurchased under our 2020 Share Repurchase Program were retired. There were no share repurchases of our common stock during the three and six months ended June 30, 2024 and 2023. Approximately $2.1 billion remained available under our 2020 Share Repurchase Program as of June 30, 2024. Accumulated Other Comprehensive Income (Loss) The following tables summarize the changes in AOCI, net of tax by component: For the Three Months Ended June 30, 2024 (In millions) Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Unrealized Gains (Losses) on Pension Benefit Obligation, Net of Tax Currency Total Balance, March 31, 2024 $ (10.3) $ (2.7) $ (147.5) $ (160.5) Other comprehensive income (loss) before reclassifications 22.3 — (5.6) 16.7 Amounts reclassified from AOCI (2.2) — — (2.2) Net current period other comprehensive income (loss) 20.1 — (5.6) 14.5 Balance, June 30, 2024 $ 9.8 $ (2.7) $ (153.1) $ (146.0) For the Six Months Ended June 30, 2024 Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Unrealized Gains (Losses) on Pension Benefit Obligation, Net of Tax Currency Total Balance, December 31, 2023 $ (25.0) $ (2.6) $ (126.1) $ (153.7) Other comprehensive income (loss) before reclassifications 37.7 (0.1) (27.0) 10.6 Amounts reclassified from AOCI (2.9) — — (2.9) Net current period other comprehensive income (loss) 34.8 (0.1) (27.0) 7.7 Balance, June 30, 2024 $ 9.8 $ (2.7) $ (153.1) $ (146.0) For the Three Months Ended June 30, 2023 (In millions) Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Unrealized Gains (Losses) on Pension Benefit Obligation, Net of Tax Currency Translation Adjustments Total Balance, March 31, 2023 $ (10.0) $ (20.3) $ (0.6) $ (141.1) $ (172.0) Other comprehensive income (loss) before reclassifications (5.5) 10.2 0.2 (3.4) 1.5 Amounts reclassified from AOCI 1.0 (3.3) — — (2.3) Net current period other comprehensive income (loss) (4.5) 6.9 0.2 (3.4) (0.8) Balance, June 30, 2023 $ (14.5) $ (13.4) $ (0.4) $ (144.5) $ (172.8) For the Six Months Ended June 30, 2023 (In millions) Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Unrealized Gains (Losses) on Pension Benefit Obligation, Net of Tax Currency Translation Adjustments Total Balance, December 31, 2022 $ (15.7) $ 15.1 $ (1.1) $ (163.2) $ (164.9) Other comprehensive income (loss) before reclassifications (0.2) (10.1) 0.7 18.7 9.1 Amounts reclassified from AOCI 1.4 (18.4) — — (17.0) Net current period other comprehensive income (loss) 1.2 (28.5) 0.7 18.7 (7.9) Balance, June 30, 2023 $ (14.5) $ (13.4) $ (0.4) $ (144.5) $ (172.8) The following table summarizes the amounts reclassified from AOCI: (In millions) Amounts Reclassified from AOCI Income Statement Location For the Three Months Ended June 30, For the Six Months Ended June 30, 2024 2023 2024 2023 Gains (losses) on securities available for sale $ — $ (1.3) $ — $ (1.8) Other (income) expense — 0.3 — 0.4 Income tax (benefit) expense Gains (losses) on cash flow hedges 7.6 2.4 10.5 20.0 Revenue (4.9) 1.4 (7.0) 0.9 Operating expense (0.2) (0.1) (0.1) (0.2) Other (income) expense (0.3) (0.4) (0.5) (2.3) Income tax (benefit) expense Total reclassifications, net of tax $ 2.2 $ 2.3 $ 2.9 $ 17.0

Earnings per Share

Earnings per Share	6 Months Ended
	Jun. 30, 2024
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 June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Numerator: Net income attributable to Biogen Inc. $ 583.6 $ 591.6 $ 977.0 $ 979.5 Denominator: Weighted average number of common shares outstanding 145.6 144.7 145.4 144.6 Effect of dilutive securities: Time-vested restricted stock units 0.2 0.7 0.4 0.7 Performance stock units settled in stock 0.1 0.1 0.1 0.1 Dilutive potential common shares 0.3 0.8 0.5 0.8 Shares used in calculating diluted earnings per share 145.9 145.5 145.9 145.4 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	6 Months Ended
	Jun. 30, 2024
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 June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Research and development $ 27.3 $ 27.9 $ 54.7 $ 59.2 Selling, general and administrative 45.6 48.4 89.6 98.5 Subtotal 72.9 76.3 144.3 157.7 Capitalized share-based compensation costs (2.3) (2.6) (5.6) (5.9) Share-based compensation expense included in total cost and expense 70.6 73.7 138.7 151.8 Income tax effect (13.4) (13.3) (26.3) (28.0) Share-based compensation expense included in net income attributable to Biogen Inc. $ 57.2 $ 60.4 $ 112.4 $ 123.8 The following table summarizes share-based compensation expense associated with each of our share-based compensation programs: For the Three Months Ended June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Time-vested restricted stock units $ 58.3 $ 59.9 $ 115.6 $ 121.6 Performance stock units settled in stock 11.7 9.8 22.2 19.3 Employee stock purchase plan 1.8 2.1 6.3 6.8 Performance stock units settled in cash 0.1 2.2 (2.0) 4.7 Stock options 0.9 1.0 1.8 1.8 Market stock units 0.1 1.3 0.4 3.5 Subtotal 72.9 76.3 144.3 157.7 Capitalized share-based compensation costs (2.3) (2.6) (5.6) (5.9) Share-based compensation expense included in total cost and expense $ 70.6 $ 73.7 $ 138.7 $ 151.8 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. 2024 Omnibus Equity Plan In June 2024 our shareholders approved the 2024 Omnibus Equity Plan for share-based awards to our prospective and current employees, non-employee directors, officers or consultants. Awards granted from the 2024 Omnibus Equity Plan may include stock options, shares of restricted stock, restricted stock units, performance shares, stock appreciation rights and other awards in such amounts and with such terms and conditions as may be determined by a committee of our Board of Directors, subject to the provisions of the plan. Shares of common stock available for grant under the 2024 Omnibus Equity Plan consist of 3.7 million shares reserved for this purpose, plus shares of common stock that remained available for grant under our 2017 Omnibus Equity Plan (including shares available by reason of a predecessor plan) on the date that our shareholders approved the 2024 Omnibus Equity Plan, plus shares that were subject to awards under the 2017 Omnibus Equity Plan (including shares available by reason of a predecessor plan) that remain unissued upon the cancellation, surrender, exchange, termination or forfeiture of such awards. The 2024 Omnibus Equity Plan provides that awards other than stock options and stock appreciation rights will be counted against the total number of shares available under the plan in a 1.5-to-1 ratio. We have not made any awards pursuant to the 2017 Omnibus Equity Plan or the Directors Plan since our shareholders approved the 2024 Omnibus Equity Plan, and do not intend to make any awards pursuant to the 2017 Omnibus Equity Plan or the Directors Plan in the future, except that unused shares under the 2017 Omnibus Equity Plan have been carried over for use under the 2024 Omnibus Equity Plan. Awards outstanding under the 2017 Omnibus Equity Plan and the Directors Plan as of the date our shareholders approved the 2024 Omnibus Equity Plan will remain outstanding and subject to the terms and conditions of the 2017 Omnibus Equity Plan and the Directors Plan, as applicable, and the relevant award agreements. 2024 Employee Stock Purchase Plan In June 2024 our shareholders approved the 2024 ESPP. The 2024 ESPP, which became effective on July 1, 2024, replaced the 2015 ESPP, which expired on June 30, 2024. The maximum number of shares of our common stock that may be purchased under the 2024 ESPP is 2.5 million.

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2024
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 June 30, For the Six Months Ended June 30, 2024 2023 2024 2023 Statutory rate 21.0 % 21.0 % 21.0 % 21.0 % State taxes 1.4 1.2 1.4 1.2 Taxes on foreign earnings (6.2) (4.7) (5.2) (5.0) Tax credits (1.6) (2.0) (2.0) (3.7) Purchased inventory valuation step-up and intangible assets 1.6 0.3 1.5 0.3 GILTI (0.9) 0.2 (1.6) 0.3 Other, including permanent items 1.2 0.2 0.9 0.3 Effective tax rate 16.5 % 16.2 % 16.0 % 14.4 % Changes in Tax Rate For the three and six months ended June 30, 2024, compared to the same periods in 2023, the changes in our effective tax rates include the impact of changes in jurisdictional profit mix, including non-cash tax effects of changes in the value of our equity investments. The increase in our effective tax rate for the six months ended June 30, 2024, also reflects the resolution of an uncertain tax matter during the first quarter of 2023 related to tax credits. 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 2019 or state, local or non-U.S. income tax examinations for years before 2013. The IRS 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, withholding taxes and other issues as we receive additional information from various taxing authorities, including reaching settlements with such authorities. We estimate that it is reasonably possible that our gross unrecognized tax benefits, exclusive of interest, could decrease by up to approximately $25.0 million in the next 12 months as a result of various audit closures, settlements and expiration of the statute of limitations.

Other Consolidated Financial St

Other Consolidated Financial Statement Detail	6 Months Ended
	Jun. 30, 2024
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 June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Interest income $ (20.1) $ (75.9) $ (37.5) $ (156.8) Interest expense 61.3 49.1 130.5 111.6 (Gains) losses on investments, net 30.7 (105.4) 61.3 (27.7) Foreign exchange (gains) losses, net 11.3 8.7 20.7 19.4 Other, net 2.0 2.3 3.9 1.7 Total other (income) expense, net $ 85.2 $ (121.2) $ 178.9 $ (51.8) The (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 relate to our equity securities held during the following periods: For the Three Months Ended June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Net (gains) losses recognized on equity securities $ 30.4 $ (106.5) $ 61.0 $ (28.4) Less: Net (gains) losses realized on equity securities 0.6 (0.7) 5.5 0.9 Net unrealized (gains) losses recognized on equity securities $ 29.8 $ (105.8) $ 55.5 $ (29.3) The net unrealized losses recognized during the three months ended June 30, 2024, primarily reflect a decrease in the aggregate fair value of our investments in Sage and Sangamo common stock of approximately $51.4 million, partially offset by an increase in the fair value of Denali common stock of approximately $27.0 million. The net unrealized gains recognized during the three months ended June 30, 2023, primarily reflect an increase in the aggregate fair value of our investments in Denali and Sage common stock of approximately $117.7 million, partially offset by a decrease in the fair value of Sangamo common stock of approximately $10.9 million. The net unrealized losses recognized during the six months ended June 30, 2024, primarily reflect a decrease in the aggregate fair value of our investments in Sage and Sangamo common stock of approximately $68.3 million, partially offset by an increase in the fair value of Denali common stock of approximately $17.6 million. The net unrealized gains recognized during the six months ended June 30, 2023, primarily reflect an increase in the aggregate fair value of our investments in Denali and Sage common stock of approximately $78.0 million, partially offset by a decrease in the fair value of Sangamo and Ionis common stock of approximately $47.4 million. Accrued Expense and Other Accrued expense and other consists of the following: (In millions) As of June 30, 2024 As of December 31, 2023 Revenue-related reserves for discounts and allowances $ 964.5 $ 926.5 Employee compensation and benefits 228.3 335.1 Collaboration expense 319.4 214.6 Royalties and licensing fees 193.0 191.5 Reata related accrued expense 77.4 117.5 Other 689.5 838.4 Total accrued expense and other $ 2,472.1 $ 2,623.6 Other Long-term Liabilities Other long-term liabilities were $556.7 million and $781.1 million as of June 30, 2024 and December 31, 2023, respectively, and included accrued income taxes totaling $191.7 million and $403.2 million, respectively.

