Collaborative and Other Relationships - Summary of Activity Related to LEQEMBI Collaboration (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
LEQEMBI \| Alzheimer's collaboration revenue
Collaborative arrangements and non-collaborative arrangement transactions
Total development expense incurred by the collaboration related to the advancement of LEQEMBI	$ 54.9	$ 90.6
Expense reflected within statements of income	27.5	45.4
LEQEMBI \| Alzheimer's collaboration revenue \| Selling, general and administrative
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income	85.7	68.9
Expense incurred by the collaboration	$ 171.2	$ 137.7
E2609 and BAN2401 \| Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Total development expense incurred by the collaboration related to the advancement of LEQEMBI			$ 107.5	$ 177.9
Expense reflected within statements of income			53.8	89.0
E2609 and BAN2401 \| Eisai \| Selling, general and administrative
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income			174.5	155.8
Expense incurred by the collaboration			$ 348.9	$ 311.6