Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 1,494.1	$ 866.6	$ 2,902.9	$ 2,039.5
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	3.3	0.6	10.2	(1.6)
Unrealized gains (losses) on cash flow hedges, net of tax	(37.9)	132.8	(21.0)	103.8
Gains (losses) on net investment hedges	11.7	0.0	25.7	0.0
Unrealized gains (losses) on pension benefit obligation, net of tax	0.1	0.9	0.7	0.4
Currency translation adjustment	(10.3)	(92.0)	(28.1)	(47.3)
Total other comprehensive income (loss), net of tax	(33.1)	42.3	(12.5)	55.3
Comprehensive income attributable to Biogen Inc.	1,461.0	908.9	2,890.4	2,094.8
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	(0.4)	48.4	(0.4)	46.7
Comprehensive income	$ 1,460.6	$ 957.3	$ 2,890.0	$ 2,141.5