Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 1,172.9	$ 747.6
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	(2.2)	(1.6)
Unrealized gains (losses) on cash flow hedges, net of tax	(29.0)	(23.8)
Unrealized gains (losses) on pension benefit obligation, net of tax	(0.5)	0.1
Currency translation adjustment	44.7	20.0
Total other comprehensive income (loss), net of tax	13.0	(5.3)
Comprehensive income attributable to Biogen Inc.	1,185.9	742.3
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	(1.7)	(0.1)
Comprehensive income	$ 1,184.2	$ 742.2