Equity - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Jun. 30, 2022	Apr. 30, 2022	Jun. 30, 2018	Feb. 29, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income tax (benefit) expense	$ 273.8	$ 135.3	$ 632.8
Revenue	9,675.9	9,835.6	10,173.4
Net income attributable to Biogen Inc.	1,632.2	1,161.1	3,046.9
Samsung Bioepis
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Ownership percentage				49.90%	49.90%	49.90%	15.00%
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.	4.2	(0.1)	145.2
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, net	0.0	(17.0)	(12.6)
Income tax (benefit) expense	0.0	3.6	2.6
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, net	(0.4)	(0.3)	(0.3)
Income tax (benefit) expense	(0.6)	(1.7)	(19.8)
Revenue	18.1	11.6	201.6
Operating expense	(12.9)	3.7	(5.5)
Gains (losses) on net investment hedges(1) | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other (income) expense, net	0.0	0.0	38.1
Currency translation adjustments | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other (income) expense, net	$ 0.0	$ 0.0	$ (58.9)