Loans and Allowance for Credit Losses - Loan Modifications by Delinquency Status (Details) - USD ($) $ in Millions	Mar. 31, 2024	Mar. 31, 2023
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	$ 3,664	$ 717
Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	3,124	591
30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	120	40
90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	420	86
Commercial | Commercial
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	597	182
Commercial | Commercial | Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	498	146
Commercial | Commercial | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	16	6
Commercial | Commercial | 90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	83	30
Commercial | Commercial real estate
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	995	40
Commercial | Commercial real estate | Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	712	6
Commercial | Commercial real estate | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	2	0
Commercial | Commercial real estate | 90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	281	34
Consumer | Residential mortgages
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	1,541	332
Consumer | Residential mortgages | Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	1,509	319
Consumer | Residential mortgages | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	17	3
Consumer | Residential mortgages | 30-89 Days Past Due | Loans purchased from GNMA mortgage pools
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	333	32
Consumer | Residential mortgages | 90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	15	10
Consumer | Residential mortgages | 90 Days or More Past Due | Loans purchased from GNMA mortgage pools
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	198	1
Consumer | Credit card
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	376	94
Consumer | Credit card | Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	275	56
Consumer | Credit card | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	67	28
Consumer | Credit card | 90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	34	10
Consumer | Other retail
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	155	69
Consumer | Other retail | Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	130	64
Consumer | Other retail | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	18	3
Consumer | Other retail | 90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	$ 7	$ 2