CREDIT QUALITY AND THE ALLOWANCE FOR CREDIT LOSSES - Accruing and Nonaccruing Past Due Amounts (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Financing Receivable, Past Due [Line Items]
Loans and leases	$ 141,632	$ 145,959
Loans and leases, nonaccrual	1,687	1,364
Loans and leases, nonaccrual with no related ACL	435	453
Guaranteed residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	1,058	1,122
Loans and leases, nonaccrual	0	0
Loans and leases, nonaccrual with no related ACL	0	0
Current
Financing Receivable, Past Due [Line Items]
Loans and leases	139,011	143,111
Current | Guaranteed residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	830	675
30-59
Financing Receivable, Past Due [Line Items]
Loans and leases	508	818
30-59 | Guaranteed residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	54	128
60-89
Financing Receivable, Past Due [Line Items]
Loans and leases	257	333
60-89 | Guaranteed residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	29	76
90+
Financing Receivable, Past Due [Line Items]
Loans and leases	169	333
90+ | Guaranteed residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	145	243
Commercial
Financing Receivable, Past Due [Line Items]
Loans and leases	71,808	74,445
Loans and leases, nonaccrual	1,071	774
Loans and leases, nonaccrual with no related ACL	112	101
Commercial | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	43,825	44,974
Loans and leases, nonaccrual	219	297
Loans and leases, nonaccrual with no related ACL	48	30
Commercial | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	27,983	29,471
Loans and leases, nonaccrual	852	477
Loans and leases, nonaccrual with no related ACL	64	71
Commercial | Current
Financing Receivable, Past Due [Line Items]
Loans and leases	70,503	73,336
Commercial | Current | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	43,542	44,591
Commercial | Current | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	26,961	28,745
Commercial | 30-59
Financing Receivable, Past Due [Line Items]
Loans and leases	126	212
Commercial | 30-59 | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	54	62
Commercial | 30-59 | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	72	150
Commercial | 60-89
Financing Receivable, Past Due [Line Items]
Loans and leases	88	77
Commercial | 60-89 | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	5	18
Commercial | 60-89 | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	83	59
Commercial | 90+
Financing Receivable, Past Due [Line Items]
Loans and leases	20	46
Commercial | 90+ | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	5	6
Commercial | 90+ | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	15	40
Retail
Financing Receivable, Past Due [Line Items]
Loans and leases	69,824	71,514
Loans and leases, nonaccrual	616	590
Loans and leases, nonaccrual with no related ACL	323	352
Retail | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	32,379	31,332
Loans and leases, nonaccrual	169	177
Loans and leases, nonaccrual with no related ACL	129	144
Retail | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	15,992	15,040
Loans and leases, nonaccrual	281	285
Loans and leases, nonaccrual with no related ACL	186	198
Retail | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	5,540	8,258
Loans and leases, nonaccrual	46	61
Loans and leases, nonaccrual with no related ACL	5	7
Retail | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	11,118	11,834
Loans and leases, nonaccrual	59	28
Loans and leases, nonaccrual with no related ACL	2	3
Retail | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	4,795	5,050
Loans and leases, nonaccrual	61	39
Loans and leases, nonaccrual with no related ACL	1	0
Retail | Current
Financing Receivable, Past Due [Line Items]
Loans and leases	68,508	69,775
Retail | Current | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	31,923	30,499
Retail | Current | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	15,591	14,640
Retail | Current | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	5,347	8,005
Retail | Current | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	10,989	11,732
Retail | Current | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	4,658	4,899
Retail | 30-59
Financing Receivable, Past Due [Line Items]
Loans and leases	382	606
Retail | 30-59 | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	93	282
Retail | 30-59 | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	93	82
Retail | 30-59 | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	109	144
Retail | 30-59 | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	43	49
Retail | 30-59 | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	44	49
Retail | 60-89
Financing Receivable, Past Due [Line Items]
Loans and leases	169	256
Retail | 60-89 | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	48	118
Retail | 60-89 | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	27	33
Retail | 60-89 | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	38	48
Retail | 60-89 | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	25	23
Retail | 60-89 | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	31	34
Retail | 90+
Financing Receivable, Past Due [Line Items]
Loans and leases	149	287
Retail | 90+ | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	146	256
Retail | 90+ | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	0	0
Retail | 90+ | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	0	0
Retail | 90+ | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	2	2
Retail | 90+ | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	$ 1	$ 29