Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 1,545.9	$ 1,444.4	$ 4,448.8	$ 3,483.9
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	0.1	(0.2)	10.3	(1.8)
Unrealized gains (losses) on cash flow hedges, net of tax	59.1	5.2	38.1	109.0
Gains (losses) on net investment hedges	21.2	0.0	46.9	0.0
Unrealized gains (losses) on pension benefit obligation, net of tax	0.8	(0.2)	1.5	0.2
Currency translation adjustment	79.4	8.5	51.3	(38.8)
Total other comprehensive income (loss), net of tax	160.6	13.3	148.1	68.6
Comprehensive income attributable to Biogen Inc.	1,706.5	1,457.7	4,596.9	3,552.5
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	0.0	(1.5)	(0.4)	45.2
Comprehensive income	$ 1,706.5	$ 1,456.2	$ 4,596.5	$ 3,597.7