Collaborative and Other Relationships - ADUHELM Collaboration Agreement (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Collaborative arrangements and non-collaborative arrangement transactions
Excess and obsolescence charges related to inventory	$ 68.7	$ 62.1
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Amounts receivable	93.2		$ 1.4
Amounts payable	$ 146.2		$ 118.4