Inventory - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Sep. 26, 2023
Inventory [Line Items]
Net Income attributable to Biogen Inc.	$ 1,292.9	$ 1,632.2	$ 1,161.5
Cost of sales, excluding amortization and impairment of acquired intangible assets	2,404.2	2,310.4	2,533.4
Excess and obsolescence charges related to inventory	29.2	101.9	124.4
Reata Pharmaceuticals, Inc
Inventory [Line Items]
Inventory	1,259.0			$ 1,300.0
Reata Pharmaceuticals, Inc | Business Combination, Pro Forma Information, Nonrecurring Adjustment, Inventory, Fair Value Adjustment
Inventory [Line Items]
Net Income attributable to Biogen Inc.	240.8	230.0	$ 31.5
Reata Pharmaceuticals, Inc | Business Combination, Pro Forma Information, Nonrecurring Adjustment, Inventory, Fair Value Adjustment | Clinical Inventory
Inventory [Line Items]
Cost of sales, excluding amortization and impairment of acquired intangible assets	$ 23.9	$ 48.5