Collaborative and Other Relatio

Collaborative and Other Relationships	6 Months Ended
	Jun. 30, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborative and Other Relationships	Note 19: Collaborative and Other Relationships Genentech, Inc. (Roche Group) We have certain business and financial rights with respect to RITUXAN for the treatment of non-Hodgkin's lymphoma, CLL and other conditions; RITUXAN HYCELA for the treatment of non-Hodgkin's lymphoma and CLL; GAZYVA for the treatment of CLL and follicular lymphoma; OCREVUS for the treatment of PPMS and RMS; LUNSUMIO for the treatment of relapsed or refractory follicular lymphoma; COLUMVI, a bispecific antibody for the treatment of non-Hodgkin's lymphoma; and have the option to add other potential anti-CD20 therapies, pursuant to our collaboration arrangements with Genentech, a wholly-owned member of the Roche Group. For purposes of this footnote, we refer to RITUXAN and RITUXAN HYCELA collectively as RITUXAN. RITUXAN Genentech and its affiliates are responsible for the worldwide manufacture of RITUXAN as well as all development and commercialization activities as follows: • U.S.: We have co-exclusively licensed our rights to develop, commercialize and market RITUXAN in the U.S. • Canada: We have co-exclusively licensed our rights to develop, commercialize and market RITUXAN in Canada. GAZYVA The Roche Group and its sub-licensees maintain sole responsibility for the development, manufacture and commercialization of GAZYVA in the U.S. The level of gross sales of GAZYVA in the U.S. has impacted our percentage of the co-promotion profits for RITUXAN and LUNSUMIO, as summarized in the table below. OCREVUS Pursuant to the terms of our collaboration arrangements with Genentech, we receive a tiered royalty on U.S. net sales from 13.5% and increasing up to 24.0% if annual net sales exceed $900.0 million. There will be a 50.0% reduction to these royalties if a biosimilar to OCREVUS is approved in the U.S. In addition, we receive a gross 3.0% royalty on net sales of OCREVUS outside the U.S., with the royalty period lasting 11 years from the first commercial sale of OCREVUS on a country-by-country basis. The commercialization of OCREVUS does not impact the percentage of the co-promotion profits we receive for RITUXAN, LUNSUMIO or GAZYVA. Genentech is solely responsible for development and commercialization of OCREVUS and funding future costs. Genentech cannot develop OCREVUS in CLL, non-Hodgkin's lymphoma or rheumatoid arthritis. OCREVUS royalty revenue is based on our estimates from third party and market research data of OCREVUS sales occurring during the corresponding period. Differences between actual and estimated royalty revenue will be adjusted for in the period in which they become known, which is generally expected to be the following quarter. LUNSUMIO (mosunetuzumab) In January 2022 we exercised our option with Genentech to participate in the joint development and commercialization of LUNSUMIO. Under our collaboration with Genentech, we were responsible for 30.0% of development costs for LUNSUMIO prior to FDA approval and will be entitled to a tiered share of co-promotion operating profits and losses in the U.S., as summarized in the table below. In addition, we receive low single-digit royalties on sales of LUNSUMIO outside the U.S. In December 2022 LUNSUMIO was granted accelerated approval by the FDA for the treatment of relapsed or refractory follicular lymphoma. Prior to regulatory approval, we record our share of the expense incurred by the collaboration for the development of anti-CD20 products in research and development expense and pre-commercialization costs within selling, general and administrative expense in our condensed consolidated statements of income. After an anti-CD20 product is approved, we record our share of the development and sales and marketing expense related to that product as a reduction of our share of pre-tax profits in revenue from anti-CD20 therapeutic programs. COLUMVI (glofitamab) In December 2022 we entered into an agreement with Genentech related to the commercialization and sharing of economics for COLUMVI, a bispecific antibody for the treatment of B-cell non-Hodgkin's lymphoma, which was subsequently granted accelerated approval by the FDA in June 2023. Under the terms of this agreement, we will have no payment obligations. Genentech will have sole decision-making rights on the commercialization of COLUMVI within the U.S. and we will receive tiered royalties in the mid-single digit range on net sales of COLUMVI in the U.S. The commercialization of COLUMVI does not impact the percentage of the co-promotion profits we receive for RITUXAN, LUNSUMIO or GAZYVA. In April 2024 Roche announced that COLUMVI, in combination with chemotherapy GemOx (glofitamab-gxbm), demonstrated a statistically significant improvement in overall survival for people with relapsed or refractory diffuse large B-cell lymphoma . Profit-sharing Formulas RITUXAN and LUNSUMIO Profit Share Our current pretax co-promotion profit-sharing formula for RITUXAN and LUNSUMIO in the U.S. provides for a 30.0% share on the first $50.0 million of combined co-promotion operating profits earned each calendar year. As a result of the FDA approval of LUNSUMIO our share of the combined annual co-promotion profits for RITUXAN and LUNSUMIO in excess of $50.0 million varies upon the following events, as summarized in the table below: After LUNSUMIO Approval until the First Threshold Date 37.5 % After First Threshold Date until the Second Threshold Date 35.0 % After Second Threshold Date 30.0 % First Threshold Date means the earlier of (i) the first day of the calendar quarter following the date U.S. gross sales of GAZYVA within any consecutive 12-month period have reached $500.0 million or (ii) the first date in any calendar year in which U.S. gross sales of LUNSUMIO have reached $150.0 million. Second Threshold Date means the later of (i) the first date the gross sales in any calendar year in which U.S. gross sales of LUNSUMIO reach $350.0 million or (ii) January 1 of the calendar year following the calendar year in which the First Threshold Date occurs. In March 2023 the First Threshold Date was achieved. As a result, beginning in April 2023 the pre-tax profit share for RITUXAN and LUNSUMIO was 35.0%. GAZYVA Profit Share Our current pretax profit-sharing formula for GAZYVA provides for a 35.0% share on the first $50.0 million of operating profits earned each calendar year. Our share of annual co-promotion profits in excess of $50.0 million varies upon the following events, as summarized in the table below: Until Second GAZYVA Threshold Date 37.5 % After Second GAZYVA Threshold Date 35.0 % Second GAZYVA Threshold Date means the first day of the calendar quarter following the date U.S. gross sales of GAZYVA within any consecutive 12-month period have reached $500.0 million. The Second GAZYVA Threshold Date can be achieved regardless of whether GAZYVA has been approved in a non-CLL indication. In March 2023 the Second GAZYVA Threshold Date was achieved. As a result, beginning in April 2023 the pre-tax profit share for GAZYVA was 35.0%. For additional information on our collaboration arrangements with Genentech, please read Note 19, Collaborative and Other Relationships , to our audited consolidated financial statements included in our 2023 Form 10-K. Eisai Co., Ltd. During the first quarter of 2023 we accrued a $31.0 million payable to Eisai related to the termination of an agreement whereby Eisai co-promoted or distributed our MS products in certain Asia-Pacific markets and settings. As of December 31, 2023, we paid approximately $16.0 million of the $31.0 million payable. The remaining portion was subsequently paid in January 2024. This termination fee is included in selling, general and administrative expense in our condensed consolidated statements of income for the six months ended June 30, 2023. LEQEMBI (lecanemab) Collaboration We have a collaboration agreement with Eisai to jointly develop and commercialize LEQEMBI (lecanemab), an anti-amyloid antibody for the treatment of Alzheimer's disease (the LEQEMBI Collaboration). Eisai serves as the lead of LEQEMBI 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. We and Eisai co-promote LEQEMBI and share profits and losses equally. We currently manufacture LEQEMBI drug substance and drug product and in March 2022 we extended our supply agreement with Eisai related to LEQEMBI from five years to ten years for the manufacture of LEQEMBI drug substance. In July 2023 the FDA granted traditional approval of LEQEMBI. Prior to receiving traditional approval, LEQEMBI had been granted accelerated approval by the FDA in January 2023, at which time it became commercially available in the U.S. Outside of the U.S., LEQEMBI is now approved in Japan (September 2023), China (January 2024), South Korea (May 2024), Hong Kong (July 2024) and Israel (July 2024). Upon commercialization of LEQEMBI in the U.S., we began recognizing our 50.0% share of LEQEMBI product revenue, net and cost of sales, including royalties, within other revenue in our condensed consolidated statements of income, as we are not the principal. Our share of LEQEMBI sales and marketing expense and development expense are recorded within selling, general and administrative expense and research and development expense, respectively, within our condensed consolidated statements of income. A summary of development and sales and marketing expense related to the LEQEMBI Collaboration is as follows: For the Three Months Ended June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Total development expense incurred by the collaboration related to the advancement of LEQEMBI $ 90.6 $ 86.2 $ 177.9 $ 194.1 Biogen's share of the LEQEMBI Collaboration development expense reflected in research and development expense in our condensed consolidated statements of income 45.4 43.0 89.0 97.0 Total sales and marketing expense incurred by the LEQEMBI Collaboration 137.7 17.1 311.6 27.6 Biogen's share of the LEQEMBI Collaboration sales and marketing expense reflected in selling, general and administrative expense in our condensed consolidated statements of income 68.9 8.5 155.8 13.8 Amounts receivable from Eisai related to the agreement s discussed above were approximately $10.3 million and $1.4 million as of June 30, 2024 and December 31, 2023, respectively. Amounts payable to Eisai related to the agreements discussed above were $171.8 million and $118.4 million as of June 30, 2024 and December 31, 2023, respectively. For additional information on our collaboration arrangements with Eisai, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2023 Form 10-K. UCB We have a collaboration agreement with UCB, effective November 2003, to jointly develop and commercialize dapirolizumab pegol, an anti-CD40L pegylated Fab, for the potential treatment of SLE 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. If marketing approval is obtained, both companies will co-promote dapirolizumab pegol and share profits and losses equally. A summary of development expense related to the UCB collaboration agreement is as follows: For the Three Months Ended June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Total UCB collaboration development expense $ 16.5 $ 14.3 $ 33.1 $ 32.6 Biogen's share of UCB collaboration development expense reflected in research and development expense in our condensed consolidated statements of income 8.3 7.1 16.6 16.3 Sage Therapeutics, Inc. In November 2020 we entered into a global collaboration and license agreement with Sage to jointly develop and commercialize ZURZUVAE (zuranolone) for the treatment of PPD and potential treatment of MDD and BIIB124 (SAGE-324) for the potential treatment of essential tremor with potential in other neurological conditions such as epilepsy. In July 2024 we and Sage announced that the Phase 2 KINETIC 2 dose-range study of BIIB124 did not meet its endpoints. Based on these results, we discontinued our further development of BIIB124 for the potential treatment in essential tremor. In August 2023 the FDA approved ZURZUVAE for adults with PPD, pending DEA scheduling, which was completed in October 2023. Upon approval, ZURZUVAE became the first and only oral, once-daily, 14-day treatment that can provide rapid improvements in depressive symptoms by day 15 for women with PPD. ZURZUVAE for PPD became commercially available in the U.S. during the fourth quarter of 2023. Additionally, the FDA issued a CRL for the NDA for zuranolone in the treatment of adults with MDD. The CRL stated that the application did not provide substantial evidence of effectiveness to support the approval of zuranolone for the treatment of MDD and that an additional study or studies would be needed. We and Sage are continuing to seek feedback from the FDA and evaluating next steps. 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. During the fourth quarter of 2023 we accrued a milestone payment due to Sage of $75.0 million upon the first commercial sale of ZURZUVAE for PPD in the U.S., which was recorded within intangible assets, net in our condensed consolidated balance sheets, and subsequently paid in January 2024. For the three and six months ended June 30, 2024, we recognized net profit-sharing expense of approximately $6.5 million and $11.5 million, respectively, to reflect Sage's 50.0% share of net collaboration results in the U.S. for ZURZUVAE for PPD, which is recognized in collaboration profit sharing/(loss reimbursement) in our condensed consolidated statements of income. A summary of development and sales and marketing expense related to the Sage collaboration is as follows: For the Three Months Ended June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Total Sage collaboration development expense $ 9.6 $ 52.1 $ 21.2 $ 86.9 Biogen's share of Sage collaboration development expense reflected in research and development expense in our condensed consolidated statements of income 4.8 26.1 10.6 43.5 Total sales and marketing expense incurred by the Sage collaboration 26.9 60.1 54.5 98.3 Biogen's share of Sage collaboration sales and marketing expense reflected in selling, general and administrative expense and collaboration profit sharing/(loss reimbursement) in our condensed consolidated statements of income 13.5 30.0 27.3 49.1 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 LRRK2 for Parkinson's disease (LRRK2 Collaboration) and also entered into a separate agreement to obtain an exclusive option to license two preclinical programs from Denali's Transport Vehicle platform, including its ATV-enabled anti-amyloid beta program and a second program utilizing its Transport Vehicle technology. In July 2024 we terminated our license with Denali for the ATV-enabled anti-amyloid beta program. This termination also results in the termination of the exclusive option agreement, as discussed above. Under the LRRK2 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. A summary of development expense related to the Denali collaboration is as follows: For the Three Months Ended June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Total Denali collaboration development expense $ 15.4 $ 22.1 $ 29.6 $ 38.7 Biogen's share of Denali collaboration development expense reflected in research and development expense in our condensed consolidated statements of income 9.3 13.2 17.8 23.2 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 and six months ended June 30, 2024, we recorded approximately $8.5 million and $16.0 million , respectively, as research and development expense in our condensed consolidated statements of income related to other research and discovery related arrangements, compared to $2.6 million and $2.8 million, respectively, in the prior year comparative periods. Samsung Bioepis Co., Ltd. 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 ranibizumab biosimilar referencing LUCENTIS, and OPUVIZ, an 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 of approximately 45.0%. During the second quarter of 2024 we accrued a $15.0 million milestone upon the FDA approval of OPUVIZ in the U.S., which was recognized in intangible assets, net in our condensed consolidated balance sheets. We may also pay Samsung Bioepis up to approximately $165.0 million in additional development, regulatory and sales-based milestones. The timing of the commercial launch of OPUVIZ in the U.S. is subject to ongoing litigation involving our collaborator, Samsung Bioepis, in which the court has entered a preliminary injunction against us and Samsung Bioepis restraining the offer for sale or sale of OPUVIZ in the U.S. 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 by August 2024, and obtained an option to acquire exclusive rights to commercialize these products in China. In July 2024 we exercised our option to extend the term of our 2013 commercial agreement for BENEPALI, IMRALDI and FLIXABI by an additional five years. In connection with this exercise, we paid Samsung Bioepis an option exercise fee of $60.0 million in July 2024. 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. Royalty payments to AbbVie on sales of IMRALDI are recognized in cost of sales within our condensed consolidated statements of income. We share 50.0% of the profit or loss related to our commercial agreement with Samsung Bioepis, which is recognized in collaboration profit sharing/(loss reimbursement) in our condensed consolidated statements of income. For the three and six months ended June 30, 2024, we recognized net profit-sharing expense of approximately $55.9 million and $116.5 million, respectively, to reflect Samsung Bioepis' 50.0% sharing of the net collaboration profits, compared to a net profit-sharing expense of $56.9 million and $114.0 million, respectively, in the prior year comparative periods. Other Services Simultaneous with the formation of Samsung Bioepis, we also entered into a license agreement with Samsung Bioepis. Under this license agreement, we granted Samsung Bioepis an exclusive license to use, develop, manufacture and commercialize biosimilar products created by Samsung Bioepis using Biogen product-specific technology. In exchange, we receive single digit royalties on biosimilar products developed and commercialized by Samsung Bioepis. Royalty revenue under the license agreement is recognized as a component of contract manufacturing, royalty and other revenue in our condensed consolidated statements of income. Amounts receivable from Samsung Bioepis related to the agreements di scussed above were $19.0 million and $9.9 million as of June 30, 2024 and December 31, 2023, respectively. Amounts payable to Samsung Bioepis related to the agreements discussed above were $93.6 million and $73.7 million as of June 30, 2024 and December 31, 2023, respectively. For additional information on our collaboration arrangements with Samsung Bioepis and our other significant collaboration arrangements, please read Note 19, Collaborative and Other Relationships,

Investments in Variable Interes

Investments in Variable Interest Entities	6 Months Ended
	Jun. 30, 2024
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 Beginning in 2007 we consolidated the results of Neurimmune as we determined we were the primary beneficiary because we had the power through the collaboration to direct the activities that most significantly impacted the entity's economic performance and we were required to fund 100.0% of the research and development costs incurred in support of the collaboration. The collaboration and license agreement with Neurimmune was for the development and commercialization of antibodies for the potential treatment of Alzheimer's disease, including ADUHELM (as amended, the Neurimmune Agreement). In November 2023 we notified Neurimmune of our decision to terminate the Neurimmune Agreement. Subsequent to the termination, we reconsidered our relationship with Neurimmune and determined that we were no longer the primary beneficiary of the variable interest entity. As a result, we recorded a net gain on the deconsolidation of Neurimmune of approximately $3.0 million, which was recorded in other (income) expense, net within our consolidated statements of income for the year ended December 31, 2023, included in our 2023 Form 10-K. 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 June 30, 2024 and December 31, 2023, the carrying value of our investments in certain biotechnology companies representing potential unconsolidated variable interest entities totaled $22.8 million and $16.4 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 20, Investments in Variable Interest Entities, to our consolidated financial statements included in our 2023 Form 10-K.

