Share-Based Payments - Share-based Compensation Expense Included in Consolidation Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$ (274.4)	$ (263.5)	$ (246.6)
Subtotal	668.4	273.6	258.8
Capitalized share-based compensation costs	(10.2)	(9.3)	(8.0)
Share-based compensation expense included in total cost and expense	658.2	264.3	250.8
Income tax effect	(132.6)	(49.2)	(46.7)
Research and development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	296.7	98.5	89.3
Selling, general and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	371.7	175.1	169.5
Share-based compensation expense included in net income attributable to Biogen Inc.
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 525.6	$ 215.1	$ 204.1