Inventory - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2022	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2019	Sep. 26, 2023
Inventory [Line Items]
Net Income attributable to Biogen Inc.		$ 1,632.2	$ 1,161.5	$ 2,961.6
Cost of sales, excluding amortization and impairment of acquired intangible assets		2,310.4	2,533.4	2,278.3
Write-downs on excess, obsolete, unmarketable or other inventory		101.9	124.4	336.2
Income (loss) from equity method investments		0.0	0.0	2.6
Reata Pharmaceuticals, Inc
Inventory [Line Items]
Inventory		1,259.0					$ 1,300.0
Inventory amortization period							4 years
Reata Pharmaceuticals, Inc | Fair Value Adjustment to Inventory
Inventory [Line Items]
Net Income attributable to Biogen Inc.		230.0	$ 31.5
Eisai | Milestone Payments
Inventory [Line Items]
Percentage of future development costs	45.00%				45.00%	45.00%
ADUHELM
Inventory [Line Items]
Cost of sales, excluding amortization and impairment of acquired intangible assets	$ 275.0
Write-downs on excess, obsolete, unmarketable or other inventory	$ 275.0
Income (loss) from equity method investments				111.0
ADUHELM | Centers for Medicare and Medicaid Service | Eisai
Inventory [Line Items]
Income (loss) from equity method investments				$ 136.0
Clinical Inventory | Reata Pharmaceuticals, Inc | Fair Value Adjustment to Inventory
Inventory [Line Items]
Cost of sales, excluding amortization and impairment of acquired intangible assets		$ 48.5