Collaborative and Other Relationships - Eisai (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Collaborative arrangements and non-collaborative arrangement transactions
Product revenues	$ 2,456.0	$ 2,589.1	$ 4,919.0	$ 5,120.9
LEQEMBI Collaboration
Collaborative arrangements and non-collaborative arrangement transactions
Product revenues	(20.7)	$ 0.0	(39.6)	$ 0.0
Payable Due To Termination Agreement	$ 31.0		$ 31.0