Litigation

Litigation	6 Months Ended
	Jun. 30, 2024
Loss Contingency, Information about Litigation Matters [Abstract]
Litigation	We are currently involved in various claims, investigations 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 2023 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. If an estimate of the possible loss or range of loss can be made at this time, it is included in the potential loss contingency description below. 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 Securities Litigation We and certain current and former officers are defendants in three securities actions, one filed by Nadia Shash and Amjad Khan in November 2020 in the District Court and related to ADUHELM, one filed by the Oklahoma Firefighters Pension and Retirement System in February 2022 in the District Court and related to ADUHELM and one filed by Thomas Allen Gray in the U.S. District Court for the District of Colorado related to statements about our compliance controls, 2023 earnings guidance and other matters. All 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. Derivative Actions We and members of the Board of Directors are named as defendants in three derivative actions, one filed by The Booth Family Trust in February 2022 in the District Court, one filed by Elaine Wang in July 2022 in the District Court and one filed by Jonathan Blaufarb in July 2024 in the U.S. District Court for the District of Colorado. The Booth and Wang actions relate to ADUHELM and other matters, and the Blaufarb action relates to statements about our compliance controls, 2023 earnings guidance and other matters. The actions allege breach of fiduciary duty, waste of corporate assets and other common law claims, and violations of the Securities Exchange Act of 1934, 15 U.S.C. §78a et seq. The actions seek declaratory and injunctive relief, monetary relief payable to Biogen, and attorneys’ fees and costs payable to the plaintiffs. The Booth and Wang actions are stayed. IMRALDI Patent Litigation In June 2024, the Technical Boards of Appeal of the European Patent Office upheld the validity of Fresenius Kabi Deutschland GmbH's (Fresenius Kabi's) European Patent 3 145 488 (the EP '488 Patent), which expires in May 2035. In June 2022 Fresenius Kabi filed a claim for damages and injunctive relief against Biogen France SAS in the Tribunal de Grande Instance de Paris alleging that IMRALDI, the adalimumab biosimilar product of Samsung Bioepis that Biogen commercializes in Europe, infringes the French counterpart of the EP ‘488 Patent. In March 2024 the Düsseldorf Regional Court dismissed Fresenius Kabi's claim of infringement of the German counterpart of the EP '488 Patent and Fresenius Kabi has appealed to the Higher Regional Court of Düsseldorf. Litigation with Former Convergence Shareholders In 2015 Biogen acquired Convergence, a U.K. company. In 2019 Shareholder Representative Services LLC, on behalf of the former shareholders of Convergence, asserted claims of $200.0 million for alleged breaches of the contract pursuant to which we acquired Convergence. In June 2023 Shareholder Representative Services LLC and 24 former shareholders filed suit against us in the High Court of Justice of England and Wales on one of the previously asserted claims, seeking payment of $49.9 million, interest and costs. Humana Patient Assistance Litigation In March 2023 the District Court dismissed the previously disclosed action filed against us by Humana in September 2020. Humana had alleged damages related to our providing MS patients with free medications and making charitable contributions to non-profit organizations that assist MS patients and had alleged violations of the federal RICO Act and state laws. In December 2023 Humana appealed to the United States Court of Appeals for the First Circuit and the appeal is pending. Genentech Litigation In February 2023 Genentech, Inc. filed suit against us in the U.S. District Court for the Northern District of California, alleging that it is owed royalties on sales of TYSABRI that occurred after the expiration of a patent licensed by Genentech to Biogen, together with interest and costs. We estimate that the royalties claimed total approximately $88.3 million. Lender Dispute In April 2024, BioPharma Credit PLC, BPCR Limited Partnership, and BioPharma Credit Investments V (Master) LP filed suit against us and Reata Pharmaceuticals, Inc. in the Supreme Court of the State of New York alleging breach of a loan agreement with Reata and seeking payment of approximately $23.2 million, plus interest, costs and attorneys' fees. Other Matters Government Investigation We have received subpoenas from the SEC seeking information relating to ADUHELM and its launch. We have also received subpoenas from the DOJ and SEC seeking information relating to our business operations in several foreign countries. The Italian Competition Authority is investigating Biogen and other companies in relation to our biosimilar product BYOOVIZ. TYSABRI Biosimilar Patent Matter In September 2022 we filed an action in the U.S. District Court for the District of Delaware against Sandoz Inc., other Sandoz entities and Polpharma Biologics S.A. under the Biologics Price Competition and Innovation Act, 42 U.S.C. §262, seeking a declaratory judgment of patent infringement. Annulment Proceedings in the General Court of the European Union relating to TECFIDERA In November 2020 Mylan Ireland filed an action in the General Court of the European Union to annul the EMA's decision not to validate its applications to market generic versions of TECFIDERA on the grounds that TECFIDERA benefits from regulatory data protection. Hatch-Waxman Act Litigation relating to VUMERITY Orange-Book Listed Patents In July 2023 Biogen and Alkermes Pharma Ireland Limited filed patent infringement proceedings relating to VUMERITY Orange-Book listed patents (U.S. Patent Nos. 8,669,281, 9,090,558 and 10,080,733) pursuant to the Drug Price Competition and Patent Term Restoration Act of 1984 (the Hatch-Waxman Act) in the U.S. District Court for the District of Delaware against Zydus Worldwide DMCC. Product Liability and Other Legal Proceedings We are also involved in product liability claims and other legal proceedings 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.

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Pay vs Performance Disclosure
Net income (loss) attributable to Biogen Inc.	$ 583.6	$ 591.6	$ 977	$ 979.5

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended
	Jun. 30, 2024
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

Summary of Significant Accoun_2

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2024
Accounting Policies [Abstract]
Business Overview	Biogen is a global biopharmaceutical company focused on discovering, developing and delivering innovative therapies for people living with serious and complex diseases worldwide. We have a broad portfolio of medicines to treat MS, have introduced the first approved treatment for SMA, co-developed treatments to address a defining pathology of Alzheimer’s disease and launched the first approved treatment to target a genetic cause of ALS. Through our 2023 acquisition of Reata we market the first and only drug approved in the U.S. and the E.U. for the treatment of Friedreich's Ataxia in adults and adolescents aged 16 years and older. We are focused on advancing our pipeline in neurology, specialized immunology and rare diseases. We support our drug discovery and development efforts through internal research and development programs, external collaborations and acquisitions. Our marketed products include TECFIDERA, VUMERITY, AVONEX, PLEGRIDY, TYSABRI and FAMPYRA for the treatment of MS; SPINRAZA for the treatment of SMA; SKYCLARYS for the treatment of Friedreich's Ataxia; QALSODY for the treatment of ALS; and FUMADERM for the treatment of severe plaque psoriasis. We also have collaborations with Eisai on the commercialization of LEQEMBI for the treatment of Alzheimer's disease and Sage on the commercialization of ZURZUVAE for the treatment of PPD and we have certain business and financial rights with respect to RITUXAN for the treatment of non-Hodgkin's lymphoma, CLL and other conditions; RITUXAN HYCELA for the treatment of non-Hodgkin's lymphoma and CLL; GAZYVA for the treatment of CLL and follicular lymphoma; OCREVUS for the treatment of PPMS and RMS; LUNSUMIO for the treatment of relapsed or refractory follicular lymphoma; COLUMVI, a bispecific antibody for the treatment of non-Hodgkin's lymphoma; and have the option to add other potential anti-CD20 therapies, pursuant to our collaboration arrangements with Genentech, a wholly-owned member of the Roche Group. We commercialize a portfolio of biosimilars of advanced biologics including BENEPALI, an etanercept biosimilar referencing ENBREL, IMRALDI, an adalimumab biosimilar referencing HUMIRA, and FLIXABI, an infliximab biosimilar referencing REMICADE, in certain countries in Europe, as well as BYOOVIZ, a ranibizumab biosimilar referencing LUCENTIS, and TOFIDENCE, a tocilizumab biosimilar referencing ACTEMRA, in the U.S. and certain international markets. We also have commercialization rights related to OPUVIZ, an aflibercept biosimilar referencing EYLEA. For additional information on our collaboration arrangements, please read Note 19, 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 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 2023 Form 10-K. Our accounting policies are described in the Notes to Consolidated Financial Statements in our 2023 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 and six months ended June 30, 2024, 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 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. In November 2023 we terminated the Neurimmune Agreement, which resulted in the deconsolidation of our variable interest entity, Neurimmune. For additional information on the deconsolidation of Neurimmune, please read Note 20, Investments in Variable Interest Entities , to these condensed consolidated financial statements.
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. Significant Accounting Policies There have been no material changes to our significant accounting policies disclosed in Note 1, Summary of Significant Accounting Policies , to our audited consolidated financial statements included in our 2023 Form 10-K.
New accounting pronouncements	From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies that we adopt as of the specified effective date. Unless otherwise discussed below, we do not believe that the adoption of recently issued standards have had or may have a material impact on our condensed consolidated financial statements or disclosures. Climate-Related Disclosures In March 2024 the SEC issued a final rule under SEC Release No. 33-11275, The Enhancement and Standardization of Climate-Related Disclosures for Investors . This new rule will require large accelerated filers to disclose material climate-related risks that are reasonably likely to have a material impact on their business, results of operations or financial condition. The required information about climate-related risks will also include disclosure of material direct greenhouse gas emissions from operations owned or controlled (Scope 1) and/or material indirect greenhouse gas emissions from purchased energy consumed in owned or controlled operations (Scope 2). Additionally, the new rules will require disclosure within the notes to the financial statements of the effects of severe weather events and other natural conditions and information on any climate-related targets or goals, subject to certain materiality thresholds. The final rule, if adopted, includes a phased-in compliance period which will begin phasing in with our annual report for the year ending December 31, 2025. In April 2024 the SEC voluntarily stayed implementation of the new climate-related disclosure requirements pending judicial review. Once the litigation is resolved, and if the rule remains in effect, the SEC will announce a new effective date. We are currently evaluating the potential impact that this new rule will have on our company's disclosures. Segment Reporting In November 2023 the FASB issued ASU No. 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosure . This standard requires disclosure of significant segment expenses that are regularly provided to the CODM and included within each reported measure of segment profit or loss, an amount and description of its composition for other segment items to reconcile to segment profit or loss and the title and position of the entity's CODM. The amendments in this update also expand the interim segment disclosure requirements. All disclosure requirements under this standard are also required for public entities with a single reportable segment. This standard is effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024. Early adoption is permitted and the amendments in this update are required to be applied on a retrospective basis. We are currently evaluating the potential impact that this new standard will have on our consolidated financial statements and related disclosures.

Acquisitions (Tables)

Acquisitions (Tables)	6 Months Ended
	Jun. 30, 2024
Business Combinations [Abstract]
Schedule of Business Acquisitions, by Acquisition	Total consideration transferred for the acquisition of Reata is summarized as follows: (In millions) As of September 26, 2023 Cash consideration paid to Reata shareholders (1) $ 6,602.9 Fair value of Reata equity compensation pre-acquisition services and related taxes (2) 590.5 Total consideration $ 7,193.4 (1) Represents cash consideration transferred of $172.50 per outstanding Reata common stock based on 38.3 million Reata shares outstanding at closing. (2) Represents the fair value of Reata stock options and stock units issued to Reata equity award holders and the related taxes attributable to pre-acquisition vesting services. (In millions) Amounts Recognized as of Acquisition Date Cash and cash equivalents $ 267.3 Accounts receivable 15.9 Inventory 1,259.0 Other current assets 53.6 Intangible assets: Completed technology for SKYCLARYS (U.S.) 4,200.0 In-process research and development (omaveloxolone) 2,300.0 Priority review voucher 100.0 Other clinical programs 40.0 Operating lease assets 121.2 Accrued expense and other (1) (106.4) Debt payable (159.9) Contingent payable to Blackstone (300.0) Deferred tax liability (1) (916.5) Operating lease liabilities (151.8) Other assets and liabilities, net (2.5) Total identifiable net assets 6,719.9 Goodwill (1) 473.5 Total assets acquired and liabilities assumed $ 7,193.4 (1) Includes measurement period adjustments recorded in the first quarter of 2024 that increased accrued expense and other by $4.9 million, deferred tax liability by $4.1 million and goodwill by $9.0 million.

Restructuring, Business Trans_2

Restructuring, Business Transformation and Other Cost Saving Initiatives (Tables)	6 Months Ended
	Jun. 30, 2024
Restructuring and Related Activities [Abstract]
Restructuring and Related Costs	Total charges incurred from our 2023 cost saving initiatives are summarized as follows: For the Three Months Ended June 30, 2024 2023 (In millions) Severance Accelerated Depreciation and Other Costs Total Severance Costs Accumulated Depreciation and Other Costs Total Selling, general and administrative $ — $ 2.0 $ 2.0 $ — $ 11.5 $ 11.5 Research and development — 2.2 2.2 — 0.5 0.5 Restructuring charges 6.3 — 6.3 17.8 16.5 34.3 Total charges $ 6.3 $ 4.2 $ 10.5 $ 17.8 $ 28.5 $ 46.3 For the Six Months Ended June 30, 2024 2023 (In millions) Severance Costs Accelerated Depreciation Total Severance Costs Accumulated Depreciation Total Selling, general and administrative $ — $ 3.4 $ 3.4 $ — $ 11.5 $ 11.5 Research and development — 7.1 7.1 — 0.5 0.5 Restructuring charges 15.6 — 15.6 24.9 16.5 41.4 Total charges $ 15.6 $ 10.5 $ 26.1 $ 24.9 $ 28.5 $ 53.4 Total charges incurred from our Reata integration are summarized as follows: For the Three Months Ended June 30, 2024 For the Six Months Ended June 30, 2024 (In millions) Severance Accelerated Depreciation and Other Costs Total Severance Costs Accumulated Depreciation and Other Costs Total Selling, general and administrative $ — $ 1.5 $ 1.5 $ — $ 3.3 $ 3.3 Research and development — 3.3 3.3 — 6.0 6.0 Restructuring charges 0.3 — 0.3 2.5 — 2.5 Total charges $ 0.3 $ 4.8 $ 5.1 $ 2.5 $ 9.3 $ 11.8
Schedule of Restructuring Reserve by Type of Cost	Charges and spending related to workforce reductions from our 2023 Fit for Growth program and Reata Integration are summarized as follows: Workforce Reductions (In millions) 2024 2023 Restructuring reserve as of January 1 $ 75.4 $ 35.9 Expense 11.5 7.1 Payment (42.2) (15.6) Foreign currency and other adjustments 0.8 0.6 Restructuring reserve as of March 31 45.5 28.0 Expense 6.6 17.8 Payment (11.9) (13.4) Foreign currency and other adjustments — (0.1) Restructuring reserve as of June 30 $ 40.2 $ 32.3

Revenues (Tables)

