Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Total revenues	$ 3,376.1	$ 3,600.1	$ 10,592.0	$ 10,706.6
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	449.1	430.0	1,314.6	1,508.3
Research and development	1,140.9	540.4	2,264.8	1,588.9
Selling, general and administrative	573.1	554.5	1,698.3	1,709.8
Amortization and impairment of acquired intangible assets	63.3	68.0	196.3	206.3
Collaboration profit (loss) sharing	73.0	60.2	166.5	181.8
Loss on divestiture of Hillerød, Denmark manufacturing operations	0.0		0.0	95.5
(Gain) loss on fair value remeasurement of contingent consideration	(29.0)	(57.8)	(23.5)	(66.3)
Restructuring charges	0.0	0.3	0.0	1.5
Acquired in-process research and development	0.0	0.0	75.0	0.0
Total cost and expenses	2,289.7	1,793.8	5,711.3	5,441.7
Income from operations	1,086.4	1,806.3	4,880.7	5,264.9
Other income (expense), net	(128.6)	(27.3)	(186.1)	132.6
Income before income tax expense and equity in loss of investee, net of tax	957.8	1,779.0	4,694.6	5,397.5
Income tax expense	240.8	211.3	979.0	881.9
Equity in (income) loss of investee, net of tax	13.1	21.8	12.7	66.8
Net income	703.9	1,545.9	3,702.9	4,448.8
Net income (loss) attributable to noncontrolling interests, net of tax	2.4	0.0	60.2	0.0
Net income attributable to Biogen Inc.	$ 701.5	$ 1,545.9	$ 3,642.7	$ 4,448.8
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 4.47	$ 8.40	$ 22.29	$ 23.38
Diluted earnings per share attributable to Biogen Inc.	$ 4.46	$ 8.39	$ 22.25	$ 23.35
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	156.9	184.0	163.4	190.3
Diluted earnings per share attributable to Biogen Inc.	157.2	184.2	163.7	190.5
Product, net
Total revenues	$ 2,690.3	$ 2,894.7	$ 8,390.6	$ 8,455.0
Revenues from anti-CD20 therapeutic programs
Total revenues	560.1	595.8	1,558.8	1,689.6
Other
Total revenues	$ 125.7	$ 109.6	$ 642.6	$ 562.0