Share-Based Payments - Share-Based Compensation Expense Included In Statements Of Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Share-based Compensation Expense included in consolidated statements of income
Subtotal	$ 87.8	$ 84.1
Capitalized share-based compensation costs	(3.8)	(3.1)
Share-based compensation expense included in total cost and expense	84.0	81.0
Income tax effect	(16.5)	(16.0)
Research and development
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	34.8	32.1
Selling, general and administrative
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	53.0	52.0
Share-based compensation expense included in net income attributable to Biogen Inc.
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	$ 67.5	$ 65.0