Loans and Allowance for Credit Losses - Loan Modifications by Delinquency Status (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	$ 3,253	$ 4,027
Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	2,992	3,213
30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	112	148
90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	149	666
Commercial | Commercial
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	517	770
Commercial | Commercial | Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	423	556
Commercial | Commercial | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	18	55
Commercial | Commercial | 90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	76	159
Commercial | Commercial real estate
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	706	1,209
Commercial | Commercial real estate | Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	688	752
Commercial | Commercial real estate | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	0	2
Commercial | Commercial real estate | 90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	18	455
Consumer | Residential mortgages
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	1,492	1,501
Consumer | Residential mortgages | Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	1,476	1,487
Consumer | Residential mortgages | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	6	4
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	410	430
Consumer | Residential mortgages | 90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	10	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	288	265
Consumer | Credit card
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	431	405
Consumer | Credit card | Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	318	298
Consumer | Credit card | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	74	70
Consumer | Credit card | 90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	39	37
Consumer | Other retail
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	107	142
Consumer | Other retail | Current
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	87	120
Consumer | Other retail | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	14	17
Consumer | Other retail | 90 Days or More Past Due
Financing Receivable, Modified [Line Items]
Loan balances modified during the prior 12 months	$ 6	$ 5