Fair Value Measurements - Fair Values of Assets and Liabilities (Significant Unobservable Level 3 Inputs Nonrecurring Basis) (Details) - Nonrecurring - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	$ 398	$ 364
Commercial loans held for sale | Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	47
Commercial loans and leases | Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	125	154
Consumer and residential mortgage loans | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	208	188
OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	6	18
Bank premises and equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	9	4
Private equity investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	3	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	395	364
Level 3 | Commercial loans held for sale | Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	47
Level 3 | Commercial loans and leases | Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	125	154
Level 3 | Consumer and residential mortgage loans | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	208	188
Level 3 | OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	6	18
Level 3 | Bank premises and equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	9	4
Level 3 | Private equity investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	0	0
Level 3 | Comparable Company Analysis | Commercial loans held for sale | Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	47
Level 3 | Comparable Company Analysis | Private equity investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	0	0
Level 3 | Appraised Value | Commercial loans and leases | Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	125	154
Level 3 | Appraised Value | Consumer and residential mortgage loans | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	208	188
Level 3 | Appraised Value | OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	6	18
Level 3 | Appraised Value | Bank premises and equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	$ 9	$ 4