Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 1,049.8	$ 927.3	$ 2,020.7	$ 1,749.8
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	4.5	(0.2)	7.0	1.1
Unrealized gains (losses) on cash flow hedges, net of tax	29.3	(96.2)	(18.3)	(8.9)
Unrealized gains (losses) on pension benefit obligation	0.7	2.9	0.9	4.1
Currency translation adjustment	(58.0)	63.0	(48.4)	(37.8)
Total other comprehensive income (loss), net of tax	(23.5)	(30.5)	(58.8)	(41.5)
Comprehensive income attributable to Biogen Inc.	1,026.3	896.8	1,961.9	1,708.3
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	(1.4)	(2.3)	(3.1)	(4.5)
Comprehensive income	$ 1,024.9	$ 894.5	$ 1,958.8	$ 1,703.7