Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Total revenue	$ 2,694.0	$ 3,534.3
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	478.1	454.3
Research and development	514.2	476.3
Selling, general and administrative	595.0	570.1
Amortization and impairment of acquired intangible assets	98.1	71.5
Collaboration profit (loss) sharing	68.5	71.8
(Gain) loss on fair value remeasurement of contingent consideration	(33.8)	(4.6)
Acquired in-process research and development	0.0	75.0
Total cost and expense	1,720.1	1,714.4
Income from operations	973.9	1,819.9
Other income (expense), net	(506.9)	(120.5)
Income before income tax expense and equity in loss of investee, net of tax	467.0	1,699.4
Income tax expense	44.2	292.0
Equity in (income) loss of investee, net of tax	18.2	14.8
Net income	404.6	1,392.6
Net income (loss) attributable to noncontrolling interests, net of tax	(5.6)	(6.5)
Net income attributable to Biogen Inc.	$ 410.2	$ 1,399.1
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 2.70	$ 8.10
Diluted earnings per share attributable to Biogen Inc.	$ 2.69	$ 8.08
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	151.9	172.8
Diluted earnings per share attributable to Biogen Inc.	152.3	173.1
Product, net
Total revenue	$ 2,211.7	$ 2,904.6
Revenue from anti-CD20 therapeutic programs
Total revenue	389.0	520.4
Other
Total revenue	$ 93.3	$ 109.3