Collaborative and Other Relationships - Summary of Activity Related to LEQEMBI Collaboration (Details) - LEQEMBI - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Alzheimer's collaboration revenue
Collaborative arrangements and non-collaborative arrangement transactions
Total development expense incurred by the collaboration related to the advancement of LEQEMBI	$ 57.5	$ 70.9
Expense reflected within statements of income	28.7	35.4
Alzheimer's collaboration revenue \| Income Statement Location [Axis]: us-gaap:SellingGeneralAndAdministrativeExpense
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income	83.9	88.3
Expense incurred by the collaboration	$ 167.7	$ 176.8
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Total development expense incurred by the collaboration related to the advancement of LEQEMBI			$ 188.3	$ 178.4
Expense reflected within statements of income			94.1	89.2
Eisai \| Income Statement Location [Axis]: us-gaap:SellingGeneralAndAdministrativeExpense
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income			246.2	262.8
Expense incurred by the collaboration			$ 492.4	$ 525.7