Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenues	$ 10,981.7	$ 13,444.6	$ 14,377.9
Cost and expense:
Cost of sales, excluding amortization and impairment of acquired intangible assets	2,109.7	1,805.2	1,955.4
Research and development	2,501.2	3,990.9	2,280.6
Selling, general and administrative	2,674.3	2,504.5	2,374.7
Amortization and impairment of acquired intangible assets	881.3	464.8	489.9
Collaboration profit (loss) sharing	7.2	232.9	241.6
(Gain) loss on divestiture of Hillerød, Denmark manufacturing operations	0.0	(92.5)	55.3
(Gain) loss on fair value remeasurement of contingent consideration	(50.7)	(86.3)	(63.7)
Acquired in-process research and development	18.0	75.0	0.0
Restructuring charges	0.0	0.0	1.5
Total cost and expense	8,141.0	8,894.5	7,335.3
Income from operations	2,840.7	4,550.1	7,042.6
Other income (expense), net	(1,095.5)	497.4	83.3
Income before income tax expense and equity in loss of investee, net of tax	1,745.2	5,047.5	7,125.9
Income tax (benefit) expense	52.5	992.3	1,158.0
Equity in (income) loss of investee, net of tax	(34.9)	(5.3)	79.4
Net income	1,727.6	4,060.5	5,888.5
Net income (loss) attributable to noncontrolling interests, net of tax	171.5	59.9	0.0
Net income attributable to Biogen Inc.	$ 1,556.1	$ 4,000.6	$ 5,888.5
Net income per share:
Basic earnings per share attributable to Biogen Inc. (in dollars per share)	$ 10.44	$ 24.86	$ 31.47
Diluted earnings per share attributable to Biogen Inc. (in dollars per share)	$ 10.40	$ 24.80	$ 31.42
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding (in shares)	149.1	160.9	187.1
Diluted earnings per share attributable to Biogen Inc. (in shares)	149.6	161.3	187.4
Product, net
Revenues	$ 8,846.9	$ 10,692.2	$ 11,379.8
Revenue from anti-CD20 therapeutic programs
Revenues	1,658.5	1,977.8	2,290.4
Other
Revenues	$ 476.3	$ 774.6	$ 707.7