Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Revenues:
Product, net	$ 2,466.0	$ 2,198.6	$ 4,775.4	$ 4,370.9
Revenues from anti-CD20 therapeutic programs	349.2	337.5	678.7	668.1
Other	79.0	55.6	166.9	107.6
Total revenues	2,894.2	2,591.6	5,621.0	5,146.6
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	370.3	286.1	683.3	598.6
Research and development	473.1	490.7	910.4	951.3
Selling, general and administrative	492.4	491.9	989.7	1,052.3
Amortization of acquired intangible assets	92.9	92.0	181.7	187.9
(Gain) loss on fair value remeasurement of contingent consideration	10.6	(2.2)	12.9	5.6
Restructuring charges	0.0	0.0	9.7	0.0
Collaboration profit (loss) sharing	(5.6)	0.0	(5.6)	0.0
Total cost and expenses	1,433.7	1,358.5	2,782.1	2,795.6
Income from operations	1,460.5	1,233.1	2,838.9	2,351.0
Other income (expense), net	(58.5)	(10.9)	(111.3)	(25.9)
Income before income tax expense and equity in loss of investee, net of tax	1,402.0	1,222.2	2,727.6	2,325.1
Income tax expense	353.6	292.5	710.0	574.4
Equity in loss of investee, net of tax	0.0	4.9	0.0	5.7
Net income	1,048.4	924.8	2,017.6	1,745.0
Net income (loss) attributable to noncontrolling interests, net of tax	(1.4)	(2.5)	(3.1)	(4.8)
Net income attributable to Biogen Inc.	$ 1,049.8	$ 927.3	$ 2,020.7	$ 1,749.8
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 4.79	$ 3.94	$ 9.23	$ 7.44
Diluted earnings per share attributable to Biogen Inc.	$ 4.79	$ 3.93	$ 9.21	$ 7.42
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	219.1	235.3	219.0	235.1
Diluted earnings per share attributable to Biogen Inc.	219.4	235.7	219.3	235.7