Equity - Reclassified From AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other (income) expense, net	$ (68.4)	$ (93.7)
Income Tax Expense (Benefit)	70.7	71.4
Total revenue	2,431.0	2,290.5
Net income attributable to Biogen Inc.	240.5	393.4
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.	10.3	0.7
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]
Other (income) expense, net	(0.1)	0.1
Income Tax Expense (Benefit)	(1.2)	(0.2)
Total revenue	10.9	2.9
Operating Expenses	$ 0.7	$ (2.1)