Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to Biogen Inc.	$ (68.1)	$ 1,134.7	$ 911.4	$ 2,496.5
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	14.5	(1.0)	15.7	(19.0)
Unrealized gains (losses) on cash flow hedges, net of tax	29.6	5.9	1.1	79.0
Gains (losses) on net investment hedges, net of tax	0.0	0.0	0.0	(25.5)
Unrealized gains (losses) on pension benefit obligation, net of tax	(0.1)	1.5	0.6	4.2
Currency translation adjustment	(30.5)	(59.6)	(11.8)	(95.6)
Total other comprehensive income (loss), net of tax	13.5	(53.2)	5.6	(56.9)
Comprehensive income (loss) attributable to Biogen Inc.	(54.6)	1,081.5	917.0	2,439.6
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	(0.2)	0.2	1.2	(84.4)
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	$ (54.8)	$ 1,081.7	$ 918.2	$ 2,355.2