Collaborative and Other Relationships - Summary of Activity Related to LEQEMBI Collaboration (Details) - LEQEMBI - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 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	$ 61.5	$ 54.9
Expense reflected within statements of income	30.8	27.5
Alzheimer's collaboration revenue \| Income Statement Location [Axis]: us-gaap:SellingGeneralAndAdministrativeExpense
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income	77.6	85.7
Expense incurred by the collaboration	$ 155.2	$ 171.2
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Total development expense incurred by the collaboration related to the advancement of LEQEMBI			$ 130.8	$ 107.5
Expense reflected within statements of income			65.4	53.8
Eisai \| Income Statement Location [Axis]: us-gaap:SellingGeneralAndAdministrativeExpense
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income			162.3	174.5
Expense incurred by the collaboration			$ 324.7	$ 348.9