Revenues (Tables)	6 Months Ended
	Jun. 30, 2024
Revenue from Contract with Customer [Abstract]
Revenues by product	Revenue by product is summarized as follows: For the Three Months Ended June 30, 2024 2023 (In millions) United Rest of Total United Rest of Total Multiple Sclerosis: TECFIDERA $ 44.1 $ 208.1 $ 252.2 $ 66.5 $ 187.7 $ 254.2 VUMERITY 144.2 21.6 165.8 130.3 15.9 146.2 Total Fumarate 188.3 229.7 418.0 196.8 203.6 400.4 AVONEX 117.2 65.6 182.8 145.9 74.4 220.3 PLEGRIDY 28.2 39.9 68.1 34.1 48.0 82.1 Total Interferon 145.4 105.5 250.9 180.0 122.4 302.4 TYSABRI 248.7 213.5 462.2 259.9 223.2 483.1 FAMPYRA — 18.7 18.7 — 23.4 23.4 Subtotal: Multiple Sclerosis 582.4 567.4 1,149.8 636.7 572.6 1,209.3 Rare Disease: SPINRAZA 157.3 271.8 429.1 155.8 281.3 437.1 SKYCLARYS (1) 75.6 24.4 100.0 — — — QALSODY (2) 4.6 0.4 5.0 0.9 — 0.9 Subtotal: Rare Disease 237.5 296.6 534.1 156.7 281.3 438.0 Biosimilars: BENEPALI — 117.3 117.3 — 109.2 109.2 IMRALDI — 53.2 53.2 — 58.8 58.8 FLIXABI — 13.1 13.1 — 20.1 20.1 BYOOVIZ (3) 10.3 3.4 13.7 7.0 — 7.0 TOFIDENCE (4) 0.8 — 0.8 — — — Subtotal: Biosimilars 11.1 187.0 198.1 7.0 188.1 195.1 Other: ZURZUVAE (5) 14.9 — 14.9 — — — Other (6) 0.8 1.9 2.7 0.6 2.8 3.4 Subtotal: Other 15.7 1.9 17.6 0.6 2.8 3.4 Total product revenue, net $ 846.7 $ 1,052.9 $ 1,899.6 $ 801.0 $ 1,044.8 $ 1,845.8 (1) SKYCLARYS was obtained as part of our acquisition of Reata in September 2023. SKYCLARYS became commercially available in the U.S. during the second quarter of 2023 and we began recognizing revenue from SKYCLARYS in the U.S. during the fourth quarter of 2023, subsequent to our acquisition. SKYCLARYS was approved and became commercially available in the E.U. during the first quarter of 2024. (2) QALSODY became commercially available in the U.S. during the second quarter of 2023 and commercially available in the E.U. during the second quarter of 2024. (3) BYOOVIZ became commercially available in certain international markets in 2023. (4) TOFIDENCE became commercially available in the U.S. during the second quarter of 2024. (5) ZURZUVAE became commercially available in the U.S. during the fourth quarter of 2023. (6) Other includes FUMADERM and ADUHELM. For the Six Months Ended June 30, 2024 2023 (In millions) United Rest of Total United Rest of Total Multiple Sclerosis: TECFIDERA $ 87.8 $ 418.7 $ 506.5 $ 141.2 $ 387.5 $ 528.7 VUMERITY 250.1 43.2 293.3 223.8 30.6 254.4 Total Fumarate 337.9 461.9 799.8 365.0 418.1 783.1 AVONEX 228.4 132.9 361.3 248.5 144.2 392.7 PLEGRIDY 56.8 76.4 133.2 64.0 91.3 155.3 Total Interferon 285.2 209.3 494.5 312.5 235.5 548.0 TYSABRI 462.5 431.0 893.5 505.3 450.6 955.9 FAMPYRA — 37.9 37.9 — 47.5 47.5 Subtotal: Multiple Sclerosis 1,085.6 1,140.1 2,225.7 1,182.8 1,151.7 2,334.5 Rare Disease: SPINRAZA 305.8 464.6 770.4 302.5 577.9 880.4 SKYCLARYS (1) 148.6 29.4 178.0 — — — QALSODY (2) 9.0 0.6 9.6 0.9 — 0.9 Subtotal: Rare Disease 463.4 494.6 958.0 303.4 577.9 881.3 Biosimilars: BENEPALI — 236.0 236.0 — 218.2 218.2 IMRALDI — 108.0 108.0 — 113.2 113.2 FLIXABI — 30.9 30.9 — 40.5 40.5 BYOOVIZ (3) 14.0 5.3 19.3 15.2 0.4 15.6 TOFIDENCE (4) 0.8 — 0.8 — — — Subtotal: Biosimilars 14.8 380.2 395.0 15.2 372.3 387.5 Other: ZURZUVAE (5) 27.3 — 27.3 — — — Other (6) 1.7 3.8 5.5 1.0 4.8 5.8 Subtotal: Other 29.0 3.8 32.8 1.0 4.8 5.8 Total product revenue, net $ 1,592.8 $ 2,018.7 $ 3,611.5 $ 1,502.4 $ 2,106.7 $ 3,609.1 (1) SKYCLARYS was obtained as part of our acquisition of Reata in September 2023. SKYCLARYS became commercially available in the U.S. during the second quarter of 2023 and we began recognizing revenue from SKYCLARYS in the U.S. during the fourth quarter of 2023, subsequent to our acquisition. SKYCLARYS was approved and became commercially available in the E.U. during the first quarter of 2024. (2) QALSODY became commercially available in the U.S. during the second quarter of 2023 and commercially available in the E.U. during the second quarter of 2024. (3) BYOOVIZ became commercially available in certain international markets in 2023. (4) TOFIDENCE became commercially available in the U.S. during the second quarter of 2024. (5) ZURZUVAE became commercially available in the U.S. during the fourth quarter of 2023. (6) Other includes FUMADERM and ADUHELM.
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, 2023 $ 173.3 $ 857.1 $ 31.6 $ 1,062.0 Current provisions relating to sales in current year 399.3 1,355.2 10.6 1,765.1 Adjustments relating to prior years 5.6 (27.3) 12.7 (9.0) Payments/credits relating to sales in current year (260.1) (797.8) (0.3) (1,058.2) Payments/credits relating to sales in prior years (155.8) (477.0) (11.1) (643.9) Balance, June 30, 2024 $ 162.3 $ 910.2 $ 43.5 $ 1,116.0
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 June 30, 2024 As of December 31, 2023 Reduction of accounts receivable $ 151.5 $ 135.5 Component of accrued expense and other 964.5 926.5 Total revenue-related reserves $ 1,116.0 $ 1,062.0
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 June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Royalty revenue on sales of OCREVUS $ 336.3 $ 325.5 $ 639.0 $ 609.1 Biogen’s share of pre-tax profits in the U.S. for RITUXAN, GAZYVA and LUNSUMIO 103.4 103.6 190.5 216.1 Other revenue from anti-CD20 therapeutic programs 4.8 4.3 9.0 7.7 Total revenue from anti-CD20 therapeutic programs $ 444.5 $ 433.4 $ 838.5 $ 832.9
Other revenues	Contract manufacturing, royalty and other revenue is summarized in the table below. For the Three Months Ended June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Contract manufacturing revenue $ 100.0 $ 183.1 $ 252.2 $ 490.0 Royalty and other revenue 20.8 (6.3) 53.2 (13.0) Total contract manufacturing, royalty and other revenue $ 120.8 $ 176.8 $ 305.4 $ 477.0

Inventory (Tables)

Inventory (Tables)	6 Months Ended
	Jun. 30, 2024
Inventory Disclosure [Abstract]
Components of inventory	The components of inventory are summarized as follows: (In millions) As of June 30, 2024 As of December 31, 2023 Raw materials $ 401.2 $ 426.9 Work in process 1,821.2 1,926.8 Finished goods 395.0 255.4 Total inventory $ 2,617.4 $ 2,609.1 Balance Sheet Classification: Inventory $ 2,506.1 $ 2,527.4 Investments and other assets 111.3 81.7 Total inventory $ 2,617.4 $ 2,609.1

Intangible Assets and Goodwill

Intangible Assets and Goodwill (Tables)	6 Months Ended
	Jun. 30, 2024
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible assets	Intangible assets, net of accumulated amortization, impairment charges and adjustments are summarized as follows: As of June 30, 2024 As of December 31, 2023 (In millions) Estimated Life Cost Accumulated Net Cost Accumulated Net Completed technology: Acquired and in-licensed rights and patents 2-22 years $ 10,515.3 $ (2,595.9) $ 7,919.4 $ 8,180.2 $ (2,440.7) $ 5,739.5 Developed technology and other 13-31 years 3,548.6 (3,439.1) 109.5 3,548.6 (3,429.1) 119.5 Total completed technology 14,063.9 (6,035.0) 8,028.9 11,728.8 (5,869.8) 5,859.0 In-process research and development Indefinite until commercialization 40.0 — 40.0 2,340.0 — 2,340.0 Priority review voucher Indefinite 100.0 — 100.0 100.0 — 100.0 Trademarks and trade names Indefinite 64.0 — 64.0 64.0 — 64.0 Total intangible assets $ 14,267.9 $ (6,035.0) $ 8,232.9 $ 14,232.8 $ (5,869.8) $ 8,363.0
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 June 30, 2024 2024 (remaining six months) $ 185.0 2025 565.0 2026 660.0 2027 680.0 2028 710.0 2029 720.0
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 June 30, 2024 Goodwill, December 31, 2023 $ 6,219.2 Goodwill resulting from Reata acquisition 9.0 Other (0.8) Goodwill, June 30, 2024 $ 6,227.4

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2024
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: Fair Value Measurements on a Recurring Basis As of June 30, 2024 (In millions) Total Quoted Prices Significant Other Significant Assets: Cash equivalents $ 1,449.7 $ — $ 1,449.7 $ — Marketable equity securities 302.7 302.7 — — Other current assets: Derivative contracts 23.4 — 23.4 — Other non-current assets: Plan assets for deferred compensation 38.5 — 38.5 — Derivative contracts 1.8 — 1.8 — Total $ 1,816.1 $ 302.7 $ 1,513.4 $ — Liabilities: Derivative contracts $ 18.6 $ — $ 18.6 $ — Total $ 18.6 $ — $ 18.6 $ — Fair Value Measurements on a Recurring Basis As of December 31, 2023 (In millions) Total Quoted Prices Significant Other Significant Assets: Cash equivalents $ 610.7 $ — $ 610.7 $ — Marketable equity securities 416.8 416.8 — — Other current assets: Receivable from Samsung BioLogics (1) 430.0 — — 430.0 Derivative contracts 11.9 — 11.9 — Other non-current assets: Plan assets for deferred compensation 37.5 — 37.5 — Total $ 1,506.9 $ 416.8 $ 660.1 $ 430.0 Liabilities: Derivative contracts $ 31.6 $ — $ 31.6 $ — Total $ 31.6 $ — $ 31.6 $ — (1) Represents the fair value of the second deferred payment due from Samsung BioLogics as a result of the sale of our 49.9% equity interest in Samsung Bioepis to Samsung BioLogics during the second quarter of 2022, for which we elected the fair value option. For additional information on the sale of our equity interest in Samsung Bioepis, please read Note 3, Dispositions, to these condensed consolidated financial statements.
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 June 30, 2024 As of December 31, 2023 (In millions) Fair Carrying Fair Carrying Current portion: 2023 Term Loan 364-day tranche $ — $ — $ 150.0 $ 150.0 Current portion of notes payable and term loan — — 150.0 150.0 Non-current portion: 2023 Term Loan three-year tranche — — 500.0 500.0 4.050% Senior Notes due September 15, 2025 1,718.7 1,747.6 1,721.5 1,746.6 2.250% Senior Notes due May 1, 2030 1,275.9 1,494.3 1,279.3 1,493.8 5.200% Senior Notes due September 15, 2045 1,018.8 1,100.8 1,089.7 1,100.7 3.150% Senior Notes due May 1, 2050 969.3 1,474.7 1,049.0 1,474.3 3.250% Senior Notes due February 15, 2051 466.2 474.6 498.2 472.8 Non-current portion of notes payable and term loan 5,448.9 6,292.0 6,137.7 6,788.2 Total notes payable and term loan $ 5,448.9 $ 6,292.0 $ 6,287.7 $ 6,938.2 In connection with our acquisition of Reata we drew $1.0 billion from our 2023 Term Loan, comprised of a $500.0 million floating rate 364-day tranche and a $500.0 million floating rate three-year tranche. As of June 30, 2024, our 2023 Term Loan was repaid in full. For additional information on our 2023 Term Loan, please read Note 13, Indebtedness , to these condensed consolidated financial statements.

Financial Instruments (Tables)

Financial Instruments (Tables)	6 Months Ended
	Jun. 30, 2024
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 June 30, 2024 As of December 31, 2023 Money market funds $ 1,449.7 $ 610.7 Total $ 1,449.7 $ 610.7
Marketable debt and equity securities	The following tables summarize our marketable equity securities, classified as available-for-sale: As of June 30, 2024 (In millions) Amortized Gross Gross Fair Marketable equity securities Marketable equity securities, non-current $ 800.2 $ — $ (497.5) $ 302.7 Total marketable equity securities $ 800.2 $ — $ (497.5) $ 302.7 As of December 31, 2023 (In millions) Amortized Gross Gross Fair Marketable equity securities Marketable equity securities, current $ 31.6 $ — $ (21.0) $ 10.6 Marketable equity securities, non-current 948.3 — (542.1) 406.2 Total marketable equity securities $ 979.9 $ — $ (563.1) $ 416.8
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: (In millions) For the Three Months Ended June 30, 2023 For the Six Months Ended June 30, 2023 Proceeds from maturities and sales $ 1,275.5 $ 1,682.2 Realized gains 0.3 0.4 Realized losses 1.4 2.1

Derivative Instruments (Tables)

Derivative Instruments (Tables)	6 Months Ended
	Jun. 30, 2024
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 and foreign currency options that were entered into to hedge forecasted revenue and operating expense is summarized as follows: Notional Amount (In millions) As of June 30, 2024 As of December 31, 2023 Euro $ 1,420.3 $ 1,169.0 British pound 69.6 — Swiss franc 142.4 — Canadian dollar 26.2 — Total foreign currency forward contracts and options $ 1,658.5 $ 1,169.0
Summary of the effect of cash flow derivatives designated as hedging instruments on the condensed consolidated statements of income	The following tables summarize the effect of foreign currency forward contracts and forward currency options designated as hedging instruments in our condensed consolidated statements of income: For the Three Months Ended June 30, Net Gains/(Losses) Net Gains/(Losses) Excluded from Effectiveness Testing and Location 2024 2023 Location 2024 2023 Revenue $ 7.6 $ 2.4 Revenue $ 0.9 $ (1.0) Operating expense (4.9) 1.4 Operating expense (0.7) (0.8) For the Six Months Ended June 30, Net Gains/(Losses) Net Gains/(Losses) Excluded from Effectiveness Testing and Location 2024 2023 Location 2024 2023 Revenue $ 10.5 $ 20.0 Revenue $ 1.1 $ 0.6 Operating expense (7.0) 0.9 Operating expense (1.9) (2.9)
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 June 30, 2024 As of December 31, 2023 Cash Flow Hedging Instruments: Asset derivative instruments Other current assets $ 17.6 $ 0.3 Investments and other assets 1.8 — Liability derivative instruments Accrued expense and other 8.3 26.5 Other Derivative Instruments: Asset derivative instruments Other current assets 5.8 11.6 Liability derivative instruments Accrued expense and other 10.3 5.1
Derivative Instruments, Unrealized Gain (Loss)	The pre-tax portion of the fair value of these foreign currency forward contracts and foreign currency options that were included in AOCI in total equity is summarized as follows: (In millions) As of June 30, 2024 As of December 31, 2023 Unrealized gains $ 12.9 $ — Unrealized (losses) (9.5) (34.8) Net unrealized gains (losses) $ 3.4 $ (34.8)

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss) (Tables)	6 Months Ended
	Jun. 30, 2024
Equity [Abstract]
Schedule of Accumulated Other Comprehensive Income (Loss)	The following tables summarize the changes in AOCI, net of tax by component: For the Three Months Ended June 30, 2024 (In millions) Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Unrealized Gains (Losses) on Pension Benefit Obligation, Net of Tax Currency Total Balance, March 31, 2024 $ (10.3) $ (2.7) $ (147.5) $ (160.5) Other comprehensive income (loss) before reclassifications 22.3 — (5.6) 16.7 Amounts reclassified from AOCI (2.2) — — (2.2) Net current period other comprehensive income (loss) 20.1 — (5.6) 14.5 Balance, June 30, 2024 $ 9.8 $ (2.7) $ (153.1) $ (146.0) For the Six Months Ended June 30, 2024 Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Unrealized Gains (Losses) on Pension Benefit Obligation, Net of Tax Currency Total Balance, December 31, 2023 $ (25.0) $ (2.6) $ (126.1) $ (153.7) Other comprehensive income (loss) before reclassifications 37.7 (0.1) (27.0) 10.6 Amounts reclassified from AOCI (2.9) — — (2.9) Net current period other comprehensive income (loss) 34.8 (0.1) (27.0) 7.7 Balance, June 30, 2024 $ 9.8 $ (2.7) $ (153.1) $ (146.0) For the Three Months Ended June 30, 2023 (In millions) Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Unrealized Gains (Losses) on Pension Benefit Obligation, Net of Tax Currency Translation Adjustments Total Balance, March 31, 2023 $ (10.0) $ (20.3) $ (0.6) $ (141.1) $ (172.0) Other comprehensive income (loss) before reclassifications (5.5) 10.2 0.2 (3.4) 1.5 Amounts reclassified from AOCI 1.0 (3.3) — — (2.3) Net current period other comprehensive income (loss) (4.5) 6.9 0.2 (3.4) (0.8) Balance, June 30, 2023 $ (14.5) $ (13.4) $ (0.4) $ (144.5) $ (172.8) For the Six Months Ended June 30, 2023 (In millions) Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Unrealized Gains (Losses) on Pension Benefit Obligation, Net of Tax Currency Translation Adjustments Total Balance, December 31, 2022 $ (15.7) $ 15.1 $ (1.1) $ (163.2) $ (164.9) Other comprehensive income (loss) before reclassifications (0.2) (10.1) 0.7 18.7 9.1 Amounts reclassified from AOCI 1.4 (18.4) — — (17.0) Net current period other comprehensive income (loss) 1.2 (28.5) 0.7 18.7 (7.9) Balance, June 30, 2023 $ (14.5) $ (13.4) $ (0.4) $ (144.5) $ (172.8)
Reclassification out of Accumulated Other Comprehensive Income	The following table summarizes the amounts reclassified from AOCI: (In millions) Amounts Reclassified from AOCI Income Statement Location For the Three Months Ended June 30, For the Six Months Ended June 30, 2024 2023 2024 2023 Gains (losses) on securities available for sale $ — $ (1.3) $ — $ (1.8) Other (income) expense — 0.3 — 0.4 Income tax (benefit) expense Gains (losses) on cash flow hedges 7.6 2.4 10.5 20.0 Revenue (4.9) 1.4 (7.0) 0.9 Operating expense (0.2) (0.1) (0.1) (0.2) Other (income) expense (0.3) (0.4) (0.5) (2.3) Income tax (benefit) expense Total reclassifications, net of tax $ 2.2 $ 2.3 $ 2.9 $ 17.0

