Loans and Related Allowance for Credit Losses (Aging Analysis) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Financing Receivable [Line Items]
Financing receivable, aging analysis	$ 3,006	$ 1,946
Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	2,147	1,595
30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	24	5
60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	5	3
90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	7	9
Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	823	334
Total commercial
Financing Receivable [Line Items]
Financing receivable, aging analysis	2,816	1,782
Total commercial | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	2,087	1,556
Total commercial | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	18
Total commercial | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	1
Total commercial | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis		3
Total commercial | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	710	223
Total commercial | Commercial and industrial
Financing Receivable [Line Items]
Financing receivable, aging analysis	1,480	960
Total commercial | Commercial and industrial | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	1,213	801
Total commercial | Commercial and industrial | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	18
Total commercial | Commercial and industrial | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	1
Total commercial | Commercial and industrial | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis		3
Total commercial | Commercial and industrial | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	248	156
Total commercial | Commercial real estate
Financing Receivable [Line Items]
Financing receivable, aging analysis	1,336	822
Total commercial | Commercial real estate | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	874	755
Total commercial | Commercial real estate | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	462	67
Total consumer
Financing Receivable [Line Items]
Financing receivable, aging analysis	190	164
Total consumer | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	60	39
Total consumer | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	6	5
Total consumer | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	4	3
Total consumer | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	7	6
Total consumer | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	113	111
Total consumer | Residential real estate
Financing Receivable [Line Items]
Financing receivable, aging analysis	92	93
Total consumer | Residential real estate | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	8	4
Total consumer | Residential real estate | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	1	1
Total consumer | Residential real estate | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis
Total consumer | Residential real estate | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis
Total consumer | Residential real estate | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	83	88
Total consumer | Home equity
Financing Receivable [Line Items]
Financing receivable, aging analysis	32	22
Total consumer | Home equity | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	5	1
Total consumer | Home equity | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis
Total consumer | Home equity | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	27	21
Total consumer | Credit card
Financing Receivable [Line Items]
Financing receivable, aging analysis	60	45
Total consumer | Credit card | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	43	31
Total consumer | Credit card | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	5	4
Total consumer | Credit card | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	4	3
Total consumer | Credit card | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	7	6
Total consumer | Credit card | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	1	1
Total consumer | Education
Financing Receivable [Line Items]
Financing receivable, aging analysis	4	3
Total consumer | Education | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	4	3
Total consumer | Education | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis
Total consumer | Education | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis
Total consumer | Other consumer
Financing Receivable [Line Items]
Financing receivable, aging analysis	2	1
Total consumer | Other consumer | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis
Total consumer | Other consumer | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	$ 2	$ 1