Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 1,632.2	$ 1,161.1	$ 3,046.9
Other comprehensive income (loss):
Unrealized gains (losses) on securities available for sale, net of tax	0.0	15.7	(13.5)
Unrealized gains (losses) on cash flow hedges, net of tax	76.6	(40.1)	(38.7)
Gains (losses) on net investment hedges, net of tax	0.0	0.0	(25.5)
Unrealized gains (losses) on pension benefit obligation, net of tax	(14.0)	(1.5)	43.7
Currency translation adjustments, net of tax	(45.1)	37.1	(24.2)
Total other comprehensive income (loss), net of tax	17.5	11.2	(58.2)
Comprehensive income (loss) attributable to Biogen Inc.	1,649.7	1,172.3	2,988.7
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	0.0	0.4	(85.3)
Comprehensive income (loss)	$ 1,649.7	$ 1,172.7	$ 2,903.4