CREDIT QUALITY AND THE ALLOWANCE FOR CREDIT LOSSES - Accruing and Nonaccruing Past Due Amounts (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Financing Receivable, Past Due [Line Items]
Loans and leases	$ 141,842	$ 145,959
Loans and leases, nonaccrual	1,527	1,364
Loans and leases, nonaccrual with no related ACL	467	453
Guaranteed residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	1,076	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	139,350	143,111
Current | Guaranteed residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	821	675
30-59
Financing Receivable, Past Due [Line Items]
Loans and leases	461	818
30-59 | Guaranteed residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	59	128
60-89
Financing Receivable, Past Due [Line Items]
Loans and leases	276	333
60-89 | Guaranteed residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	28	76
90+
Financing Receivable, Past Due [Line Items]
Loans and leases	228	333
90+ | Guaranteed residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	168	243
Commercial
Financing Receivable, Past Due [Line Items]
Loans and leases	71,934	74,445
Loans and leases, nonaccrual	939	774
Loans and leases, nonaccrual with no related ACL	136	101
Commercial | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	43,623	44,974
Loans and leases, nonaccrual	261	297
Loans and leases, nonaccrual with no related ACL	25	30
Commercial | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	28,311	29,471
Loans and leases, nonaccrual	678	477
Loans and leases, nonaccrual with no related ACL	111	71
Commercial | Current
Financing Receivable, Past Due [Line Items]
Loans and leases	70,735	73,336
Commercial | Current | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	43,311	44,591
Commercial | Current | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	27,424	28,745
Commercial | 30-59
Financing Receivable, Past Due [Line Items]
Loans and leases	99	212
Commercial | 30-59 | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	27	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	118	77
Commercial | 60-89 | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	17	18
Commercial | 60-89 | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	101	59
Commercial | 90+
Financing Receivable, Past Due [Line Items]
Loans and leases	43	46
Commercial | 90+ | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	7	6
Commercial | 90+ | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	36	40
Retail
Financing Receivable, Past Due [Line Items]
Loans and leases	69,908	71,514
Loans and leases, nonaccrual	588	590
Loans and leases, nonaccrual with no related ACL	331	352
Retail | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	31,890	31,332
Loans and leases, nonaccrual	153	177
Loans and leases, nonaccrual with no related ACL	126	144
Retail | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	15,534	15,040
Loans and leases, nonaccrual	279	285
Loans and leases, nonaccrual with no related ACL	195	198
Retail | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	6,383	8,258
Loans and leases, nonaccrual	44	61
Loans and leases, nonaccrual with no related ACL	6	7
Retail | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	11,265	11,834
Loans and leases, nonaccrual	52	28
Loans and leases, nonaccrual with no related ACL	3	3
Retail | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	4,836	5,050
Loans and leases, nonaccrual	60	39
Loans and leases, nonaccrual with no related ACL	1	0
Retail | Current
Financing Receivable, Past Due [Line Items]
Loans and leases	68,615	69,775
Retail | Current | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	31,417	30,499
Retail | Current | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	15,147	14,640
Retail | Current | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	6,190	8,005
Retail | Current | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	11,154	11,732
Retail | Current | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	4,707	4,899
Retail | 30-59
Financing Receivable, Past Due [Line Items]
Loans and leases	362	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	79	82
Retail | 30-59 | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	113	144
Retail | 30-59 | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	37	49
Retail | 30-59 | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	40	49
Retail | 60-89
Financing Receivable, Past Due [Line Items]
Loans and leases	158	256
Retail | 60-89 | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	45	118
Retail | 60-89 | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	29	33
Retail | 60-89 | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	36	48
Retail | 60-89 | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	20	23
Retail | 60-89 | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	28	34
Retail | 90+
Financing Receivable, Past Due [Line Items]
Loans and leases	185	287
Retail | 90+ | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	182	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