Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues:
Product, net	$ 9,188.5	$ 8,203.4	$ 5,542.3
Unconsolidated joint business	1,339.2	1,195.4	1,126.0
Other	236.1	304.5	263.9
Total revenues	10,763.8	9,703.3	6,932.2
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	1,240.4	1,171.0	857.7
Research and development	2,012.8	1,893.4	1,444.1
Selling, general and administrative	2,113.1	2,232.3	1,712.1
Amortization of acquired intangible assets	382.6	489.8	342.9
Restructuring charges	93.4	0.0	0.0
Collaboration profit sharing	0.0	0.0	85.4
(Gain) loss on fair value remeasurement of contingent consideration	30.5	(38.9)	(0.5)
Total cost and expenses	5,872.8	5,747.7	4,441.6
Gain on sale of rights	0.0	16.8	24.9
Income from operations	4,891.0	3,972.4	2,515.5
Other income (expense), net	(123.7)	(25.8)	(34.9)
Income before income tax expense and equity in loss of investee, net of tax	4,767.3	3,946.6	2,480.6
Income tax expense	1,161.6	989.9	601.0
Equity in loss of investee, net of tax	12.5	15.1	17.2
Net income	3,593.2	2,941.6	1,862.3
Net income attributable to noncontrolling interests, net of tax	46.2	6.8	0.0
Net income attributable to Biogen Inc.	$ 3,547.0	$ 2,934.8	$ 1,862.3
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 15.38	$ 12.42	$ 7.86
Diluted earnings per share attributable to Biogen Inc.	$ 15.34	$ 12.37	$ 7.81
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	230.7	236.4	236.9
Diluted earnings per share attributable to Biogen Inc.	231.2	237.2	238.3