Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 3,547.0	$ 2,934.8	$ 1,862.3
Other comprehensive income:
Unrealized gains (losses) recognized during the period, net of tax	(1.7)	0.4	11.8
Less: reclassification adjustment for (gains) losses included in net income, net of tax	1.3	(6.4)	(10.4)
Unrealized gains (losses) on securities available for sale, net of tax	(0.4)	(6.0)	1.4
Unrealized gains (losses) recognized during the period, net of tax	110.8	101.7	(26.7)
Less: reclassification adjustment for (gains) losses included in net income, net of tax	(172.3)	(6.3)	13.7
Unrealized gains (losses) on cash flow hedges, net of tax	(61.5)	95.4	(13.0)
Unrealized gains (losses) on pension benefit obligation	(6.2)	(12.0)	2.1
Currency translation adjustment	(96.4)	(109.2)	37.1
Total other comprehensive income (loss), net of tax	(164.5)	(31.8)	27.6
Comprehensive income attributable to Biogen Inc.	3,382.5	2,903.0	1,889.9
Comprehensive income attributable to noncontrolling interests, net of tax	46.2	6.8	0.0
Comprehensive income	$ 3,428.7	$ 2,909.8	$ 1,889.9