Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 240.5	$ 393.4
Other comprehensive income (loss):
Unrealized gains (losses) on cash flow hedges, net of tax	(57.8)	14.7
Unrealized gains (losses) on pension benefit obligation, net of tax	0.4	(0.1)
Currency translation adjustments, net of tax	19.1	(21.4)
Total other comprehensive income (loss), net of tax	(38.3)	(6.8)
Comprehensive income (loss) attributable to Biogen Inc.	$ 202.2	$ 386.6