ALLOWANCE FOR CREDIT LOSSES, NONACCRUAL LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Accruing and Nonaccruing Past Due Amounts (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Loans and leases	$ 128,163	$ 123,090
Nonaccrual	702	1,019
Nonaccrual with no ACL	386	380
Financing Receivables, 1 to 29 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	126,308	121,328
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	412	544
Financing Receivables, 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	166	138
Financing Receivables, 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	575	61
Commercial
Financing Receivable, Past Due [Line Items]
Loans and leases	60,350	60,793
Nonaccrual	183	458
Nonaccrual with no ACL	37	58
Commercial | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	44,500	44,173
Nonaccrual	171	280
Nonaccrual with no ACL	36	56
Commercial | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	14,264	14,652
Nonaccrual	11	176
Nonaccrual with no ACL	1	2
Commercial | Leases
Financing Receivable, Past Due [Line Items]
Loans and leases	1,586	1,968
Nonaccrual	1	2
Nonaccrual with no ACL	0	0
Commercial | Financing Receivables, 1 to 29 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	60,064	60,097
Commercial | Financing Receivables, 1 to 29 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	44,247	43,666
Commercial | Financing Receivables, 1 to 29 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	14,247	14,475
Commercial | Financing Receivables, 1 to 29 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Loans and leases	1,570	1,956
Commercial | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	67	208
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	47	198
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	6	1
Commercial | Financing Receivables, 30 to 59 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Loans and leases	14	9
Commercial | Financing Receivables, 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	27	9
Commercial | Financing Receivables, 60 to 89 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	26	9
Commercial | Financing Receivables, 60 to 89 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans and leases	0	0
Commercial | Financing Receivables, 60 to 89 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Loans and leases	1	0
Commercial | Financing Receivables, 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	9	21
Commercial | Financing Receivables, 90 Days or More Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans and leases	9	20
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	0	1
Retail
Financing Receivable, Past Due [Line Items]
Loans and leases	67,813	62,297
Nonaccrual	519	561
Nonaccrual with no ACL	349	322
Retail | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	22,822	19,539
Nonaccrual with no ACL	137	96
Retail | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	12,015	12,149
Nonaccrual with no ACL	186	207
Retail | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	14,549	12,153
Nonaccrual with no ACL	22	17
Retail | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	12,997	12,308
Nonaccrual with no ACL	2	2
Retail | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	5,430	6,148
Nonaccrual with no ACL	2	0
Retail | FHA, VA And USDA Loans
Financing Receivable, Past Due [Line Items]
Loans and leases	544	21
Retail | Financing Receivables, 1 to 29 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	66,244	61,231
Retail | Financing Receivables, 1 to 29 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	21,918	19,272
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	11,745	11,799
Retail | Financing Receivables, 1 to 29 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	14,324	11,870
Retail | Financing Receivables, 1 to 29 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	12,926	12,245
Retail | Financing Receivables, 1 to 29 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	5,331	6,045
Retail | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	345	336
Retail | Financing Receivables, 30 to 59 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	102	55
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	38	54
Retail | Financing Receivables, 30 to 59 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	131	157
Retail | Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	34	31
Retail | Financing Receivables, 30 to 59 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	40	39
Retail | Financing Receivables, 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	139	129
Retail | Financing Receivables, 60 to 89 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	52	15
Retail | Financing Receivables, 60 to 89 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	12	20
Retail | Financing Receivables, 60 to 89 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	39	54
Retail | Financing Receivables, 60 to 89 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	13	12
Retail | Financing Receivables, 60 to 89 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	23	28
Retail | Financing Receivables, 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Loans and leases	566	40
Retail | Financing Receivables, 90 Days or More Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Loans and leases	549	30
Nonaccrual	201	167
Retail | Financing Receivables, 90 Days or More Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Loans and leases	0	0
Nonaccrual	220	276
Retail | Financing Receivables, 90 Days or More Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Loans and leases	0	0
Nonaccrual	55	72
Retail | Financing Receivables, 90 Days or More Past Due | Education
Financing Receivable, Past Due [Line Items]
Loans and leases	1	2
Nonaccrual	23	18
Retail | Financing Receivables, 90 Days or More Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Loans and leases	16	8
Nonaccrual	$ 20	$ 28