Consolidated Statements of Income - USD ($) $ in Thousands, shares in Millions	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Revenues	$ 9,890,600	$ 9,675,900	$ 9,835,600
Cost and expense:
Cost of sales, excluding amortization and impairment of acquired intangible assets	2,404,200	2,310,400	2,533,400
Research and development	1,778,600	1,980,300	2,445,400
Acquired in-process research and development, upfront and milestone expense	471,800	61,500	16,600
Selling, general and administrative	2,433,600	2,403,700	2,549,700
Amortization and impairment of acquired intangible assets	515,000	446,700	240,600
Collaboration profit sharing/(loss reimbursement)	290,200	254,400	218,800
(Gain) loss on fair value remeasurement of contingent consideration	33,600	27,700	0
Impairment of ROU asset	52,900	0	0
Restructuring charges	48,600	30,200	218,800
Gain on sale of priority review voucher, net	0	(88,600)	0
Other (income) expense, net	305,600	343,600	315,500
Total cost and expense	8,334,100	7,769,900	8,538,800
Income before income tax (benefit) expense	1,556,500	1,906,000	1,296,800
Income tax (benefit) expense	263,600	273,800	135,300
Net income	1,292,900	1,632,200	1,161,500
Net income attributable to noncontrolling interests, net of tax	0	0	400
Net income attributable to Biogen Inc.	$ 1,292,900	$ 1,632,200	$ 1,161,100
Net income per share:
Basic earnings per share attributable to Biogen Inc. (in dollars per share)	$ 8.83	$ 11.21	$ 8.02
Diluted earnings per share attributable to Biogen Inc. (in dollars per share)	$ 8.79	$ 11.18	$ 7.97
Weighted-average shares used in calculating:
Weighted-average number of common shares outstanding (in shares)	146.5	145.6	144.7
Diluted earnings per share attributable to Biogen Inc. (in shares)	147.1	145.9	145.6
Product revenue, net
Revenues	$ 7,119,400	$ 7,213,500	$ 7,246,700
Revenue from anti-CD20 therapeutic programs
Revenues	1,860,600	1,749,900	1,689,600
Alzheimer's collaboration revenue
Revenues	177,700	59,900	0
Contract manufacturing, royalty and other revenue
Revenues	$ 732,900	$ 652,600	$ 899,300

Amortization And Impairment Of Acquired Intangible Assets

No definition available.

Name:	biib_AmortizationAndImpairmentOfAcquiredIntangibleAssets
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Payments are made to Elan for their share of rest of world net operating profits to effect an equal sharing of collaboration operating profit. These payments include the reimbursement of our portion of third-party royalties that Elan pays on behalf of the collaboration, relating to sales outside of the U.S. These amounts are reflected in the collaboration profit sharing line in our consolidated statement of income. As sales of TYSABRI outside the U.S. increase, our collaboration profit sharing expense will increase.

+ References

No definition available.