Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Product, net	$ 10,354.7	$ 9,817.9	$ 9,188.5
Revenues from anti-CD20 therapeutic programs	1,559.2	1,314.5	1,339.2
Other	360.0	316.4	236.1
Total revenues	12,273.9	11,448.8	10,763.8
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	(1,630.0)	(1,478.7)	(1,240.4)
Research and development	(2,253.6)	(1,973.3)	(2,012.8)
Selling, general and administrative	(1,935.5)	(1,947.9)	(2,113.1)
Amortization of acquired intangible assets	(814.7)	(385.6)	(382.6)
Acquired in-process research and development	120.0	0.0	0.0
Collaboration profit (loss) sharing	112.3	10.2	0.0
Loss (gain) on fair value remeasurement of contingent consideration	62.7	14.8	30.5
Restructuring charges	(0.9)	(33.1)	(93.4)
TECFIDERA litigation settlement charge	0.0	454.8	0.0
Total cost and expenses	6,929.7	6,298.4	5,872.8
Income from operations	5,344.2	5,150.4	4,891.0
Other income (expense), net	(215.4)	(217.4)	(123.7)
Income before income tax expense and equity in loss of investee, net of tax	5,128.8	4,933.0	4,767.3
Income tax expense	2,458.7	1,237.3	1,161.6
Equity in loss of investee, net of tax	0.0	0.0	12.5
Net income	2,670.1	3,695.7	3,593.2
Net income (loss) attributable to noncontrolling interests, net of tax	131.0	(7.1)	46.2
Net income attributable to Biogen Inc.	$ 2,539.1	$ 3,702.8	$ 3,547.0
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 11.94	$ 16.96	$ 15.38
Diluted earnings per share attributable to Biogen Inc.	$ 11.92	$ 16.93	$ 15.34
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	212.6	218.4	230.7
Diluted earnings per share attributable to Biogen Inc.	213.0	218.8	231.2