Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 319.5	$ 240.5
Other comprehensive income (loss):
Unrealized gains (losses) on securities available for sale, net of tax	(1.8)	0.0
Unrealized gains (losses) on cash flow hedges, net of tax	52.6	(57.8)
Unrealized gains (losses) on pension benefit obligation, net of tax	(0.5)	0.4
Currency translation adjustments, net of tax	(8.5)	19.1
Total other comprehensive income (loss), net of tax	41.8	(38.3)
Comprehensive income (loss) attributable to Biogen Inc.	$ 361.3	$ 202.2