Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues:
Product, net	$ 2,622.5	$ 2,539.6	$ 7,642.3	$ 7,315.0
Revenues from anti-CD20 therapeutic programs	406.5	317.6	1,144.2	996.3
Other	48.8	98.6	180.4	265.5
Total revenues	3,077.8	2,955.8	8,966.9	8,576.8
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	370.0	416.9	1,120.8	1,100.2
Research and development	446.4	529.0	1,666.0	1,439.4
Selling, general and administrative	433.8	462.7	1,363.1	1,452.4
Amortization of acquired intangible assets	108.9	99.7	674.9	281.4
Acquired in-process research and development	0.0	0.0	120.0	0.0
Collaboration profit (loss) sharing	35.2	4.7	82.5	(0.9)
(Gain) loss on fair value remeasurement of contingent consideration	30.0	5.9	61.2	18.8
Restructuring charges	0.0	11.6	0.0	21.3
Total cost and expenses	1,424.3	1,530.5	5,088.5	4,312.6
Income from operations	1,653.5	1,425.3	3,878.4	4,264.2
Other income (expense), net	(43.6)	(58.1)	(149.4)	(169.4)
Income before income tax expense and equity in loss of investee, net of tax	1,609.9	1,367.2	3,729.0	4,094.8
Income tax expense	383.8	337.0	892.6	1,047.0
Equity in loss of investee, net of tax	0.0	0.0	0.0	0.0
Net income	1,226.1	1,030.2	2,836.4	3,047.8
Net income (loss) attributable to noncontrolling interests, net of tax	0.0	(2.7)	(0.1)	(5.8)
Net income attributable to Biogen Inc.	$ 1,226.1	$ 1,032.9	$ 2,836.5	$ 3,053.6
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 5.80	$ 4.72	$ 13.32	$ 13.95
Diluted earnings per share attributable to Biogen Inc.	$ 5.79	$ 4.71	$ 13.30	$ 13.92
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	211.4	218.9	213.0	219.0
Diluted earnings per share attributable to Biogen Inc.	211.8	219.4	213.3	219.4