Reclassification out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Mar. 31, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other (income) expense	$ (85.2)	$ 121.2	$ (178.9)	$ 51.8
Income Tax Expense (Benefit)	115.1	114.8	186.5	165.5
Product revenues	2,464.9	2,456.0	4,755.4	4,919.0
Net income (loss) attributable to Biogen Inc.	$ 583.6	591.6	$ 977.0	979.5
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.	$ 2.2	2.3	$ 2.9	17.0
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	(1.3)	0.0	(1.8)
Income Tax Expense (Benefit)	0.0	0.3	0.0	0.4
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	(0.2)	(0.1)	(0.1)	(0.2)
Income Tax Expense (Benefit)	(0.3)	(0.4)	(0.5)	(2.3)
Product revenues	7.6	2.4	10.5	20.0
Operating Expenses	$ (4.9)	$ 1.4	$ (7.0)	$ 0.9