Equity - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Mar. 31, 2022	Feb. 29, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income tax (benefit) expense	$ 135.3	$ 632.8	$ 52.5
Revenue	9,835.6	10,173.4	10,981.7
Net income attributable to Biogen Inc.	1,161.1	3,046.9	1,556.1
Samsung Bioepis
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income tax (benefit) expense		31.2
Ownership percentage				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.	(0.1)	145.2	(58.2)
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	(17.0)	(12.6)	(3.8)
Income tax (benefit) expense	3.6	2.6	0.8
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	(0.3)	(0.3)	0.2
Income tax (benefit) expense	(1.7)	(19.8)	6.0
Revenue	11.6	201.6	(60.0)
Operating expense	3.7	(5.5)	(0.8)
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	0.0	38.1	(0.6)
Accumulated Foreign Currency Adjustment Including Portion Attributable to Noncontrolling Interest | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other (income) expense	$ 0.0	$ (58.9)	$ 0.0