Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 866.6	$ 862.8	$ 2,039.5	$ 1,610.4
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	0.6	7.2	(1.6)	5.6
Unrealized gains (losses) on cash flow hedges, net of tax	132.8	(103.0)	103.8	(126.8)
Unrealized gains (losses) on pension benefit obligation, net of tax	0.9	(0.6)	0.4	(0.5)
Currency translation adjustment, net of tax	(92.0)	82.8	(47.3)	102.8
Total other comprehensive income (loss), net of tax	42.3	(13.6)	55.3	(18.9)
Comprehensive income attributable to Biogen Inc.	908.9	849.2	2,094.8	1,591.5
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	48.4	0.0	46.7	(0.1)
Comprehensive income	$ 957.3	$ 849.2	$ 2,141.5	$ 1,591.4