v3.26.3
Equity - Reclassified From AOCI (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]        
Total revenue $ 2,804.1 $ 2,534.7 $ 8,017.9 $ 7,611.2
Other (income) expense, net (137.4) (34.1) (175.6) (151.2)
Income tax (benefit) expense 11.6 90.8 104.7 270.7
Net (loss) income attributable to Biogen Inc. (20.6) 466.5 396.4 1,341.8
Reclassification out of Accumulated Other Comprehensive Income        
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]        
Net (loss) income attributable to Biogen Inc. 1.2 (20.2) (28.7) (24.5)
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 3.8 (29.1) (29.7) (41.1)
Operating Expenses (2.4) 6.7 (1.9) 14.0
Other (income) expense, net 0.0 (0.1) 0.0 (0.3)
Income tax (benefit) expense (0.2) 2.3 3.9 2.9
Gains (losses) on securities available for sale | Reclassification out of Accumulated Other Comprehensive Income        
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]        
Other (income) expense, net 0.0 0.0 (1.2) 0.0
Income tax (benefit) expense $ 0.0 $ 0.0 $ 0.2 $ 0.0