Collaborative and Other Relationships - Summary of Activity Related to BAN2401 and Elenbecestat Collaboration (Details) - Eisai - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
E2609 and BAN2401
Collaborative arrangements and non-collaborative arrangement transactions
Total development expense incurred by the collaboration related to the advancement of LEQEMBI	$ 88.5	$ 109.3	$ 282.6	$ 264.9
Expense reflected within statements of income	44.3	54.7	141.3	132.5
Selling, general and administrative | E2609 and BAN2401
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income	81.7	14.5	95.5	34.2
Expense incurred by the collaboration	$ 163.4	29.0	$ 191.0	68.3
Selling, general and administrative | ADUHELM
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income		(3.9)		68.1
Expense incurred by the collaboration		$ (8.0)		$ 127.8