Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Total revenue	$ 2,775.0	$ 3,681.6	$ 5,469.0	$ 7,215.9
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	459.7	411.1	937.8	865.5
Research and development	585.1	647.6	1,099.3	1,123.9
Selling, general and administrative	637.3	555.1	1,232.3	1,125.2
Amortization and impairment of acquired intangible assets	604.1	61.5	702.2	133.0
Collaboration profit sharing	(15.2)	21.8	53.3	93.5
(Gain) loss on fair value remeasurement of contingent consideration	0.3	10.0	(33.5)	5.5
Acquired in-process research and development	18.0	0.0	18.0	75.0
Total cost and expense	2,289.3	1,707.1	4,009.4	3,421.6
Income from operations	485.7	1,974.5	1,459.6	3,794.3
Other income (expense), net	96.4	63.0	(410.5)	(57.5)
Income before income tax expense and equity in loss of investee, net of tax	582.1	2,037.5	1,049.1	3,736.8
Income tax (benefit) expense	(409.1)	446.1	(364.9)	738.2
Equity in (income) loss of investee, net of tax	(34.3)	(15.1)	(16.1)	(0.4)
Net income	1,025.5	1,606.5	1,430.1	2,999.0
Net income (loss) attributable to noncontrolling interests, net of tax	577.0	64.4	571.4	57.8
Net income attributable to Biogen Inc.	$ 448.5	$ 1,542.1	$ 858.7	$ 2,941.2
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 3.00	$ 9.60	$ 5.70	$ 17.65
Diluted earnings per share attributable to Biogen Inc.	$ 2.99	$ 9.59	$ 5.68	$ 17.61
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	149.7	160.6	150.8	166.7
Diluted earnings per share attributable to Biogen Inc.	150.1	160.9	151.2	167.0
Product, net
Total revenue	$ 2,236.0	$ 2,795.7	$ 4,447.7	$ 5,700.3
Revenue from anti-CD20 therapeutic programs
Total revenue	440.0	478.3	829.0	998.7
Other
Total revenue	$ 99.0	$ 407.6	$ 192.3	$ 516.9