Equity - Reclassified From AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Total revenue	$ 2,477.8	$ 2,431.0
Other (income) expense, net	(19.7)	(68.4)
Income Tax Expense (Benefit)	58.2	70.7
Net income attributable to Biogen Inc.	319.5	240.5
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.	(20.7)	10.3
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	(24.3)	10.9
Operating Expenses	0.7	0.7
Other (income) expense, net	0.0	(0.1)
Income Tax Expense (Benefit)	$ 2.9	$ (1.2)