Fair Value Measurements - Fair Values of Assets and Liabilities (Significant Unobservable Level 3 Inputs Nonrecurring Basis) (Details) - Nonrecurring - USD ($) $ in Millions	Mar. 31, 2025	Mar. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	$ 423	$ 354
Commercial loans held for sale | Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value		25
Commercial loans and leases | Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	261	130
Consumer and residential mortgage loans | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	145	197
OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	4	1
Bank premises and equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value		1
Private equity investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	13	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	410	354
Level 3 | Commercial loans held for sale | Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value		25
Level 3 | Commercial loans and leases | Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	261	130
Level 3 | Consumer and residential mortgage loans | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	145	197
Level 3 | OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	4	1
Level 3 | Bank premises and equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value		1
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		25
Level 3 | Comparable Company Analysis | Private equity investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value		0
Level 3 | Appraised Value | Commercial loans and leases | Commercial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	261	130
Level 3 | Appraised Value | Consumer and residential mortgage loans | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	145	197
Level 3 | Appraised Value | OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value	$ 4	1
Level 3 | Appraised Value | Bank premises and equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets, fair value		$ 1