Loans - Wholesale Loan Portfolio, Payment Status of FDMs (Details) - Wholesale - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Secured by real estate
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	$ 346	$ 125
Secured by real estate | Criticized nonaccrual
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	64	8
Secured by real estate | Current and less than 30 days past due and still accruing
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	281	117
Secured by real estate | 30–89 days past due and still accruing
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	1	0
Secured by real estate | 90 or more days past due and still accruing
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	0	0
Commercial and industrial
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	1,609	1,028
Commercial and industrial | Criticized nonaccrual
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	507	315
Commercial and industrial | Current and less than 30 days past due and still accruing
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	1,077	703
Commercial and industrial | 30–89 days past due and still accruing
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	21	0
Commercial and industrial | 90 or more days past due and still accruing
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	4	10
Other
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	543	837
Other | Criticized nonaccrual
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	167	561
Other | Current and less than 30 days past due and still accruing
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	367	248
Other | 30–89 days past due and still accruing
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	9	28
Other | 90 or more days past due and still accruing
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	$ 0	$ 0