Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Revenues	$ 9,675.9	$ 9,835.6	$ 10,173.4
Cost and expense:
Cost of sales, excluding amortization and impairment of acquired intangible assets	2,310.4	2,533.4	2,278.3
Research and development	2,041.8	2,462.0	2,231.1
Selling, general and administrative	2,403.7	2,549.7	2,403.6
Amortization and impairment of acquired intangible assets	446.7	240.6	365.9
Collaboration profit sharing/(loss reimbursement)	254.4	218.8	(7.4)
(Gain) loss on fair value remeasurement of contingent consideration	27.7	0.0	(209.1)
Restructuring charges	30.2	218.8	131.1
Gain on sale of priority review voucher, net	(88.6)	0.0	0.0
Gain on sale of building, net	0.0	0.0	(503.7)
Other (income) expense, net	343.6	315.5	(108.2)
Total cost and expense	7,769.9	8,538.8	6,581.6
Income before income tax (benefit) expense and equity in (income) loss of investee, net of tax	1,906.0	1,296.8	3,591.8
Income tax (benefit) expense	273.8	135.3	632.8
Equity in (income) loss of investee, net of tax	0.0	0.0	(2.6)
Net income	1,632.2	1,161.5	2,961.6
Net income (loss) attributable to noncontrolling interests, net of tax	0.0	0.4	(85.3)
Net income attributable to Biogen Inc.	$ 1,632.2	$ 1,161.1	$ 3,046.9
Net income per share:
Basic earnings per share attributable to Biogen Inc. (in dollars per share)	$ 11.21	$ 8.02	$ 20.96
Diluted earnings per share attributable to Biogen Inc. (in dollars per share)	$ 11.18	$ 7.97	$ 20.87
Weighted-average shares used in calculating:
Weighted-average number of common shares outstanding (in shares)	145.6	144.7	145.3
Diluted earnings per share attributable to Biogen Inc. (in shares)	145.9	145.6	146.0
Product revenue, net
Revenues	$ 7,213.5	$ 7,246.7	$ 7,987.8
Revenue from anti-CD20 therapeutic programs
Revenues	1,749.9	1,689.6	1,700.5
Alzheimer's collaboration revenue
Revenues	59.9	0.0	0.0
Contract manufacturing, royalty and other revenue
Revenues	$ 652.6	$ 899.3	$ 485.1