Collaborative and Other Relationships - ADUHELM Collaboration Agreement (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Collaborative arrangements and non-collaborative arrangement transactions
Excess and obsolescence charges related to inventory	$ 27.5	$ 17.4
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Amounts receivable	15.9		$ 1.4
Amounts payable	$ 180.7		$ 118.4