Collaborative and Other Relationships - Summary of Activity Related to Aducanumab Collaboration (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Collaborative arrangements and non-collaborative arrangement transactions
Biogen's share of reimbursement from Eisai of ADUHELM milestone payments reflected in collaboration profit sharing/(loss reimbursement) in our consolidated statements of income	$ 0.0	$ 2.6	$ 34.9
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income	$ 186.0	173.6	161.5
Total ADUHELM Collaboration third party milestones		0.0	100.0
Biogen's share of reimbursement from Eisai of ADUHELM milestone payments reflected in collaboration profit sharing/(loss reimbursement) in our consolidated statements of income		0.0	45.0
Aducanumab | Research and development | Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration		149.4	183.7
Expense reflected within statements of income		82.2	101.1
Aducanumab | Selling, general and administrative | Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Expense incurred by the collaboration		134.2	562.3
Expense reflected within statements of income		$ 71.5	$ 301.4