Loans and Allowance for Credit Losses - Loan Modifications by Delinquency Status (Details) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	$ 4,027	$ 2,299
Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	3,213	1,957
30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	148	96
90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	666	246
Commercial | Commercial
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	770	248
Commercial | Commercial | Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	556	223
Commercial | Commercial | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	55	11
Commercial | Commercial | 90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	159	14
Commercial | Commercial real estate
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	1,209	537
Commercial | Commercial real estate | Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	752	347
Commercial | Commercial real estate | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	2	1
Commercial | Commercial real estate | 90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	455	189
Consumer | Residential mortgages
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	1,501	1,118
Consumer | Residential mortgages | Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	1,487	1,089
Consumer | Residential mortgages | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	4	15
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	430	263
Consumer | Residential mortgages | 90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	10	14
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	265	64
Consumer | Credit card
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	405	268
Consumer | Credit card | Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	298	192
Consumer | Credit card | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	70	54
Consumer | Credit card | 90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	37	22
Consumer | Other retail
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	142	128
Consumer | Other retail | Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	120	106
Consumer | Other retail | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	17	15
Consumer | Other retail | 90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	$ 5	$ 7