Collaborative and Other Relationships - Summary of Activity Related to BAN2401 and Elenbecestat Collaboration (Details) - E2609 and BAN2401 - Eisai - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Collaborative arrangements and non-collaborative arrangement transactions
Total development expense incurred by the collaboration related to the advancement of LEQEMBI	$ 77.7	$ 88.5	$ 255.6	$ 282.6
Expense reflected within statements of income	38.8	44.3	127.8	141.3
Selling, general and administrative
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income	82.3	81.7	238.1	95.5
Expense incurred by the collaboration	$ 164.6	$ 163.4	$ 476.2	$ 191.0