ALLOWANCE FOR CREDIT LOSSES, NONACCRUAL LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Accruing and Nonaccruing Past Due Amounts (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Financing Receivable, Past Due [Line Items]
Loans and leases	$ 131,305	$ 128,163
Loans and leases, nonaccrual	789	702
Loans and leases, nonaccrual with no related ACL	413	386
Financial Asset, 1 to 29 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	129,184	126,308
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	369	412
Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	137	166
Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	826	575
Commercial
Financing Receivable, Past Due [Line Items]
Loans and leases	61,521	60,350
Loans and leases, nonaccrual	212	183
Loans and leases, nonaccrual with no related ACL	32	37
Commercial | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	45,724	44,500
Loans and leases, nonaccrual	200	171
Loans and leases, nonaccrual with no related ACL	31	36
Commercial | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	14,268	14,264
Loans and leases, nonaccrual	11	11
Loans and leases, nonaccrual with no related ACL	1	1
Commercial | Leases
Financing Receivable, Past Due [Line Items]
Loans and leases	1,529	1,586
Loans and leases, nonaccrual	1	1
Loans and leases, nonaccrual with no related ACL	0	0
Commercial | Financial Asset, 1 to 29 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	61,211	60,064
Commercial | Financial Asset, 1 to 29 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	45,477	44,247
Commercial | Financial Asset, 1 to 29 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	14,256	14,247
Commercial | Financial Asset, 1 to 29 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Loans and leases	1,478	1,570
Commercial | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	63	67
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	18	47
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	0	6
Commercial | Financing Receivables, 30 to 59 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Loans and leases	45	14
Commercial | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	17	27
Commercial | Financing Receivables 60 To 89 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	16	26
Commercial | Financing Receivables 60 To 89 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	1	0
Commercial | Financing Receivables 60 To 89 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Loans and leases	0	1
Commercial | Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	18	9
Commercial | Financing Receivables 90 Days or More Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	13	9
Commercial | Financing Receivables 90 Days or More Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	0	0
Commercial | Financing Receivables 90 Days or More Past Due | Leases
Financing Receivable, Past Due [Line Items]
Loans and leases	5	0
Retail
Financing Receivable, Past Due [Line Items]
Loans and leases	69,784	67,813
Loans and leases, nonaccrual	577	519
Loans and leases, nonaccrual with no related ACL	381	349
Retail | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	24,211	22,822
Loans and leases, nonaccrual with no related ACL	179	137
Retail | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	12,264	12,015
Loans and leases, nonaccrual with no related ACL	188	186
Retail | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	14,439	14,549
Loans and leases, nonaccrual with no related ACL	10	22
Retail | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	13,306	12,997
Loans and leases, nonaccrual with no related ACL	3	2
Retail | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	5,564	5,430
Loans and leases, nonaccrual with no related ACL	1	2
Retail | FHA, VA And USDA Loans
Financing Receivable, Past Due [Line Items]
Loans and leases, 90 days or more past due and still accruing	792	544
Retail | Financial Asset, 1 to 29 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	67,973	66,244
Retail | Financial Asset, 1 to 29 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	23,077	21,918
Retail | Financial Asset, 1 to 29 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	11,969	11,745
Retail | Financial Asset, 1 to 29 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	14,236	14,324
Retail | Financial Asset, 1 to 29 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	13,244	12,926
Retail | Financial Asset, 1 to 29 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	5,447	5,331
Retail | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	306	345
Retail | Financing Receivables, 30 to 59 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	57	102
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	42	38
Retail | Financing Receivables, 30 to 59 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	120	131
Retail | Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	26	34
Retail | Financing Receivables, 30 to 59 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	61	40
Retail | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	120	139
Retail | Financing Receivables 60 To 89 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	42	52
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	14	12
Retail | Financing Receivables 60 To 89 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	31	39
Retail | Financing Receivables 60 To 89 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	11	13
Retail | Financing Receivables 60 To 89 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	22	23
Retail | Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	808	566
Retail | Financing Receivables 90 Days or More Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	792	549
Loans and leases, nonaccrual	243	201
Retail | Financing Receivables 90 Days or More Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	0	0
Loans and leases, nonaccrual	239	220
Retail | Financing Receivables 90 Days or More Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	0	0
Loans and leases, nonaccrual	52	55
Retail | Financing Receivables 90 Days or More Past Due | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	2	1
Loans and leases, nonaccrual	23	23
Retail | Financing Receivables 90 Days or More Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	14	16
Loans and leases, nonaccrual	$ 20	$ 20