Earnings per Share (Tables)

Earnings per Share (Tables)	6 Months Ended
	Jun. 30, 2024
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 June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Numerator: Net income attributable to Biogen Inc. $ 583.6 $ 591.6 $ 977.0 $ 979.5 Denominator: Weighted average number of common shares outstanding 145.6 144.7 145.4 144.6 Effect of dilutive securities: Time-vested restricted stock units 0.2 0.7 0.4 0.7 Performance stock units settled in stock 0.1 0.1 0.1 0.1 Dilutive potential common shares 0.3 0.8 0.5 0.8 Shares used in calculating diluted earnings per share 145.9 145.5 145.9 145.4

Share-Based Payments (Tables)

Share-Based Payments (Tables)	6 Months Ended
	Jun. 30, 2024
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 June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Research and development $ 27.3 $ 27.9 $ 54.7 $ 59.2 Selling, general and administrative 45.6 48.4 89.6 98.5 Subtotal 72.9 76.3 144.3 157.7 Capitalized share-based compensation costs (2.3) (2.6) (5.6) (5.9) Share-based compensation expense included in total cost and expense 70.6 73.7 138.7 151.8 Income tax effect (13.4) (13.3) (26.3) (28.0) Share-based compensation expense included in net income attributable to Biogen Inc. $ 57.2 $ 60.4 $ 112.4 $ 123.8
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 June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Time-vested restricted stock units $ 58.3 $ 59.9 $ 115.6 $ 121.6 Performance stock units settled in stock 11.7 9.8 22.2 19.3 Employee stock purchase plan 1.8 2.1 6.3 6.8 Performance stock units settled in cash 0.1 2.2 (2.0) 4.7 Stock options 0.9 1.0 1.8 1.8 Market stock units 0.1 1.3 0.4 3.5 Subtotal 72.9 76.3 144.3 157.7 Capitalized share-based compensation costs (2.3) (2.6) (5.6) (5.9) Share-based compensation expense included in total cost and expense $ 70.6 $ 73.7 $ 138.7 $ 151.8

Income Taxes (Tables)

Income Taxes (Tables)	6 Months Ended
	Jun. 30, 2024
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 June 30, For the Six Months Ended June 30, 2024 2023 2024 2023 Statutory rate 21.0 % 21.0 % 21.0 % 21.0 % State taxes 1.4 1.2 1.4 1.2 Taxes on foreign earnings (6.2) (4.7) (5.2) (5.0) Tax credits (1.6) (2.0) (2.0) (3.7) Purchased inventory valuation step-up and intangible assets 1.6 0.3 1.5 0.3 GILTI (0.9) 0.2 (1.6) 0.3 Other, including permanent items 1.2 0.2 0.9 0.3 Effective tax rate 16.5 % 16.2 % 16.0 % 14.4 %

Other Consolidated Financial _2

Other Consolidated Financial Statement Detail (Tables)	6 Months Ended
	Jun. 30, 2024
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 June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Interest income $ (20.1) $ (75.9) $ (37.5) $ (156.8) Interest expense 61.3 49.1 130.5 111.6 (Gains) losses on investments, net 30.7 (105.4) 61.3 (27.7) Foreign exchange (gains) losses, net 11.3 8.7 20.7 19.4 Other, net 2.0 2.3 3.9 1.7 Total other (income) expense, net $ 85.2 $ (121.2) $ 178.9 $ (51.8)
Gain (loss) on investments in equity securities	The following table summarizes our (gains) losses on investments, net that relate to our equity securities held during the following periods: For the Three Months Ended June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Net (gains) losses recognized on equity securities $ 30.4 $ (106.5) $ 61.0 $ (28.4) Less: Net (gains) losses realized on equity securities 0.6 (0.7) 5.5 0.9 Net unrealized (gains) losses recognized on equity securities $ 29.8 $ (105.8) $ 55.5 $ (29.3) The net unrealized losses recognized during the three months ended June 30, 2024, primarily reflect a decrease in the aggregate fair value of our investments in Sage and Sangamo common stock of approximately $51.4 million, partially offset by an increase in the fair value of Denali common stock of approximately $27.0 million. The net unrealized gains recognized during the three months ended June 30, 2023, primarily reflect an increase in the aggregate fair value of our investments in Denali and Sage common stock of approximately $117.7 million, partially offset by a decrease in the fair value of Sangamo common stock of approximately $10.9 million. The net unrealized losses recognized during the six months ended June 30, 2024, primarily reflect a decrease in the aggregate fair value of our investments in Sage and Sangamo common stock of approximately $68.3 million, partially offset by an increase in the fair value of Denali common stock of approximately $17.6 million. The net unrealized gains recognized during the six months ended June 30, 2023, primarily reflect an increase in the aggregate fair value of our investments in Denali and Sage common stock of approximately $78.0 million, partially offset by a decrease in the fair value of Sangamo and Ionis common stock of approximately $47.4 million.
Accrued expenses and other	Accrued expense and other consists of the following: (In millions) As of June 30, 2024 As of December 31, 2023 Revenue-related reserves for discounts and allowances $ 964.5 $ 926.5 Employee compensation and benefits 228.3 335.1 Collaboration expense 319.4 214.6 Royalties and licensing fees 193.0 191.5 Reata related accrued expense 77.4 117.5 Other 689.5 838.4 Total accrued expense and other $ 2,472.1 $ 2,623.6

Collaborative and Other Relat_2

Collaborative and Other Relationships (Tables)	6 Months Ended
	Jun. 30, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Co-promotion profit sharing formula	As a result of the FDA approval of LUNSUMIO our share of the combined annual co-promotion profits for RITUXAN and LUNSUMIO in excess of $50.0 million varies upon the following events, as summarized in the table below: After LUNSUMIO Approval until the First Threshold Date 37.5 % After First Threshold Date until the Second Threshold Date 35.0 % After Second Threshold Date 30.0 %
Pretax profit sharing formula	Our share of annual co-promotion profits in excess of $50.0 million varies upon the following events, as summarized in the table below: Until Second GAZYVA Threshold Date 37.5 % After Second GAZYVA Threshold Date 35.0 %
Summary of activity related to the UCB collaboration	A summary of development expense related to the UCB collaboration agreement is as follows: For the Three Months Ended June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Total UCB collaboration development expense $ 16.5 $ 14.3 $ 33.1 $ 32.6 Biogen's share of UCB collaboration development expense reflected in research and development expense in our condensed consolidated statements of income 8.3 7.1 16.6 16.3
Summary of Activity Related to Denali Therapeutics Collaboration	A summary of development expense related to the Denali collaboration is as follows: For the Three Months Ended June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Total Denali collaboration development expense $ 15.4 $ 22.1 $ 29.6 $ 38.7 Biogen's share of Denali collaboration development expense reflected in research and development expense in our condensed consolidated statements of income 9.3 13.2 17.8 23.2
Summary of Activity Related to BAN2401 and Elenbecestat Collaboration	A summary of development and sales and marketing expense related to the LEQEMBI Collaboration is as follows: For the Three Months Ended June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Total development expense incurred by the collaboration related to the advancement of LEQEMBI $ 90.6 $ 86.2 $ 177.9 $ 194.1 Biogen's share of the LEQEMBI Collaboration development expense reflected in research and development expense in our condensed consolidated statements of income 45.4 43.0 89.0 97.0 Total sales and marketing expense incurred by the LEQEMBI Collaboration 137.7 17.1 311.6 27.6 Biogen's share of the LEQEMBI Collaboration sales and marketing expense reflected in selling, general and administrative expense in our condensed consolidated statements of income 68.9 8.5 155.8 13.8
Summary of Activity Related to Sage Therapeutics	A summary of development and sales and marketing expense related to the Sage collaboration is as follows: For the Three Months Ended June 30, For the Six Months Ended June 30, (In millions) 2024 2023 2024 2023 Total Sage collaboration development expense $ 9.6 $ 52.1 $ 21.2 $ 86.9 Biogen's share of Sage collaboration development expense reflected in research and development expense in our condensed consolidated statements of income 4.8 26.1 10.6 43.5 Total sales and marketing expense incurred by the Sage collaboration 26.9 60.1 54.5 98.3 Biogen's share of Sage collaboration sales and marketing expense reflected in selling, general and administrative expense and collaboration profit sharing/(loss reimbursement) in our condensed consolidated statements of income 13.5 30.0 27.3 49.1

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Details)	6 Months Ended
	Jun. 30, 2024 segment
Accounting Policies [Abstract]
Number of reportable segments	1
Interest in subsidiary (less than given percentage)	100%

Acquisitions - Narrative (Detai

Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions			3 Months Ended			6 Months Ended		12 Months Ended
	Jul. 02, 2024 USD ($)	Sep. 26, 2023 USD ($) ft² $ / shares	Sep. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Business Acquisition [Line Items]
Selling, general and administrative				$ 553.8	$ 548	$ 1,135.3	$ 1,153
Goodwill				6,227.4		6,227.4		$ 6,219.2
Net income				(583.6)	(593.3)	(977)	(980.9)
Cost of sales, excluding amortization and impairment of acquired intangible assets				546	$ 592.7	1,088.2	$ 1,255.5
Forecast
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross, Outstanding Equity Awards			$ 983.9
Reata Pharmaceuticals, Inc
Business Acquisition [Line Items]
Price per share | $ / shares		$ 172.50
Consideration transferred		$ 7,193.4
Total transaction value		6,602.9
Selling, general and administrative		196.4
Research and development expense asset acquired		197
Operating expenses		393.4
Inventory		1,300		1,259		1,259
Operating lease liabilities		151.8		151.8		151.8
Operating lease assets		$ 121.2		121.2		121.2
Lease term		16 years
Operating Lease Area | ft²		327,400
Lessee, Operating Lease, Remaining Lease Term		15 years
Fair Value Indefinite Lived Intangible Assets Discount Rate		14.30%
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory Amortization Period		3 years
Goodwill		$ 473.5		473.5		473.5
Business Combination, Consideration Transferred, Equity Compensation Pre-acquisition Services		590.5
Reata Pharmaceuticals, Inc | Fair Value Adjustment to Inventory
Business Acquisition [Line Items]
Net income				90.1		134.2
Reata Pharmaceuticals, Inc | Clinical Inventory | Fair Value Adjustment to Inventory
Business Acquisition [Line Items]
Cost of sales, excluding amortization and impairment of acquired intangible assets				46
Reata Pharmaceuticals, Inc | Secured Debt | The Credit Facility | Line of Credit
Business Acquisition [Line Items]
Long-term debt, gross		1,000
Reata Pharmaceuticals, Inc | Selling, general and administrative
Business Acquisition [Line Items]
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Cost Expensed								$ 28.4
Reata Pharmaceuticals, Inc | Completed technology for SKYCLARYS (U.S.)
Business Acquisition [Line Items]
Intangible assets:		4,200		4,200		4,200
Reata Pharmaceuticals, Inc | In-process research and development
Business Acquisition [Line Items]
Intangible assets:		2,300		2,300		2,300
Reata Pharmaceuticals, Inc | Priority review voucher
Business Acquisition [Line Items]
Intangible assets:		100		100		100
Reata Pharmaceuticals, Inc | Other clinical programs
Business Acquisition [Line Items]
Intangible assets:		$ 40		$ 40		$ 40
Human Immunology Biosciences | Subsequent Event
Business Acquisition [Line Items]
Consideration transferred	$ 1,150
Contingent consideration obligations	$ 650

Acquisitions - Purchase Price C

Acquisitions - Purchase Price Consideration (Details) - Reata Pharmaceuticals, Inc $ / shares in Units, shares in Millions, $ in Millions	Sep. 26, 2023 USD ($) $ / shares shares
Business Acquisition [Line Items]
Total transaction value	$ 6,602.9
Business Combination, Consideration Transferred, Equity Compensation Pre-acquisition Services	590.5
Consideration transferred	$ 7,193.4
Price per share | $ / shares	$ 172.50
Number of common stock shares acquired | shares	38.3

Acquisitions Preliminaty Purcha

Acquisitions Preliminaty Purchase Price Allocation (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 26, 2023
Business Acquisition [Line Items]
Goodwill		$ 6,227.4	$ 6,219.2
Reata Pharmaceuticals, Inc
Business Acquisition [Line Items]
Cash and cash equivalents		267.3
Accounts receivable		15.9
Inventory		1,259		$ 1,300
Other current assets		53.6
Operating lease assets		121.2		121.2
Accrued expense and other(1)		(106.4)
Debt payable		(159.9)
Contingent payable to Blackstone		(300)
Deferred tax liabilities		(916.5)
Operating lease liabilities		(151.8)		(151.8)
Other assets and liabilities, net		(2.5)
Total identifiable net assets		6,719.9
Goodwill		473.5		473.5
Total assets acquired and liabilities assumed		7,193.4
Adjustment, accrued expense and other	$ 4.9
Deferred tax liability	4.1
Adjustment, goodwill	$ 9
Reata Pharmaceuticals, Inc | Completed technology for SKYCLARYS (U.S.)
Business Acquisition [Line Items]
Intangible assets:		4,200		4,200
Reata Pharmaceuticals, Inc | In-process research and development
Business Acquisition [Line Items]
Intangible assets:		2,300		2,300
Reata Pharmaceuticals, Inc | Priority review voucher
Business Acquisition [Line Items]
Intangible assets:		100		100
Reata Pharmaceuticals, Inc | Other clinical programs
Business Acquisition [Line Items]
Intangible assets:		$ 40		$ 40

Dispositions (Details Textual)

Dispositions (Details Textual) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		6 Months Ended
	Apr. 30, 2024	Apr. 30, 2022	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Mar. 31, 2022
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accounts payable			$ 354.5		$ 354.5		$ 403.3
Gain on sale of priority review voucher, net			$ 88.6	$ 0	$ 88.6	$ 0
Ionis
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of intangible assets	$ 103
Accounts payable	$ 14.4
Samsung Bioepis
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity, ownership interest			49.90%		49.90%			49.90%
Equity Method Investment, Amount Sold		$ 2,300
Samsung Bioepis
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from divestiture of interest in joint venture		1,000
Payables to divestiture of interest in joint venture		1,300
Samsung Bioepis | Payment Due At First Anniversary
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payables to divestiture of interest in joint venture		812.5
Gain (loss) on sale				2.6		13.7
Samsung Bioepis | Payment Due At Second Anniversary
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payables to divestiture of interest in joint venture		$ 437.5
Gain (loss) on sale			$ 1.4	$ 3.6	$ 7.5	$ 9.8

