Consolidated Statements of Income - USD ($) shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Revenues	$ 10,173.4	$ 10,981.7	$ 13,444.6
Cost and expense:
Cost of sales, excluding amortization and impairment of acquired intangible assets	2,278.3	2,109.7	1,805.2
Research and development	2,231.1	2,501.2	3,990.9
Selling, general and administrative	2,403.6	2,674.3	2,504.5
Amortization and impairment of acquired intangible assets	365.9	881.3	464.8
Collaboration profit (loss) sharing	(7.4)	7.2	232.9
(Gain) loss on divestiture of Hillerød, Denmark manufacturing operations	0.0	0.0	(92.5)
(Gain) loss on fair value remeasurement of contingent consideration	(209.1)	(50.7)	(86.3)
Acquired in-process research and development	0.0	18.0	75.0
Restructuring charges	131.1	0.0	0.0
Gain on sale of building	(503.7)	0.0	0.0
Other (income) expense, net	(108.2)	1,095.5	(497.4)
Total cost and expense	6,581.6	9,236.5	8,397.1
Income before income tax expense and equity in loss of investee, net of tax	3,591.8	1,745.2	5,047.5
Income tax (benefit) expense	632.8	52.5	992.3
Equity in (income) loss of investee, net of tax	(2.6)	(34.9)	(5.3)
Net income	2,961.6	1,727.6	4,060.5
Net income (loss) attributable to noncontrolling interests, net of tax	(85.3)	171.5	59.9
Net income attributable to Biogen Inc.	$ 3,046.9	$ 1,556.1	$ 4,000.6
Net income per share:
Basic earnings per share attributable to Biogen Inc. (in dollars per share)	$ 20.96	$ 10.44	$ 24.86
Diluted earnings per share attributable to Biogen Inc. (in dollars per share)	$ 20.87	$ 10.40	$ 24.80
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding (in shares)	145,300	149,100	160,900
Diluted earnings per share attributable to Biogen Inc. (in shares)	146,000	149,600	161,300
Product, net
Revenues	$ 7,987.8	$ 8,846.9	$ 10,692.2
Revenue from anti-CD20 therapeutic programs
Revenues	1,700.5	1,658.5	1,977.8
Other
Revenues	$ 485.1	$ 476.3	$ 774.6