Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenues:
Product, net	$ 2,639.7	$ 2,466.0	$ 5,019.8	$ 4,775.4
Revenues from anti-CD20 therapeutic programs	397.1	349.2	737.7	678.7
Other	41.6	79.0	131.6	166.9
Total revenues	3,078.4	2,894.2	5,889.1	5,621.0
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	366.2	370.3	750.8	683.3
Research and development	796.2	473.1	1,219.6	910.4
Selling, general and administrative	430.2	492.4	929.3	989.7
Amortization of acquired intangible assets	117.5	92.9	566.0	181.7
Acquired in-process research and development	120.0	0.0	120.0	0.0
Collaboration profit (loss) sharing	26.5	(5.6)	47.3	(5.6)
(Gain) loss on fair value remeasurement of contingent consideration	21.2	10.6	31.2	12.9
Restructuring charges	0.0	0.0	0.0	9.7
Total cost and expenses	1,877.8	1,433.7	3,664.2	2,782.1
Income from operations	1,200.6	1,460.5	2,224.9	2,838.9
Other income (expense), net	(68.2)	(58.5)	(105.8)	(111.3)
Income before income tax expense and equity in loss of investee, net of tax	1,132.4	1,402.0	2,119.1	2,727.6
Income tax expense	269.6	353.6	508.8	710.0
Equity in loss of investee, net of tax	0.0	0.0	0.0	0.0
Net income	862.8	1,048.4	1,610.3	2,017.6
Net income (loss) attributable to noncontrolling interests, net of tax	0.0	(1.4)	(0.1)	(3.1)
Net income attributable to Biogen Inc.	$ 862.8	$ 1,049.8	$ 1,610.4	$ 2,020.7
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 4.07	$ 4.79	$ 7.53	$ 9.23
Diluted earnings per share attributable to Biogen Inc.	$ 4.07	$ 4.79	$ 7.52	$ 9.21
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	211.9	219.1	213.7	219.0
Diluted earnings per share attributable to Biogen Inc.	212.2	219.4	214.0	219.3