Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Total revenue	$ 2,589.1	$ 2,775.0	$ 5,120.9	$ 5,469.0
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	484.0	459.7	1,237.9	937.8
Research and development	528.6	585.1	1,080.3	1,099.3
Selling, general and administrative	572.6	637.3	1,207.5	1,232.3
Amortization and impairment of acquired intangible assets	67.5	604.1	134.4	702.2
Collaboration profit (loss) sharing	29.4	(15.2)	(87.9)	53.3
(Gain) loss on fair value remeasurement of contingent consideration	(4.5)	0.3	(11.6)	(33.5)
Acquired in-process research and development	0.0	18.0	0.0	18.0
Restructuring charges	70.6	0.0	108.7	0.0
Other (income) expense, net	(428.6)	(96.4)	(165.3)	410.5
Total cost and expense	1,319.6	2,192.9	3,504.0	4,419.9
Income before income tax expense and equity in loss of investee, net of tax	1,269.5	582.1	1,616.9	1,049.1
Income tax (benefit) expense	216.7	(409.1)	342.3	(364.9)
Equity in (income) loss of investee, net of tax	(5.9)	(34.3)	(2.6)	(16.1)
Net income	1,058.7	1,025.5	1,277.2	1,430.1
Net income (loss) attributable to noncontrolling interests, net of tax	0.7	577.0	(84.6)	571.4
Net income attributable to Biogen Inc.	$ 1,058.0	$ 448.5	$ 1,361.8	$ 858.7
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 7.25	$ 3.00	$ 9.30	$ 5.70
Diluted earnings per share attributable to Biogen Inc.	$ 7.24	$ 2.99	$ 9.27	$ 5.68
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	145,900	149,700	146,500	150,800
Diluted earnings per share attributable to Biogen Inc.	146,200	150,100	146,800	151,200
Product, net
Total revenue	$ 2,054.9	$ 2,236.0	$ 4,121.2	$ 4,447.7
Revenue from anti-CD20 therapeutic programs
Total revenue	436.3	440.0	835.7	829.0
Other
Total revenue	$ 97.9	$ 99.0	$ 164.0	$ 192.3