Reclassification out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other (income) expense	$ (93.7)	$ (69.4)
Income Tax Expense (Benefit)	71.4	50.7
Product revenues	2,290.5	2,463.0
Net income (loss) attributable to Biogen Inc.	$ 393.4	387.9
Samsung Bioepis
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Equity, ownership interest	49.90%		49.90%
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income (loss) attributable to Biogen Inc.	$ 0.7	14.7
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	0.0	(0.5)
Income Tax Expense (Benefit)	0.0	0.1
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	0.1	(0.1)
Income Tax Expense (Benefit)	(0.2)	(1.9)
Product revenues	2.9	17.6
Operating Expenses	$ (2.1)	$ (0.5)