v3.25.3
Equity - Reclassified From AOCI (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]        
Total revenue $ 2,534.7 $ 2,465.8 $ 7,611.2 $ 7,221.2
Other (income) expense, net (34.1) (14.8) (151.2) (193.7)
Income Tax Expense (Benefit) 90.8 62.5 270.7 249.0
Net income attributable to Biogen Inc. 466.5 388.5 1,341.8 1,365.5
Reclassification out of Accumulated Other Comprehensive Income        
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]        
Net income attributable to Biogen Inc. (20.2) (8.1) (24.5) (5.2)
Gains (losses) on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income        
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]        
Total revenue (29.1) (7.6) (41.1) 2.9
Operating Expenses 6.7 (1.5) 14.0 (8.5)
Other (income) expense, net (0.1) (0.1) (0.3) (0.2)
Income Tax Expense (Benefit) $ 2.3 $ 1.1 $ 2.9 $ 0.6