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, 2022	Dec. 31, 2021	Dec. 31, 2020
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		$ 2.6	$ 34.9	$ 3.3
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income		173.6	161.5	109.6
Total ADUHELM Collaboration third party milestones		0.0	100.0	75.0
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	0.0	45.0	33.8
Aducanumab | Research and development | Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration		149.4	183.7	152.0
Expense reflected within statements of income		82.2	101.1	83.6
Aducanumab | Selling, general and administrative | Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration		134.2	562.3	353.0
Expense reflected within statements of income		$ 71.5	$ 301.4	$ 193.7