Equity - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income (expense)	$ 1,095.5	$ (497.4)	$ (83.3)
Income tax benefit (expense)	52.5	992.3	1,158.0
Revenues	(10,981.7)	(13,444.6)	(14,377.9)
Net income attributable to Biogen Inc.	(1,556.1)	(4,000.6)	(5,888.5)
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.	(58.2)	18.2	125.6
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)	(3.8)	(8.2)	4.5
Income tax benefit (expense)	0.8	1.7	(0.9)
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	0.3	0.3
Income tax benefit (expense)	6.0	(0.1)	(0.6)
Revenues	(60.0)	18.3	118.6
Operating expense	(0.8)	3.3	(3.3)
Gains (losses) on net investment hedge | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income (expense)	$ (0.6)	$ 2.9	$ 7.0