Share-Based Payments - Share-based Compensation Expense Included in Consolidation Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$ (246.6)	$ (204.5)	$ (191.2)
Subtotal	258.8	211.3	225.4
Capitalized share-based compensation costs	(8.0)	(6.2)	(8.9)
Share-based compensation expense included in total cost and expense	250.8	205.1	216.5
Income tax effect	(46.7)	(33.5)	(35.7)
Research and development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	89.3	80.0	77.1
Selling, general and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	169.5	131.3	148.3
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	$ 204.1	$ 171.6	$ 180.8