Loans - Wholesale Loan Portfolio, Payment Status of FDMs (Details) - Wholesale - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Secured by real estate
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	$ 783	$ 346
Secured by real estate | Criticized nonaccrual
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	375	64
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	408	281
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	0	1
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	2,506	1,609
Commercial and industrial | Criticized nonaccrual
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	800	507
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	1,683	1,077
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	10	21
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	13	4
Other
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	91	543
Other | Criticized nonaccrual
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	12	167
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	79	367
Other | 30–89 days past due and still accruing
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	0	9
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