Reclassification out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Mar. 31, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other (income) expense	$ (14.8)	$ (300.0)	$ (193.7)	$ (248.2)
Income Tax Expense (Benefit)	62.5	(72.9)	249.0	92.6
Product revenues	2,465.8	2,530.3	7,221.2	7,449.3
Net income (loss) attributable to Biogen Inc.	$ 388.5	(68.1)	$ 1,365.5	911.4
Samsung Bioepis
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Equity, ownership interest	49.90%		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.	$ (8.1)	(12.6)	$ (5.2)	4.4
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	(15.2)	0.0	(17.0)
Income Tax Expense (Benefit)	0.0	3.2	0.0	3.6
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)	(0.2)	(0.3)
Income Tax Expense (Benefit)	1.1	0.0	0.6	(2.3)
Product revenues	(7.6)	(1.7)	2.9	18.3
Operating Expenses	$ (1.5)	$ 1.2	$ (8.5)	$ 2.1