Restructuring, Business Trans_3

Restructuring, Business Transformation and Other Cost Saving Initiatives - FIt for Growth Initiative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 6.6	$ 34.4	$ 18.1	$ 44
2023 Cost Saving Initiatives
Restructuring Cost and Reserve [Line Items]
Severance Costs	6.3	17.8	15.6	24.9
Accelerated Depreciation and Other Costs	4.2	28.5	10.5	28.5
Restructuring charges	10.5	46.3	26.1	53.4
Reata Integration
Restructuring Cost and Reserve [Line Items]
Severance Costs	0.3		2.5
Accelerated Depreciation and Other Costs	4.8		9.3
Restructuring charges	5.1		11.8
Selling, general and administrative | 2023 Cost Saving Initiatives
Restructuring Cost and Reserve [Line Items]
Severance Costs	0	0	0	0
Accelerated Depreciation and Other Costs	2	11.5	3.4	11.5
Restructuring charges	2	11.5	3.4	11.5
Selling, general and administrative | Reata Integration
Restructuring Cost and Reserve [Line Items]
Severance Costs	0		0
Accelerated Depreciation and Other Costs	1.5		3.3
Restructuring charges	1.5		3.3
Research and development | 2023 Cost Saving Initiatives
Restructuring Cost and Reserve [Line Items]
Severance Costs	0	0	0	0
Accelerated Depreciation and Other Costs	2.2	0.5	7.1	0.5
Restructuring charges	2.2	0.5	7.1	0.5
Research and development | Reata Integration
Restructuring Cost and Reserve [Line Items]
Severance Costs	0		0
Accelerated Depreciation and Other Costs	3.3		6
Restructuring charges	3.3		6
Restructuring charges | 2023 Cost Saving Initiatives
Restructuring Cost and Reserve [Line Items]
Severance Costs	6.3	17.8	15.6	24.9
Accelerated Depreciation and Other Costs	0	16.5	0	16.5
Restructuring charges	6.3	$ 34.3	15.6	$ 41.4
Restructuring charges | Reata Integration
Restructuring Cost and Reserve [Line Items]
Severance Costs	0.3		2.5
Accelerated Depreciation and Other Costs	0		0
Restructuring charges	$ 0.3		$ 2.5

Restructuring, Business Trans_4

Restructuring, Business Transformation and Other Cost Saving Initiatives - Narrative (Details) $ in Millions		3 Months Ended		6 Months Ended
	Sep. 26, 2023 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) Employee	Jun. 30, 2023 USD ($)	Jul. 31, 2023 USD ($)
Minimum
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Expected Cost Remaining						$ 260
Maximum
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Expected Cost Remaining						$ 280
2023 Cost Saving Initiatives
Restructuring Cost and Reserve [Line Items]
Expected operating expense reduction	$ 1,000
Expected number of positions eliminated | Employee				1,000
Severance Costs		$ 6.3	$ 17.8	$ 15.6	$ 24.9
2023 Cost Saving Initiatives | Restructuring charges
Restructuring Cost and Reserve [Line Items]
Severance Costs		6.3	$ 17.8	15.6	$ 24.9
Reata Integration
Restructuring Cost and Reserve [Line Items]
Severance Costs		0.3		2.5
Reata Integration | Minimum
Restructuring Cost and Reserve [Line Items]
Expected cost		35		35
Reata Integration | Maximum
Restructuring Cost and Reserve [Line Items]
Expected cost		40		40
Reata Integration | Restructuring charges
Restructuring Cost and Reserve [Line Items]
Severance Costs		$ 0.3		$ 2.5

Restructuring, Business Trans_5

Restructuring, Business Transformation and Other Cost Saving Initiatives - Total Charges cost saving initiative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 6.6	$ 34.4	$ 18.1	$ 44

Restructuring, Business Trans_6

Restructuring, Business Transformation and Other Cost Saving Initiatives - Restructuring Reserve Roll Forward (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023
Restructuring Cost and Reserve [Line Items]
Expense	$ 6.6		$ 34.4		$ 18.1	$ 44
2023 Cost Saving Initiatives
Restructuring Cost and Reserve [Line Items]
Expense	10.5		46.3		26.1	53.4
Severance Costs | 2023 and 2022 Cost Saving Initiatives
Restructuring Cost and Reserve [Line Items]
Restructuring reserve, beginning	45.5	$ 75.4	28	$ 35.9	75.4	35.9
Expense	6.6	11.5	17.8	7.1
Payment	(11.9)	(42.2)	(13.4)	(15.6)
Foreign currency and other adjustments	0	0.8	(0.1)	0.6
Restructuring reserve, ending	$ 40.2	$ 45.5	$ 32.3	$ 28	$ 40.2	$ 32.3

Revenues - Revenues by product

Revenues - Revenues by product (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation of Revenue [Line Items]
Product revenues	$ 2,464.9	$ 2,456	$ 4,755.4	$ 4,919
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	1,899.6	1,845.8	3,611.5	3,609.1
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	846.7	801	1,592.8	1,502.4
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	1,052.9	1,044.8	2,018.7	2,106.7
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	1,149.8	1,209.3	2,225.7	2,334.5
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	582.4	636.7	1,085.6	1,182.8
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	567.4	572.6	1,140.1	1,151.7
Fumarate
Disaggregation of Revenue [Line Items]
Product revenues	418	400.4	799.8	783.1
Fumarate | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	188.3	196.8	337.9	365
Fumarate | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	229.7	203.6	461.9	418.1
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	252.2	254.2	506.5	528.7
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	44.1	66.5	87.8	141.2
TECFIDERA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	208.1	187.7	418.7	387.5
VUMERITY
Disaggregation of Revenue [Line Items]
Product revenues	165.8	146.2	293.3	254.4
VUMERITY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	144.2	130.3	250.1	223.8
VUMERITY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	21.6	15.9	43.2	30.6
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	250.9	302.4	494.5	548
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	145.4	180	285.2	312.5
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	105.5	122.4	209.3	235.5
AVONEX
Disaggregation of Revenue [Line Items]
Product revenues	182.8	220.3	361.3	392.7
AVONEX | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	117.2	145.9	228.4	248.5
AVONEX | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	65.6	74.4	132.9	144.2
PLEGRIDY
Disaggregation of Revenue [Line Items]
Product revenues	68.1	82.1	133.2	155.3
PLEGRIDY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	28.2	34.1	56.8	64
PLEGRIDY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	39.9	48	76.4	91.3
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	462.2	483.1	893.5	955.9
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	248.7	259.9	462.5	505.3
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	213.5	223.2	431	450.6
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	18.7	23.4	37.9	47.5
FAMPYRA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0	0	0
FAMPYRA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	18.7	23.4	37.9	47.5
Rare Disease Product Revenue
Disaggregation of Revenue [Line Items]
Product revenues	534.1	438	958	881.3
Rare Disease Product Revenue | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	237.5	156.7	463.4	303.4
Rare Disease Product Revenue | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	296.6	281.3	494.6	577.9
QALSODY
Disaggregation of Revenue [Line Items]
Product revenues	5		9.6	0.9
QALSODY | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	4.6		9	0.9
QALSODY | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0.4		0.6	0
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	429.1	437.1	770.4	880.4
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	157.3	155.8	305.8	302.5
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	271.8	281.3	464.6	577.9
Completed technology for SKYCLARYS (U.S.)
Disaggregation of Revenue [Line Items]
Product revenues	100		178	0
Completed technology for SKYCLARYS (U.S.) | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	75.6		148.6	0
Completed technology for SKYCLARYS (U.S.) | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	24.4		29.4	0
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	198.1	195.1	395	387.5
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	11.1	7	14.8	15.2
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	187	188.1	380.2	372.3
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	117.3	109.2	236	218.2
BENEPALI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0	0	0
BENEPALI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	117.3	109.2	236	218.2
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	53.2	58.8	108	113.2
IMRALDI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0	0	0
IMRALDI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	53.2	58.8	108	113.2
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	13.1	20.1	30.9	40.5
FLIXABI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0	0	0
FLIXABI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	13.1	20.1	30.9	40.5
BYOOVIZ
Disaggregation of Revenue [Line Items]
Product revenues	13.7	7	19.3	15.6
BYOOVIZ | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	10.3	7	14	15.2
BYOOVIZ | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	3.4	0	5.3	0.4
TOFIDENCE
Disaggregation of Revenue [Line Items]
Product revenues	0.8	0	0.8	0
TOFIDENCE | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.8	0	0.8	0
TOFIDENCE | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0	0	0	0
Other
Disaggregation of Revenue [Line Items]
Product revenues	17.6	3.4	32.8	5.8
Other | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	15.7	0.6	29	1
Other | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	1.9	2.8	3.8	4.8
ZURZUVAE
Disaggregation of Revenue [Line Items]
Product revenues	14.9	0	27.3	0
ZURZUVAE | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	14.9	0	27.3	0
ZURZUVAE | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	0	0	0	0
FUMADERM And ADUHELM
Disaggregation of Revenue [Line Items]
Product revenues	2.7	3.4	5.5	5.8
FUMADERM And ADUHELM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.8	0.6	1.7	1
FUMADERM And ADUHELM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1.9	$ 2.8	$ 3.8	$ 4.8

Revenues Reserves for Discounts

Revenues Reserves for Discounts and Allowances (Details 1) $ in Millions	6 Months Ended
	Jun. 30, 2024 USD ($)
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	$ 1,062
Current Provisions Relating To Sales In Current Year	1,765.1
Adjustments Relating To Prior Years	(9)
Payments/Returns Relating To Sales in Current Year	(1,058.2)
Payments/Returns Relating To Sales in Prior Year	(643.9)
Ending Balance	1,116
Discounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	173.3
Current Provisions Relating To Sales In Current Year	399.3
Adjustments Relating To Prior Years	5.6
Payments/Returns Relating To Sales in Current Year	(260.1)
Payments/Returns Relating To Sales in Prior Year	(155.8)
Ending Balance	162.3
Contractual adjustments
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	857.1
Current Provisions Relating To Sales In Current Year	1,355.2
Adjustments Relating To Prior Years	(27.3)
Payments/Returns Relating To Sales in Current Year	(797.8)
Payments/Returns Relating To Sales in Prior Year	(477)
Ending Balance	910.2
Returns
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	31.6
Current Provisions Relating To Sales In Current Year	10.6
Adjustments Relating To Prior Years	12.7
Payments/Returns Relating To Sales in Current Year	(0.3)
Payments/Returns Relating To Sales in Prior Year	(11.1)
Ending Balance	$ 43.5

Revenues- Reserves for Discount

Revenues- Reserves for Discounts and Allowances (Details 2) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Total Reserves	$ 1,116	$ 1,062
Reduction of accounts receivable
Total Reserves	151.5	135.5
Component of accrued expenses and other
Total Reserves	$ 964.5	$ 926.5

Revenues - Revenues from Anti-C

Revenues - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 2,464.9	$ 2,456	$ 4,755.4	$ 4,919
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	103.4	103.6	190.5	216.1
Other revenues from anti-CD20 therapeutic programs	4.8	4.3	9	7.7
Revenue from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	444.5	433.4	838.5	832.9
Royalty Attributed To OCREVUS
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 336.3	$ 325.5	$ 639	$ 609.1

Revenues - Other Revenues (Deta

Revenues - Other Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,464.9	$ 2,456	$ 4,755.4	$ 4,919
Contract manufacturing revenue
Disaggregation of Revenue [Line Items]
Total revenue	100	183.1	252.2	490
Royalty and other revenue
Disaggregation of Revenue [Line Items]
Total revenue	20.8	(6.3)	53.2	(13)
Contract manufacturing, royalty and other revenue
Disaggregation of Revenue [Line Items]
Total revenue	$ 120.8	$ 176.8	$ 305.4	$ 477

Revenues - Narrative (Details)

Revenues - Narrative (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024 USD ($) wholesaler	Jun. 30, 2023 USD ($) wholesaler	Jun. 30, 2024 USD ($) wholesaler	Jun. 30, 2023 USD ($) wholesaler
Disaggregation of Revenue [Line Items]
Number of wholesalers | wholesaler	2	2	2	2
Product revenues	$ 2,464.9	$ 2,456	$ 4,755.4	$ 4,919
Distributor One
Disaggregation of Revenue [Line Items]
Percentage of revenues from major distributors	25.40%	27%	25.50%	27.20%
Distributor Two
Disaggregation of Revenue [Line Items]
Percentage of revenues from major distributors	12.30%	9.30%	12%	8.40%
Contract manufacturing, royalty and other revenue
Disaggregation of Revenue [Line Items]
Product revenues	$ 120.8	$ 176.8	$ 305.4	$ 477
LEQEMBI Collaboration
Disaggregation of Revenue [Line Items]
Share of non-controling interest recognized	50%		50%

Inventory - Components of Inven

Inventory - Components of Inventory (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Inventory Disclosure [Abstract]
Raw materials	$ 401.2	$ 426.9
Work in process	1,821.2	1,926.8
Finished goods	395	255.4
Inventory	2,617.4	2,609.1
Inventory, current	2,506.1	2,527.4
Inventory, noncurrent	$ 111.3	$ 81.7

Inventory - Narrative (Details)

Inventory - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Sep. 26, 2023
Inventory [Line Items]
Net income	$ 583.6	$ 593.3	$ 977	$ 980.9
Cost of sales, excluding amortization and impairment of acquired intangible assets	546	$ 592.7	1,088.2	$ 1,255.5
Reata Pharmaceuticals, Inc
Inventory [Line Items]
Inventory	1,259		1,259		$ 1,300
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory Amortization Period					3 years
Reata Pharmaceuticals, Inc | Fair Value Adjustment to Inventory
Inventory [Line Items]
Net income	$ (90.1)		$ (134.2)

Intangible Assets and Goodwil_2

Intangible Assets and Goodwill - Summary of Intangible Assets (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Finite-Lived Intangible Assets, Net [Abstract]
Finite-Lived Intangible Assets, Gross	$ 14,063.9	$ 11,728.8
Accumulated Amortization	(6,035)	(5,869.8)
Net	8,028.9	5,859
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Cost	14,267.9	14,232.8
Accumulated Amortization	6,035	5,869.8
Net	8,232.9	8,363
In-process research and development
Indefinite-Lived Intangible Assets [Line Items]
Cost and net	40	2,340
Priority review voucher
Indefinite-Lived Intangible Assets [Line Items]
Cost and net	100	100
Trademarks and trade names
Indefinite-Lived Intangible Assets [Line Items]
Cost and net	64	64
Acquired and in-licensed rights and patents
Finite-Lived Intangible Assets, Net [Abstract]
Finite-Lived Intangible Assets, Gross	10,515.3	8,180.2
Accumulated Amortization	(2,595.9)	(2,440.7)
Net	7,919.4	5,739.5
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Accumulated Amortization	$ 2,595.9	2,440.7
Acquired and in-licensed rights and patents | Minimum
Finite-Lived Intangible Assets [Line Items]
Estimated life (in years)	2 years
Acquired and in-licensed rights and patents | Maximum
Finite-Lived Intangible Assets [Line Items]
Estimated life (in years)	22 years
Developed technology and other
Finite-Lived Intangible Assets, Net [Abstract]
Finite-Lived Intangible Assets, Gross	$ 3,548.6	3,548.6
Accumulated Amortization	(3,439.1)	(3,429.1)
Net	109.5	119.5
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Accumulated Amortization	$ 3,439.1	$ 3,429.1
Developed technology and other | Minimum
Finite-Lived Intangible Assets [Line Items]
Estimated life (in years)	13 years
Developed technology and other | Maximum
Finite-Lived Intangible Assets [Line Items]
Estimated life (in years)	31 years

