Revenue - Schedule of Estimated Investment Advisory, Administration Fees Expected to be Recognized in Future, Related to Unsatisfied Portion of Performance Obligations (Detail) - Investment Advisory, Administration Fees and Securities Lending Revenue [Member] - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Schedule of Investment Advisory Administration Fees and Performance Fees by Type [line Items]
Revenue, Remaining Performance Obligation	$ 591	$ 573
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2023-10-01
Schedule of Investment Advisory Administration Fees and Performance Fees by Type [line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period		3 months
Revenue, Remaining Performance Obligation		$ 51
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2024-01-01
Schedule of Investment Advisory Administration Fees and Performance Fees by Type [line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period		1 year
Revenue, Remaining Performance Obligation		$ 173
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2024-10-01
Schedule of Investment Advisory Administration Fees and Performance Fees by Type [line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	3 months
Revenue, Remaining Performance Obligation	$ 52
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2025-01-01
Schedule of Investment Advisory Administration Fees and Performance Fees by Type [line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year	1 year
Revenue, Remaining Performance Obligation	$ 191	$ 141
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2026-01-01
Schedule of Investment Advisory Administration Fees and Performance Fees by Type [line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year	1 year
Revenue, Remaining Performance Obligation	$ 172	$ 119
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2027-01-01
Schedule of Investment Advisory Administration Fees and Performance Fees by Type [line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year	1 year
Revenue, Remaining Performance Obligation	$ 128	$ 89
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2028-01-01
Schedule of Investment Advisory Administration Fees and Performance Fees by Type [line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Revenue, Remaining Performance Obligation	$ 48