Asset Quality - Schedule of Performance of Loans That Have Been Modified in the Last 12 Months (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	$ 431	$ 430
Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	410	373
30-89 Days  Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	8	22
90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	13	35
Total commercial loans
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	398	400
Total commercial loans | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	380	348
Total commercial loans | 30-89 Days  Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	6	18
Total commercial loans | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	12	34
Total commercial loans | Commercial and Industrial
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	116	153
Total commercial loans | Commercial and Industrial | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	102	132
Total commercial loans | Commercial and Industrial | 30-89 Days  Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	4	18
Total commercial loans | Commercial and Industrial | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	10	3
Total commercial loans | Total commercial real estate loans
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	282	400
Total commercial loans | Total commercial real estate loans | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	278	348
Total commercial loans | Total commercial real estate loans | 30-89 Days  Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	2	18
Total commercial loans | Total commercial real estate loans | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	2	34
Total commercial loans | Commercial mortgage
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	248	220
Total commercial loans | Commercial mortgage | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	244	189
Total commercial loans | Commercial mortgage | 30-89 Days  Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	2	0
Total commercial loans | Commercial mortgage | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	2	31
Total commercial loans | Construction
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	34	27
Total commercial loans | Construction | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	34	27
Total commercial loans | Construction | 30-89 Days  Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total commercial loans | Construction | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total commercial loans | Commercial lease financing
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total commercial loans | Commercial lease financing | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total commercial loans | Commercial lease financing | 30-89 Days  Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total commercial loans | Commercial lease financing | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total consumer loans
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	33	30
Total consumer loans | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	30	25
Total consumer loans | 30-89 Days  Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	2	4
Total consumer loans | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	1	1
Total consumer loans | Real estate — residential mortgage
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	13	12
Total consumer loans | Real estate — residential mortgage | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	12	9
Total consumer loans | Real estate — residential mortgage | 30-89 Days  Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	1	3
Total consumer loans | Real estate — residential mortgage | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total consumer loans | Home equity loans
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	11	11
Total consumer loans | Home equity loans | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	9	9
Total consumer loans | Home equity loans | 30-89 Days  Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	1	1
Total consumer loans | Home equity loans | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	1	1
Total consumer loans | Other consumer loans
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	6	3
Total consumer loans | Other consumer loans | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	6	3
Total consumer loans | Other consumer loans | 30-89 Days  Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total consumer loans | Other consumer loans | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total consumer loans | Credit cards
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	3	4
Total consumer loans | Credit cards | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	3	4
Total consumer loans | Credit cards | 30-89 Days  Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total consumer loans | Credit cards | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	$ 0	$ 0