Intangible Assets and Goodwil_3

Intangible Assets and Goodwill - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Sep. 26, 2023
Goodwill [Line Items]
Amortization and impairment of acquired intangible assets	$ 86,900,000	$ 52,900,000	$ 165,200,000	$ 103,100,000
Impairment of intangible assets	0	$ 0	0	$ 0
Accumulated impairment losses related to goodwill	$ 0		$ 0
Acquired and in-licensed rights and patents
Goodwill [Line Items]
Weighted-Average remaining useful life	12 years		12 years
Acquired and in-licensed rights and patents | Minimum
Goodwill [Line Items]
Estimated life (in years)	2 years		2 years
Acquired and in-licensed rights and patents | Maximum
Goodwill [Line Items]
Estimated life (in years)	22 years		22 years
Developed technology and other
Goodwill [Line Items]
Weighted-Average remaining useful life	10 years		10 years
Developed technology and other | Minimum
Goodwill [Line Items]
Estimated life (in years)	13 years		13 years
Developed technology and other | Maximum
Goodwill [Line Items]
Estimated life (in years)	31 years		31 years
Completed Technology | Minimum
Goodwill [Line Items]
Estimated life (in years)	2 years		2 years
Completed Technology | Maximum
Goodwill [Line Items]
Estimated life (in years)	31 years		31 years
Reata Pharmaceuticals, Inc | In-process research and development
Goodwill [Line Items]
Intangible assets:	$ 2,300,000,000		$ 2,300,000,000		$ 2,300,000,000
Reata Pharmaceuticals, Inc | Completed technology for SKYCLARYS (U.S.)
Goodwill [Line Items]
Intangible assets:	4,200,000,000		4,200,000,000		4,200,000,000
Reata Pharmaceuticals, Inc | Priority review voucher
Goodwill [Line Items]
Intangible assets:	$ 100,000,000		$ 100,000,000		$ 100,000,000

Intangible Assets and Goodwil_4

Intangible Assets and Goodwill - Estimated Future Amortization of Intangible Assets (Details) $ in Millions	Jun. 30, 2024 USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2024 (remaining six months)	$ 185
2025	565
2026	660
2027	680
2028	710
2029	$ 720

Intangible Assets and Goodwil_5

Intangible Assets and Goodwill - Changes in Goodwill (Details)	6 Months Ended
	Jun. 30, 2024 USD ($)
Summary of roll forward of the changes in goodwill
Goodwill, beginning of period	$ 6,219,200,000
Goodwill resulting from Reata acquisition	9,000,000
Other	(800,000)
Goodwill, end of period	6,227,400,000
Accumulated impairment losses related to goodwill	$ 0

Fair Value Measurements (Detail

Fair Value Measurements (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023	Mar. 31, 2022
Assets:
Derivative contracts	$ 1.8
Samsung Bioepis
Liabilities:
Equity, ownership interest	49.90%		49.90%
Fair Value, Measurements Recurring
Assets:
Cash equivalents	$ 1,449.7	$ 610.7
Marketable equity securities	302.7	416.8
Derivative contracts	23.4	11.9
Plan assets for deferred compensation	38.5	37.5
Total	1,816.1	1,506.9
Liabilities:
Derivative contracts	18.6	31.6
Total	18.6	31.6
Fair Value, Measurements Recurring | Samsung Bioepis
Assets:
Other current assets, receivable		430
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements Recurring
Assets:
Cash equivalents	0	0
Marketable equity securities	302.7	416.8
Derivative contracts	0	0
Plan assets for deferred compensation	0	0
Derivative contracts	0
Total	302.7	416.8
Liabilities:
Derivative contracts	0	0
Total	0	0
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements Recurring | Samsung Bioepis
Assets:
Other current assets, receivable		0
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements Recurring
Assets:
Cash equivalents	1,449.7	610.7
Marketable equity securities	0	0
Derivative contracts	23.4	11.9
Plan assets for deferred compensation	38.5	37.5
Derivative contracts	1.8
Total	1,513.4	660.1
Liabilities:
Derivative contracts	18.6	31.6
Total	18.6	31.6
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements Recurring | Samsung Bioepis
Assets:
Other current assets, receivable		0
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
Derivative contracts	0
Total	0	430
Liabilities:
Derivative contracts	0	0
Total	$ 0	0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements Recurring | Samsung Bioepis
Assets:
Other current assets, receivable		$ 430

Fair Value Measurements (Deta_2

Fair Value Measurements (Details Textual) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2024	Jun. 30, 2024
Debt Instrument [Line Items]
Asset impairment charges	$ 0	$ 0

Fair Value Measurements (Deta_3

Fair Value Measurements (Details 1) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Sep. 26, 2023
Debt Instrument [Line Items]
Notes payable, fair value	$ 5,448,900	$ 6,287,700
Current portion of term loan	0	150,000
Notes payable	6,292,000	6,938,200
4.050% Senior Notes due 2025
Debt Instrument [Line Items]
Notes payable, fair value		1,721,500
4.050% Senior Notes due 2025 | Senior Notes
Debt Instrument [Line Items]
Notes payable, fair value	1,718,700
Notes payable	1,747,600	1,746,600
2.250% Senior Notes due May 1, 2030
Debt Instrument [Line Items]
Notes payable, fair value		1,089,700
2.250% Senior Notes due May 1, 2030 | Senior Notes
Debt Instrument [Line Items]
Notes payable, fair value	1,018,800
Notes payable	1,100,800	1,100,700
5.200% Senior Notes due 2045
Debt Instrument [Line Items]
Notes payable, fair value		1,279,300
5.200% Senior Notes due 2045 | Senior Notes
Debt Instrument [Line Items]
Notes payable, fair value	1,275,900
Notes payable	1,494,300	1,493,800
3.250% Senior Notes, Due February 15, 2051
Debt Instrument [Line Items]
Notes payable, fair value		498,200
3.250% Senior Notes, Due February 15, 2051 | Senior Notes
Debt Instrument [Line Items]
Notes payable, fair value	466,200
Notes payable	474,600	472,800
3.150% Senior Notes due May 1, 2050
Debt Instrument [Line Items]
Notes payable, fair value		1,049,000
3.150% Senior Notes due May 1, 2050 | Senior Notes
Debt Instrument [Line Items]
Notes payable, fair value	969,300
Notes payable	1,474,700	1,474,300
3.625% Senior Notes due 2022 | Senior Notes
Debt Instrument [Line Items]
Notes payable, fair value	0	150,000
Current portion of term loan	0	150,000
Current Portion Of Notes Payable And Term Loan | Senior Notes
Debt Instrument [Line Items]
Notes payable, fair value	0	150,000
Current portion of term loan	0	150,000
2023 Term Loan Three-Year Tranche | Senior Notes
Debt Instrument [Line Items]
Notes payable, fair value	0	500,000
Notes payable	0	500,000
Non-Current Portion Of Notes Payable And Term Loan | Senior Notes
Debt Instrument [Line Items]
Notes payable, fair value	5,448,900	6,137,700
Notes payable	$ 6,292,000	$ 6,788,200
The Credit Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity			$ 1,500,000
The Credit Facility | Line of Credit | Reata Pharmaceuticals, Inc | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross			1,000,000
Credit Facility Floating Rate 364-Day Tranche | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity			500,000
The Credit Facility Floating Rate Three-Year Tranche | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity			$ 500,000

Financial Instruments (Details)

Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 1,449.7	$ 610.7
Money market funds
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 1,449.7	$ 610.7

Financial Instruments (Details

Financial Instruments (Details 1) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Marketable equity securities
Amortized Cost	$ 800.2	$ 979.9
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(497.5)	(563.1)
Fair Value	302.7	416.8
Equity Securities, Current
Marketable equity securities
Amortized Cost		31.6
Gross Unrealized Gains		0
Gross Unrealized Losses		(21)
Fair Value		10.6
Equity Securities, Non-Current
Marketable equity securities
Amortized Cost	800.2	948.3
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(497.5)	(542.1)
Fair Value	$ 302.7	$ 406.2

Financial Instruments (Detail_2

Financial Instruments (Details 3) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2023	Jun. 30, 2023
Investments, Debt and Equity Securities [Abstract]
Proceeds from maturities and sales	$ 1,275.5	$ 1,682.2
Realized gains	0.3	0.4
Realized losses	$ 1.4	$ 2.1

Financial Instruments Financial

Financial Instruments Financial Instruments (Details Textual 2) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Other noncurrent assets | Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio	$ 352.3	$ 460.7

Derivative Instruments (Details

Derivative Instruments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Mar. 31, 2022
Derivatives [Line Items]
Unrealized Gain on Derivatives			$ 12.9	$ 0
Unrealized Loss on Derivatives			(9.5)	(34.8)
Unrealized Gain (Loss) on Derivatives			3.4	(34.8)
Foreign Currency Cash Flow Hedge Gain (Loss) To Be Reclassified During Next 15 Months	$ 3.4		3.4
Foreign Currency Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months	1.9		1.9
Gain (loss) on foreign currency derivative instruments not designated as hedging instruments	$ (5.7)	$ 7.2	$ (30)	5.4
Minimum
Derivatives [Line Items]
Range of durations of foreign currency forward contracts	1 month		1 month		1 month
Maximum
Derivatives [Line Items]
Range of durations of foreign currency forward contracts	15 months		15 months		12 months
Not designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount	$ 1,199.3		$ 1,199.3		$ 1,301.5
Other current assets | Designated as hedging instrument
Derivatives [Line Items]
Derivative asset, fair value	17.6		17.6		0.3
Other current assets | Not designated as hedging instrument
Derivatives [Line Items]
Derivative asset, fair value	5.8		5.8		11.6
Accrued expenses and other | Designated as hedging instrument
Derivatives [Line Items]
Derivative liability, fair value	8.3		8.3		26.5
Accrued expenses and other | Not designated as hedging instrument
Derivatives [Line Items]
Derivative liability, fair value	10.3		10.3		5.1
Other noncurrent assets | Designated as hedging instrument
Derivatives [Line Items]
Derivative asset, fair value	1.8		1.8		0
Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount	1,658.5		1,658.5		1,169
Revenue | Foreign exchange contract
Derivatives [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	7.6	2.4	10.5	20
Revenue | Cash flows, revenue | Foreign exchange contract
Derivatives [Line Items]
Gain on interest rate swap	0.9	(1)	1.1	0.6
Operating expense | Foreign exchange contract
Derivatives [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	(4.9)	1.4	(7)	0.9
Operating expense | Cash flows, operating expenses | Foreign exchange contract
Derivatives [Line Items]
Gain on interest rate swap	(0.7)	$ (0.8)	(1.9)	$ (2.9)
Euro | Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount	1,420.3		1,420.3		1,169
Swiss franc | Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount	69.6		69.6		0
Japan, Yen | Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount	26.2		26.2		0
Canadian dollar | Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Derivative, Notional Amount	$ 142.4		$ 142.4		$ 0
Samsung Bioepis
Derivatives [Line Items]
Equity, ownership interest	49.90%		49.90%			49.90%

Property, Plant and Equipment (

Property, Plant and Equipment (Details) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) ft²	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 2,537.4			$ 2,537.4		$ 2,402.5
Depreciation	71.9		$ 64.5	141.2	$ 126.6
Property, Plant and Equipment, Net	$ 3,249.3			$ 3,249.3		3,309.7
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						$ 728.8
Property, Plant and Equipment, Additions		$ 717.3

Leases (Details Textual)

Leases (Details Textual) - Reata Pharmaceuticals, Inc $ in Millions	Jun. 30, 2024 USD ($)	Sep. 26, 2023 USD ($) ft²
Operating Leased Assets [Line Items]
Operating Lease Area | ft²		327,400
Lease term		16 years
Operating lease liabilities	$ 151.8	$ 151.8
Lessee, Operating Lease, Remaining Lease Term		15 years
Operating lease assets	$ 121.2	$ 121.2

Indebtedness (Details)

Indebtedness (Details) - Line of Credit - USD ($) $ in Thousands	Sep. 26, 2023	Jun. 30, 2024
Credit Facility Floating Rate 364-Day Tranche
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 500,000
The Credit Facility Floating Rate Three-Year Tranche
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	500,000
The Credit Facility Floating Rate Three-Year Tranche | Floating Rate 364-Day Tranche
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 250,000
The Credit Facility, Terminated
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	500,000
The Credit Facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	1,500,000
The Credit Facility | Secured Debt | Reata Pharmaceuticals, Inc
Debt Instrument [Line Items]
Long-term debt, gross	1,000,000
2023 Term Loan | Floating Rate 364-Day Tranche
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		150,000
Repayments of Short-Term Debt, Maturing in More than Three Months	$ 350,000
2023 Term Loan | 2023 Term Loan Three-Year Tranche
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 250,000

Share Repurchases (Details)

Share Repurchases (Details) - 2020 Share Repurchase Program - USD ($)	Jun. 30, 2024	Oct. 31, 2020
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount		$ 5,000,000,000
Amount remaining under 2019 Share Repurchase Program	$ 2,100,000,000

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	$ (160.5)	$ (172)	$ (153.7)	$ (164.9)
Balance, January 1, 2018	(146)	(172.8)	(146)	(172.8)
Other comprehensive income (loss), before reclassifications, net of tax	16.7	1.5	10.6	9.1
Amounts reclassified from accumulated other comprehensive income, net of tax	(2.2)	(2.3)	(2.9)	(17)
Other comprehensive income (loss), net of tax	14.5	(0.8)	7.7	(7.9)
Accumulated other comprehensive income (loss), net of tax ending balance	(146)	(172.8)	(146)	(172.8)
Other comprehensive income (loss), net of tax	14.5	(0.8)	7.7	(7.9)
Gains (losses) on securities available for sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	(10.3)	(10)	(25)	(15.7)
Balance, January 1, 2018	9.8	(14.5)	9.8	(14.5)
Other comprehensive income (loss), before reclassifications, net of tax	22.3	(5.5)	37.7	(0.2)
Amounts reclassified from accumulated other comprehensive income, net of tax	(2.2)	1	(2.9)	1.4
Other comprehensive income (loss), net of tax	20.1	(4.5)	34.8	1.2
Accumulated other comprehensive income (loss), net of tax ending balance	9.8	(14.5)	9.8	(14.5)
Gains (losses) on cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	(2.7)	(20.3)	(2.6)	15.1
Balance, January 1, 2018	(2.7)	(13.4)	(2.7)	(13.4)
Other comprehensive income (loss), before reclassifications, net of tax	0	10.2	(0.1)	(10.1)
Amounts reclassified from accumulated other comprehensive income, net of tax	0	(3.3)	0	(18.4)
Other comprehensive income (loss), net of tax	0	6.9	(0.1)	(28.5)
Accumulated other comprehensive income (loss), net of tax ending balance	(2.7)	(13.4)	(2.7)	(13.4)
Unfunded status of postretirement benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance		(0.6)		(1.1)
Balance, January 1, 2018		(0.4)		(0.4)
Other comprehensive income (loss), before reclassifications, net of tax		0.2		0.7
Amounts reclassified from accumulated other comprehensive income, net of tax		0		0
Other comprehensive income (loss), net of tax		0.2		0.7
Accumulated other comprehensive income (loss), net of tax ending balance		(0.4)		(0.4)
Currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance	(147.5)	(141.1)	(126.1)	(163.2)
Balance, January 1, 2018	(153.1)	(144.5)	(153.1)	(144.5)
Other comprehensive income (loss), before reclassifications, net of tax	(5.6)	(3.4)	(27)	18.7
Amounts reclassified from accumulated other comprehensive income, net of tax	0	0	0	0
Other comprehensive income (loss), net of tax	(5.6)	(3.4)	(27)	18.7
Accumulated other comprehensive income (loss), net of tax ending balance	$ (153.1)	$ (144.5)	$ (153.1)	$ (144.5)

