Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 701.5	$ 1,545.9	$ 3,642.7	$ 4,448.8
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	0.1	0.1	1.0	10.3
Unrealized gains (losses) on cash flow hedges, net of tax	(83.7)	59.1	(101.1)	38.1
Gains (losses) on net investment hedges	(11.3)	21.2	5.5	46.9
Unrealized gains (losses) on pension benefit obligation, net of tax	(0.5)	0.8	0.4	1.5
Currency translation adjustment	50.9	79.4	3.9	51.3
Total other comprehensive income (loss), net of tax	(44.5)	160.6	(90.3)	148.1
Comprehensive income	657.0	1,706.5	3,552.4	4,596.9
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	1.5	0.0	61.1	(0.4)
Comprehensive income attributable to Biogen Inc.	$ 658.5	$ 1,706.5	$ 3,613.5	$ 4,596.5