Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues:
Product, net	$ 2,523.5	$ 2,380.1
Revenues from anti-CD20 therapeutic programs	443.2	340.6
Other	164.4	90.0
Total revenues	3,131.1	2,810.7
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	446.0	384.6
Research and development	496.7	423.4
Selling, general and administrative	501.3	498.7
Amortization of acquired intangible assets	103.9	448.5
Collaboration profit (loss) sharing	42.5	20.8
Acquired in-process research and development	10.0	0.0
(Gain) loss on fair value remeasurement of contingent consideration	(5.6)	10.0
Restructuring charges	1.6	0.0
Total cost and expenses	1,596.4	1,786.0
Income from operations	1,534.7	1,024.7
Other income (expense), net	(41.0)	(38.0)
Income before income tax expense and equity in loss of investee, net of tax	1,493.7	986.7
Income tax expense	322.5	239.2
Equity in loss of investee, net of tax	0.0	0.0
Net income	1,171.2	747.5
Net income (loss) attributable to noncontrolling interests, net of tax	(1.7)	(0.1)
Net income attributable to Biogen Inc.	$ 1,172.9	$ 747.6
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 5.55	$ 3.47
Diluted earnings per share attributable to Biogen Inc.	$ 5.54	$ 3.46
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	211.4	215.6
Diluted earnings per share attributable to Biogen Inc.	211.7	215.9