Reclassification out of Accumul

Reclassification out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Mar. 31, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other (income) expense	$ (85.2)	$ 121.2	$ (178.9)	$ 51.8
Income Tax Expense (Benefit)	115.1	114.8	186.5	165.5
Product revenues	2,464.9	2,456	4,755.4	4,919
Net income (loss) attributable to Biogen Inc.	$ 583.6	591.6	$ 977	979.5
Samsung Bioepis
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Equity, ownership interest	49.90%		49.90%		49.90%
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income (loss) attributable to Biogen Inc.	$ 2.2	2.3	$ 2.9	17
Gains (losses) on securities available for sale | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other (income) expense	0	(1.3)	0	(1.8)
Income Tax Expense (Benefit)	0	0.3	0	0.4
Gains (losses) on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other (income) expense	(0.2)	(0.1)	(0.1)	(0.2)
Income Tax Expense (Benefit)	(0.3)	(0.4)	(0.5)	(2.3)
Product revenues	7.6	2.4	10.5	20
Operating Expenses	$ (4.9)	$ 1.4	$ (7)	$ 0.9

Earnings per Share (Details)

Earnings per Share (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Numerator:
Net income (loss) attributable to Biogen Inc.	$ 583.6	$ 591.6	$ 977	$ 979.5
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	145,600	144,700	145,400	144,600
Effect of dilutive securities:
Dilutive potential common shares	300	800	500	800
Shares used in calculating diluted earnings per share	145,900	145,500	145,900	145,400
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	200	700	400	700
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	100	100	100	100

Share-Based Payments - Share-Ba

Share-Based Payments - Share-Based Compensation Expense Included In Statements Of Income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Sep. 26, 2023	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense		$ (72.9)	$ (74.1)	$ (149)	$ (153)
Subtotal		72.9	76.3	144.3	157.7
Capitalized share-based compensation costs		(2.3)	(2.6)	(5.6)	(5.9)
Share-based compensation expense included in total costs and expenses		70.6	73.7	138.7	151.8
Income tax effect		(13.4)	(13.3)	(26.3)	(28)
Selling, General and Administrative Expense		553.8	548	1,135.3	1,153
Reata Pharmaceuticals, Inc
Share-based Compensation Expense included in consolidated statements of income
Operating expenses	$ 393.4
Selling, General and Administrative Expense	196.4
Research and development expense asset acquired	$ 197
Research and development
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense		27.3	27.9	54.7	59.2
Selling, general and administrative
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense		45.6	48.4	89.6	98.5
Total share-based compensation expense, net of tax
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense		$ 57.2	$ 60.4	$ 112.4	$ 123.8

Share-Based Payments - Share-_2

Share-Based Payments - Share-Based Compensation Expense by Share-Based Compensation Program (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	$ (72.9)	$ (74.1)	$ (149)	$ (153)
Subtotal	72.9	76.3	144.3	157.7
Capitalized share-based compensation costs	(2.3)	(2.6)	(5.6)	(5.9)
Share-based compensation expense included in total costs and expenses	70.6	73.7	138.7	151.8
Market stock units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	0.1	1.3	0.4	3.5
Time-vested restricted stock units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	58.3	59.9	115.6	121.6
Performance stock units settled in stock
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	11.7	9.8	22.2	19.3
Performance stock units settled in cash
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	0.1	2.2	(2)	4.7
Employee stock purchase plan
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	1.8	2.1	6.3	6.8
Options
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	$ 0.9	$ 1	$ 1.8	$ 1.8

Share-based Payments - Narrativ

Share-based Payments - Narrative (Details) shares in Millions	1 Months Ended
	Jun. 30, 2024 shares
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Employee stock purchase plans (in shares)	2.5
Omnibus Equity Plan 2024
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share based compensation arrangement, authorized (in shares)	3.7
Ratio of total number of shares reserved under the plan	1.5

Income Taxes (Details Textual)

Income Taxes (Details Textual) $ in Millions	Jun. 30, 2024 USD ($)
Income Tax Contingency [Line Items]
Decrease in unrecognized tax benefits is reasonably possible	$ 25

Income Taxes (Details)

Income Taxes (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Reconciliation between the U.S. federal statutory tax rate and effective tax rate
Statutory rate	21%	21%	21%	21%
State taxes	1.40%	1.20%	1.40%	1.20%
Taxes on foreign earnings	(6.20%)	(4.70%)	(5.20%)	(5.00%)
Credits and net operating loss utilization	(1.60%)	(2.00%)	(2.00%)	(3.70%)
Purchased intangible assets	1.60%	0.30%	1.50%	0.30%
GILTI	(0.90%)	0.20%	(1.60%)	0.30%
Other	1.20%	0.20%	0.90%	0.30%
Effective tax rate	16.50%	16.20%	16%	14.40%

Other Consolidated Financial _3

Other Consolidated Financial Statement Detail (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Other Income (Expense), Net
Interest income	$ (20.1)	$ (75.9)	$ (37.5)	$ (156.8)
Interest expense	61.3	49.1	130.5	111.6
(Gains) losses on investments, net	30.7	(105.4)	61.3	(27.7)
Foreign exchange (gains) losses, net	11.3	8.7	20.7	19.4
Other, net	2	2.3	3.9	1.7
Other (income) expense	85.2	(121.2)	178.9	(51.8)
Equity Securities, FV-NI, Gain (Loss), Alternative [Abstract]
Net (gains) losses recognized on equity securities	30.4	(106.5)	61	(28.4)
Less: Net (gains) losses realized on equity securities	0.6	(0.7)	5.5	0.9
Net unrealized (gains) losses recognized on equity securities	29.8	(105.8)	55.5	(29.3)
Accrued Expenses and Other
Revenue-related reserves for discounts and allowances	1,116		1,116		$ 1,062
Employee compensation and benefits	228.3		228.3		335.1
Collaboration expense	319.4		319.4		214.6
Royalties and licensing fees	193		193		191.5
Reata related accrued expense	77.4		77.4		117.5
Other	689.5		689.5		838.4
Total accrued expense and other	2,472.1		2,472.1		2,623.6
Ionis, Sangamo, Denali and Sage
Equity Securities, FV-NI, Gain (Loss), Alternative [Abstract]
Net unrealized (gains) losses recognized on equity securities	51.4	$ 117.7	68.3	$ 78
Component of accrued expenses and other
Accrued Expenses and Other
Revenue-related reserves for discounts and allowances	$ 964.5		$ 964.5		$ 926.5

Other Consolidated Financial _4

Other Consolidated Financial Statement (Details Textual) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Sep. 26, 2023	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Net gains recognized on the increase in fair value of equity securities		$ 29.8	$ (105.8)	$ 55.5	$ (29.3)
Net gains (losses) realized during the period on equity securities		0.6	(0.7)	5.5	0.9
Other long-term liabilities		556.7		556.7		$ 781.1
Accrued income taxes		191.7		191.7		$ 403.2
Reata Pharmaceuticals, Inc
Total transaction value	$ 6,602.9
Ionis, Sangamo, Denali and Sage
Net gains recognized on the increase in fair value of equity securities		51.4	117.7	68.3	78
Sangamo
Net gains recognized on the increase in fair value of equity securities		$ 27		$ 17.6
Sage Therapeutics
Net gains recognized on the increase in fair value of equity securities			$ 10.9		$ 47.4

Collaborative and Other Relat_3

Collaborative and Other Relationships - OCREVUS (Details) - USD ($) $ in Millions	1 Months Ended	6 Months Ended
	Mar. 31, 2017	Jun. 30, 2024
OCREVUS
Collaborative arrangements and non-collaborative arrangement transactions
Royalty operating profit threshold for highest royalty rate percentage	$ 900
Reduction in royalty rate	50%
Collaboration agreement term	11 years
OCREVUS | Foreign Tax Authority
Collaborative arrangements and non-collaborative arrangement transactions
Royalty percentage to be received	3%
OCREVUS | Minimum
Collaborative arrangements and non-collaborative arrangement transactions
Royalty percentage to be received	13.50%
OCREVUS | Maximum
Collaborative arrangements and non-collaborative arrangement transactions
Royalty percentage to be received	24%
Mosunetuzumab
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of future development costs		30%

Collaborative and Other Relat_4

Collaborative and Other Relationships - Profit Sharing (Details) $ in Millions	6 Months Ended
	Jun. 30, 2024 USD ($)
RITUXAN
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of co promotion operating profits first fifty million	30%
Until Second GAZYVA Threshold Date	37.50%
After Second GAZYVA Threshold Date	35%
Percentage Of Co Promotion Operating Profits Greater Than First Fifty Million Option Two Sub Option Four	30%
GAZYVA
Collaborative arrangements and non-collaborative arrangement transactions
Co-promotion operating profit threshold for Rituxan in US and Canada to determine share of co promotion operating profit prior to amendment	$ 50
Until Second GAZYVA Threshold Date	37.50%
After Second GAZYVA Threshold Date	35%
Limit of gross sale of GAZYVA to be achieved in preceding 12 months under option one	$ 500
Limit of gross sale of GAZYVA to be achieved in any 12 months under option one	150
Sales trigger gross sales threshold	$ 350

Collaborative and Other Relat_5

Collaborative and Other Relationships - Eisai (Details) $ in Millions	Dec. 31, 2023 USD ($)
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Payments Made To Termination Agreement	$ 16
LEQEMBI Collaboration
Collaborative arrangements and non-collaborative arrangement transactions
Payable Due To Termination Agreement	$ 31

Collaborative and Other Relat_6

Collaborative and Other Relationships - Summary of Activity Related to BAN2401 and Elenbecestat Collaboration (Details) - E2609 and BAN2401 - Eisai - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Collaborative arrangements and non-collaborative arrangement transactions
Total development expense incurred by the collaboration related to the advancement of LEQEMBI	$ 90.6	$ 86.2	$ 177.9	$ 194.1
Expense reflected within statements of income	45.4	43	89	97
Selling, general and administrative
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income	68.9	8.5	155.8	13.8
Expense incurred by the collaboration	$ 137.7	$ 17.1	$ 311.6	$ 27.6

Collaborative and Other Relat_7

Collaborative and Other Relationships - ADUHELM Collaboration Agreement (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Collaborative arrangements and non-collaborative arrangement transactions
Excess and obsolescence charges related to inventory	$ 44.4	$ 36.8
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Amounts receivable	10.3		$ 1.4
Amounts payable	$ 171.8		$ 118.4

Collaborative and Other Relat_8

Collaborative and Other Relationships - Summary of Activity Related to ADUHELM Collaboration (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Collaborative arrangements and non-collaborative arrangement transactions
Selling, general and administrative	$ 553.8	$ 548	$ 1,135.3	$ 1,153
LEQEMBI Collaboration
Collaborative arrangements and non-collaborative arrangement transactions
Share of non-controling interest recognized	50%		50%

Collaborative and Other Relat_9

Collaborative and Other Relationships - Summary of Activity Related to the UCB Collaboration (Details) - Research and development - UCB Pharma S.A. - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration	$ 16.5	$ 14.3	$ 33.1	$ 32.6
Expense reflected within statements of income	$ 8.3	$ 7.1	$ 16.6	$ 16.3

Collaborative and Other Rela_10

Collaborative and Other Relationships - Summary of Activity Related to Sage Therapeutics (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2024
Collaborative arrangements and non-collaborative arrangement transactions
Share of net profit from sage therapeutics, Percent	50%		50%
Collaboration profit sharing/(loss reimbursement)	$ 62.4	$ 56.9	$ 128	$ 114
Sage Therapeutics
Collaborative arrangements and non-collaborative arrangement transactions
Collaboration profit sharing/(loss reimbursement)	6.5		11.5
Sage Therapeutics | Forecast
Collaborative arrangements and non-collaborative arrangement transactions
Potential future milestone payments commitment to third party					$ 75
Research and development | Sage Therapeutics
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration	9.6	52.1	21.2	86.9
Expense reflected within statements of income	4.8	26.1	10.6	43.5
Selling, general and administrative | Sage Therapeutics
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration	26.9	60.1	54.5	98.3
Expense reflected within statements of income	$ 13.5	$ 30	$ 27.3	$ 49.1

Collaborative and Other Rela_11

Collaborative and Other Relationships - Summary of Activity Related To Denali Therapeutics (Details) - Research and development - Denali Therapeutics - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration	$ 15.4	$ 22.1	$ 29.6	$ 38.7
Expense reflected within statements of income	$ 9.3	$ 13.2	$ 17.8	$ 23.2

Collaborative and Other Rela_12

Collaborative and Other Relationships - Other Arrangements (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Collaborative arrangements and non-collaborative arrangement transactions
Research and development	$ 513.9	$ 584.2	$ 966.8	$ 1,154.8
Other research and discovery
Collaborative arrangements and non-collaborative arrangement transactions
Research and development	$ 8.5	$ 2.6	$ 16	$ 2.8

Collaborative and Other Rela_13

Collaborative and Other Relationships - Samsung Bioepis (Details) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Dec. 31, 2019 product	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Collaborative arrangements and non-collaborative arrangement transactions
Collaboration profit sharing/(loss reimbursement)		$ 62.4	$ 56.9	$ 128	$ 114
Samsung Bioepis
Collaborative arrangements and non-collaborative arrangement transactions
Collaboration profit sharing/(loss reimbursement)		55.9	$ 56.9	$ 116.5	$ 114
Samsung Bioepis
Collaborative arrangements and non-collaborative arrangement transactions
Contingent Commercialized Rights, Number Of Products | product	2
Expected profit share percentage	45%			50%
Accrued milestone payments		15
Estimated additional payments upon achievement of development and commercial milestones				$ 165
Collaboration agreement term				5 years
Option exercise fee				$ 60
Option exercise fee paid				60
Samsung Bioepis | Related Party
Collaborative arrangements and non-collaborative arrangement transactions
Accounts receivable		19		19		$ 9.9
Accounts payable		$ 93.6		$ 93.6		$ 73.7

Investments in Variable Inter_2

Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Variable Interest Entity [Line Items]
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities	$ 22.8	$ 16.4
Neurimmune
Variable Interest Entity [Line Items]
Research and development costs, percentage	100%
Deconsolidation, gain (loss), amount	$ 3

Litigation (Details)

Litigation (Details) - USD ($) $ in Millions		1 Months Ended
	Sep. 26, 2023	Feb. 28, 2023	Sep. 30, 2023	Dec. 31, 2019
Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss			$ 49.9	$ 200
Genentech
Loss Contingencies [Line Items]
Brazil tax assessment, including interest and penalties		$ 88.3
Lender Dispute
Loss Contingencies [Line Items]
Brazil tax assessment, including interest and penalties	$ 23.2