v3.26.1
Equity - Reclassified From AOCI (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]        
Total revenue $ 2,736.0 $ 2,645.5 $ 5,213.8 $ 5,076.5
Other (income) expense, net (18.5) (48.7) (38.2) (117.1)
Income Tax Expense (Benefit) 34.9 109.2 93.1 179.9
Net income attributable to Biogen Inc. 97.5 634.8 417.0 875.3
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. (9.2) (14.6) (29.9) (4.3)
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 (9.2) (22.9) (33.5) (12.0)
Operating Expenses (0.2) 6.6 0.5 7.3
Other (income) expense, net 0.0 (0.1) 0.0 (0.2)
Income Tax Expense (Benefit) 1.2 1.8 4.1 0.6
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 (1.2) 0.0 (1.2) 0.0
Income Tax Expense (Benefit) $ 0.2 $ 0.0 $ 0.2 $ 0.0