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Share-Based Payments - Share-based Compensation Expense Included in Consolidation Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$ (204.5)	$ (191.2)	$ (168.7)
Subtotal	211.3	225.4	181.6
Capitalized share-based compensation costs	(6.2)	(8.9)	(11.5)
Share-based compensation expense included in total cost and expenses	205.1	216.5	170.1
Income tax effect	(33.5)	(35.7)	(27.5)
Research and development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	80.0	77.1	75.8
Selling, general and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	131.3	148.3	105.8
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	$ 171.6	$ 180.8	$ 142.6

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Share based compensation expense included in costs and expenses.

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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