Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Total revenues	$ 3,439.0	$ 3,077.8	$ 9,926.6	$ 8,966.9
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	460.8	370.0	1,327.8	1,120.8
Research and development	507.9	446.4	1,985.6	1,666.0
Selling, general and administrative	497.7	433.4	1,515.2	1,361.9
Amortization of acquired intangible assets	281.9	108.9	493.2	674.9
Collaboration profit (loss) sharing	47.5	35.2	129.2	82.5
Acquired in-process research and development	27.5	0.0	112.5	120.0
Restructuring charges	6.0	0.0	9.2	0.0
(Gain) loss on fair value remeasurement of contingent consideration	(87.9)	30.0	(91.6)	61.2
Total cost and expenses	1,741.4	1,423.9	5,481.1	5,087.3
Income from operations	1,697.6	1,653.9	4,445.5	3,879.6
Other income (expense), net	115.1	(44.0)	39.6	(150.6)
Income before income tax expense and equity in loss of investee, net of tax	1,812.7	1,609.9	4,485.1	3,729.0
Income tax expense	369.8	383.8	956.0	892.6
Equity in loss of investee, net of tax	0.0	0.0	0.0	0.0
Net income	1,442.9	1,226.1	3,529.1	2,836.4
Net income (loss) attributable to noncontrolling interests, net of tax	(1.5)	0.0	45.2	(0.1)
Net income attributable to Biogen Inc.	$ 1,444.4	$ 1,226.1	$ 3,483.9	$ 2,836.5
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 7.17	$ 5.80	$ 16.86	$ 13.32
Diluted earnings per share attributable to Biogen Inc.	$ 7.15	$ 5.79	$ 16.83	$ 13.30
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	201.4	211.4	206.6	213.0
Diluted earnings per share attributable to Biogen Inc.	201.9	211.8	207.0	213.3
Product, net
Total revenues	$ 2,780.1	$ 2,622.5	$ 8,061.1	$ 7,642.3
Revenues from anti-CD20 therapeutic programs
Total revenues	511.7	406.5	1,445.3	1,144.2
Other
Total revenues	$ 147.2	$ 48.8	$ 420.2	$ 180.4