CREDIT QUALITY AND THE ALLOWANCE FOR CREDIT LOSSES - Accruing and Nonaccruing Past Due Amounts (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Financing Receivable, Past Due [Line Items]
Loans and leases	$ 143,188	$ 145,959
Loans and leases, nonaccrual	1,469	1,364
Loans and leases, nonaccrual with no related ACL	418	453
Guaranteed residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	1,091	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	140,248	143,111
Current | Guaranteed residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	714	675
30-59
Financing Receivable, Past Due [Line Items]
Loans and leases	915	818
30-59 | Guaranteed residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	116	128
60-89
Financing Receivable, Past Due [Line Items]
Loans and leases	256	333
60-89 | Guaranteed residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	59	76
90+
Financing Receivable, Past Due [Line Items]
Loans and leases	300	333
90+ | Guaranteed residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	202	243
Commercial
Financing Receivable, Past Due [Line Items]
Loans and leases	72,823	74,445
Loans and leases, nonaccrual	891	774
Loans and leases, nonaccrual with no related ACL	79	101
Commercial | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	43,951	44,974
Loans and leases, nonaccrual	294	297
Loans and leases, nonaccrual with no related ACL	25	30
Commercial | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	28,872	29,471
Loans and leases, nonaccrual	597	477
Loans and leases, nonaccrual with no related ACL	54	71
Commercial | Current
Financing Receivable, Past Due [Line Items]
Loans and leases	71,468	73,336
Commercial | Current | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	43,530	44,591
Commercial | Current | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	27,938	28,745
Commercial | 30-59
Financing Receivable, Past Due [Line Items]
Loans and leases	367	212
Commercial | 30-59 | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	76	62
Commercial | 30-59 | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	291	150
Commercial | 60-89
Financing Receivable, Past Due [Line Items]
Loans and leases	35	77
Commercial | 60-89 | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	28	18
Commercial | 60-89 | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	7	59
Commercial | 90+
Financing Receivable, Past Due [Line Items]
Loans and leases	62	46
Commercial | 90+ | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	23	6
Commercial | 90+ | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	39	40
Retail
Financing Receivable, Past Due [Line Items]
Loans and leases	70,365	71,514
Loans and leases, nonaccrual	578	590
Loans and leases, nonaccrual with no related ACL	339	352
Retail | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	31,512	31,332
Loans and leases, nonaccrual	174	177
Loans and leases, nonaccrual with no related ACL	136	144
Retail | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	15,113	15,040
Loans and leases, nonaccrual	288	285
Loans and leases, nonaccrual with no related ACL	192	198
Retail | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	7,277	8,258
Loans and leases, nonaccrual	47	61
Loans and leases, nonaccrual with no related ACL	6	7
Retail | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	11,646	11,834
Loans and leases, nonaccrual	29	28
Loans and leases, nonaccrual with no related ACL	4	3
Retail | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	4,817	5,050
Loans and leases, nonaccrual	40	39
Loans and leases, nonaccrual with no related ACL	1	0
Retail | Current
Financing Receivable, Past Due [Line Items]
Loans and leases	68,780	69,775
Retail | Current | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	30,742	30,499
Retail | Current | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	14,720	14,640
Retail | Current | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	7,085	8,005
Retail | Current | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	11,556	11,732
Retail | Current | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	4,677	4,899
Retail | 30-59
Financing Receivable, Past Due [Line Items]
Loans and leases	548	606
Retail | 30-59 | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	271	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	115	144
Retail | 30-59 | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	40	49
Retail | 30-59 | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	43	49
Retail | 60-89
Financing Receivable, Past Due [Line Items]
Loans and leases	221	256
Retail | 60-89 | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	116	118
Retail | 60-89 | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	26	33
Retail | 60-89 | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	30	48
Retail | 60-89 | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	19	23
Retail | 60-89 | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	30	34
Retail | 90+
Financing Receivable, Past Due [Line Items]
Loans and leases	238	287
Retail | 90+ | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	209	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	$ 27	$ 29