Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Total revenues	$ 3,681.6	$ 3,616.7	$ 7,215.9	$ 7,106.5
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	411.1	476.3	865.5	1,078.3
Research and development	647.6	484.8	1,123.9	1,048.5
Selling, general and administrative	555.1	587.6	1,125.2	1,155.3
Amortization and impairment of acquired intangible assets	61.5	70.1	133.0	138.3
Collaboration profit (loss) sharing	21.8	63.5	93.5	121.6
Loss on divestiture of Hillerød, Denmark manufacturing operations	0.0		0.0	113.2
(Gain) loss on fair value remeasurement of contingent consideration	10.0	(20.0)	5.5	(8.5)
Restructuring charges	0.0	0.8	0.0	1.2
Acquired in-process research and development	0.0	0.0	75.0	0.0
Total cost and expenses	1,707.1	1,660.8	3,421.6	3,647.9
Income from operations	1,974.5	1,955.9	3,794.3	3,458.6
Other income (expense), net	63.0	(197.4)	(57.5)	159.9
Income before income tax expense and equity in loss of investee, net of tax	2,037.5	1,758.5	3,736.8	3,618.5
Income tax expense	446.1	248.1	738.2	670.6
Equity in loss of investee, net of tax	(15.1)	16.3	(0.4)	45.0
Net income	1,606.5	1,494.1	2,999.0	2,902.9
Net income (loss) attributable to noncontrolling interests, net of tax	64.4	0.0	57.8	0.0
Net income attributable to Biogen Inc.	$ 1,542.1	$ 1,494.1	$ 2,941.2	$ 2,902.9
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 9.60	$ 7.85	$ 17.65	$ 15.01
Diluted earnings per share attributable to Biogen Inc.	$ 9.59	$ 7.85	$ 17.61	$ 14.99
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	160.6	190.3	166.7	193.4
Diluted earnings per share attributable to Biogen Inc.	160.9	190.4	167.0	193.7
Product, net
Total revenues	$ 2,795.7	$ 2,880.3	$ 5,700.3	$ 5,560.3
Revenues from anti-CD20 therapeutic programs
Total revenues	478.3	576.4	998.7	1,093.8
Other
Total revenues	$ 407.6	$ 160.0	$ 516.9	$ 452.4