Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Total revenue	$ 2,778.9	$ 3,376.1	$ 8,247.9	$ 10,592.0
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	511.8	449.1	1,449.6	1,314.6
Research and development	702.4	1,140.9	1,801.7	2,264.8
Selling, general and administrative	654.1	573.1	1,886.4	1,698.3
Amortization and impairment of acquired intangible assets	111.0	82.6	813.2	215.6
Collaboration profit sharing	21.2	73.0	74.5	166.5
(Gain) loss on fair value remeasurement of contingent consideration	(15.6)	(29.0)	(49.1)	(23.5)
Acquired in-process research and development	0.0	0.0	18.0	75.0
Total cost and expense	1,984.9	2,289.7	5,994.3	5,711.3
Income from operations	794.0	1,086.4	2,253.6	4,880.7
Other income (expense), net	(502.9)	(128.6)	(913.4)	(186.1)
Income before income tax expense and equity in loss of investee, net of tax	291.1	957.8	1,340.2	4,694.6
Income tax (benefit) expense	(25.9)	240.8	(390.7)	979.0
Equity in (income) loss of investee, net of tax	(1.1)	13.1	(17.2)	12.7
Net income	318.1	703.9	1,748.1	3,702.9
Net income (loss) attributable to noncontrolling interests, net of tax	(11.1)	2.4	560.2	60.2
Net income attributable to Biogen Inc.	$ 329.2	$ 701.5	$ 1,187.9	$ 3,642.7
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 2.22	$ 4.47	$ 7.93	$ 22.29
Diluted earnings per share attributable to Biogen Inc.	$ 2.22	$ 4.46	$ 7.90	$ 22.25
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	148.0	156.9	149.9	163.4
Diluted earnings per share attributable to Biogen Inc.	148.6	157.2	150.3	163.7
Product, net
Total revenue	$ 2,205.7	$ 2,690.3	$ 6,653.4	$ 8,390.6
Revenue from anti-CD20 therapeutic programs
Total revenue	415.4	560.1	1,244.4	1,558.8
Other
Total revenue	$ 157.8	$ 125.7	$ 350.1	$ 642.6