Collaborative and Other Relationships - Summary of Activity Related to Aducanumab Collaboration (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Collaborative arrangements and non-collaborative arrangement transactions
Biogen's share of reimbursement from Eisai of ADUHELM milestone payments reflected in collaboration profit (loss) sharing in our consolidated statements of income			$ 34.9	$ 3.3	$ (77.4)
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income			161.5	109.6	174.3
Total ADUHELM collaboration third-party milestones		$ 75.0	100.0	75.0	0.0
Biogen's share of reimbursement from Eisai of ADUHELM milestone payments reflected in collaboration profit (loss) sharing in our consolidated statements of income	$ 33.8	$ 33.8	45.0	33.8	0.0
Aducanumab | Research and development | Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration			183.7	152.0	179.4
Expense reflected within statements of income			101.1	83.6	98.7
Aducanumab | Selling, general and administrative | Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration			562.3	353.0	27.4
Expense reflected within statements of income			$ 301.4	$ 193.7	$ 15.1