Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Total revenues	$ 3,356.5	$ 3,078.4	$ 6,487.6	$ 5,889.1
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	421.0	366.2	867.0	750.8
Research and development	981.0	796.2	1,477.7	1,219.6
Selling, general and administrative	516.2	429.8	1,017.5	928.5
Amortization of acquired intangible assets	107.4	117.5	211.3	566.0
Acquired in-process research and development	75.0	120.0	85.0	120.0
Collaboration profit (loss) sharing	39.2	26.5	81.7	47.3
Loss (gain) on fair value remeasurement of contingent consideration	1.9	21.2	(3.7)	31.2
Restructuring charges	1.6	0.0	3.2	0.0
Total cost and expenses	2,143.3	1,877.4	3,739.7	3,663.4
Income from operations	1,213.2	1,201.0	2,747.9	2,225.7
Other income (expense), net	(34.5)	(68.6)	(75.5)	(106.6)
Income before income tax expense and equity in loss of investee, net of tax	1,178.7	1,132.4	2,672.4	2,119.1
Income tax expense	263.7	269.6	586.2	508.8
Equity in loss of investee, net of tax	0.0	0.0	0.0	0.0
Net income	915.0	862.8	2,086.2	1,610.3
Net income (loss) attributable to noncontrolling interests, net of tax	48.4	0.0	46.7	(0.1)
Net income attributable to Biogen Inc.	$ 866.6	$ 862.8	$ 2,039.5	$ 1,610.4
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 4.18	$ 4.07	$ 9.75	$ 7.53
Diluted earnings per share attributable to Biogen Inc.	$ 4.18	$ 4.07	$ 9.73	$ 7.52
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	207.1	211.9	209.2	213.7
Diluted earnings per share attributable to Biogen Inc.	207.3	212.2	209.5	214.0
Product, net
Total revenues	$ 2,757.5	$ 2,639.7	$ 5,281.0	$ 5,019.8
Revenues from anti-CD20 therapeutic programs
Total revenues	490.4	397.1	933.6	737.7
Other
Total revenues	$ 108.6	$ 41.6	$ 273.0	$ 131.6