Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Total revenues	$ 3,600.1	$ 3,439.0	$ 10,706.6	$ 9,926.6
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	430.0	460.8	1,508.3	1,327.8
Research and development	540.4	507.9	1,588.9	1,985.6
Selling, general and administrative	554.5	497.7	1,709.8	1,515.2
Amortization and impairment of acquired intangible assets	283.9	281.9	422.2	493.2
Collaboration profit (loss) sharing	60.2	47.5	181.8	129.2
Loss on divestiture of Hillerød, Denmark manufacturing operations	(17.7)	0.0	95.5	0.0
(Gain) loss on fair value remeasurement of contingent consideration	(57.8)	(87.9)	(66.3)	(91.6)
Restructuring charges	0.3	6.0	1.5	9.2
Acquired in-process research and development	0.0	27.5	0.0	112.5
Total cost and expenses	1,793.8	1,741.4	5,441.7	5,481.1
Income from operations	1,806.3	1,697.6	5,264.9	4,445.5
Other income (expense), net	(27.3)	115.1	132.6	39.6
Income before income tax expense and equity in loss of investee, net of tax	1,779.0	1,812.7	5,397.5	4,485.1
Income tax expense	211.3	369.8	881.9	956.0
Equity in loss of investee, net of tax	21.8	0.0	66.8	0.0
Net income	1,545.9	1,442.9	4,448.8	3,529.1
Net income (loss) attributable to noncontrolling interests, net of tax	0.0	(1.5)	0.0	45.2
Net income attributable to Biogen Inc.	$ 1,545.9	$ 1,444.4	$ 4,448.8	$ 3,483.9
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 8.40	$ 7.17	$ 23.38	$ 16.86
Diluted earnings per share attributable to Biogen Inc.	$ 8.39	$ 7.15	$ 23.35	$ 16.83
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	184.0	201.4	190.3	206.6
Diluted earnings per share attributable to Biogen Inc.	184.2	201.9	190.5	207.0
Product, net
Total revenues	$ 2,894.7	$ 2,780.1	$ 8,455.0	$ 8,061.1
Revenues from anti-CD20 therapeutic programs
Total revenues	595.8	511.7	1,689.6	1,445.3
Other
Total revenues	$ 109.6	$ 147.2	$ 562.0	$ 420.2