v3.25.3
Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Statement of Comprehensive Income [Abstract]        
Net income attributable to Biogen Inc. $ 466.5 $ 388.5 $ 1,341.8 $ 1,365.5
Other comprehensive income (loss):        
Unrealized gains (losses) on cash flow hedges, net of tax 26.5 (41.0) (137.1) (6.2)
Unrealized gains (losses) on pension benefit obligation, net of tax 0.0 0.3 1.0 0.2
Currency translation adjustments, net of tax (3.2) 41.0 57.5 14.0
Total other comprehensive income (loss), net of tax 23.3 0.3 (78.6) 8.0
Comprehensive income (loss) attributable to Biogen Inc. $ 489.8 $ 388.8 $ 1,263.2 $ 1,373.5