Reclassification out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Mar. 31, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other (income) expense	$ 121.2	$ 428.6	$ 51.8	$ 165.3
Income Tax Expense (Benefit)	114.8	216.7	165.5	342.3
Product revenues	2,456.0	2,589.1	4,919.0	5,120.9
Net income attributable to Biogen Inc.	$ 591.6	1,058.0	$ 979.5	1,361.8
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 attributable to Biogen Inc.	$ 2.3	17.6	$ 17.0	34.5
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	(1.3)	(0.8)	(1.8)	(1.4)
Income Tax Expense (Benefit)	0.3	0.2	0.4	0.3
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	(0.1)	(0.1)	(0.2)	(0.2)
Income Tax Expense (Benefit)	(0.4)	(4.2)	(2.3)	(6.2)
Product revenues	2.4	44.6	20.0	65.5
Operating Expenses	1.4	(2.4)	0.9	(2.7)
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	0.0	39.2	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	$ 0.0	$ (58.9)	$ 0.0	$ (58.9)