Equity - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other (income) expense	$ (108.2)	$ 1,095.5	$ (497.4)
Income tax (benefit) expense	632.8	52.5	992.3
Revenue	10,173.4	10,981.7	13,444.6
Net income attributable to Biogen Inc.	3,046.9	1,556.1	4,000.6
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.	145.2	(58.2)	18.2
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]
Income tax (benefit) expense	2.6	0.8	1.7
Other Operating Income (Expense), Net	(12.6)	(3.8)	(8.2)
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]
Income tax (benefit) expense	(19.8)	6.0	(0.1)
Revenue	201.6	(60.0)	18.3
Operating expense	(5.5)	(0.8)	3.3
Other Operating Income (Expense), Net	(0.3)	0.2	0.3
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 Operating Income (Expense), Net	38.1	(0.6)	2.9
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 Operating Income (Expense), Net	$ (58.9)	$ 0.0	$ 0.0