ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Recorded Investment in Retail Loans by Delinquency Status (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 1,257	$ 1,456
Total loans and leases	116,660	110,617
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	493	501
Financing Receivables, 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	177	163
Financing Receivables, 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	587	792
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans and leases	59,877	58,586
Retail | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	18,664	16,714
Past Due	183	184
Total loans and leases	18,978	17,045
Retail | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	945	1,212
Past Due	53	78
Total loans and leases	1,073	1,392
Retail | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	12,042	12,756
Past Due	282	289
Total loans and leases	12,710	13,483
Retail | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	355	477
Past Due	23	36
Total loans and leases	399	542
Retail | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	79	116
Past Due	10	12
Total loans and leases	104	149
Retail | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	10,729	11,596
Past Due	338	335
Total loans and leases	12,106	13,204
Retail | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	8,694	7,898
Past Due	47	76
Total loans and leases	8,900	8,134
Retail | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,894	1,747
Past Due	44	38
Total loans and leases	1,991	1,848
Retail | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	3,481	2,679
Past Due	59	42
Total loans and leases	3,616	2,789
Retail | Total retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	56,883	55,195
Total loans and leases	59,877	58,586
Retail | Financing Receivables, 1 to 29 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	131	147
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	75	102
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	386	438
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	21	29
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	15	21
Retail | Financing Receivables, 1 to 29 Days Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,039	1,273
Retail | Financing Receivables, 1 to 29 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	159	160
Retail | Financing Receivables, 1 to 29 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	53	63
Retail | Financing Receivables, 1 to 29 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	76	68
Retail | Financing Receivables, 1 to 29 Days Past Due | Total retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,955	2,301
Retail | Financing Receivables, 30 to 59 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	37	46
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	12	20
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	65	78
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	7	10
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2	4
Retail | Financing Receivables, 30 to 59 Days Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	207	220
Retail | Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	23	23
Retail | Financing Receivables, 30 to 59 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	14	12
Retail | Financing Receivables, 30 to 59 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	26	20
Retail | Financing Receivables, 30 to 59 Days Past Due | Total retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	393	433
Retail | Financing Receivables, 60 to 89 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13	18
Retail | Financing Receivables, 60 to 89 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3	4
Retail | Financing Receivables, 60 to 89 Days Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	22	23
Retail | Financing Receivables, 60 to 89 Days Past Due | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3	4
Retail | Financing Receivables, 60 to 89 Days Past Due | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1	1
Retail | Financing Receivables, 60 to 89 Days Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	59	55
Retail | Financing Receivables, 60 to 89 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13	12
Retail | Financing Receivables, 60 to 89 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	10	9
Retail | Financing Receivables, 60 to 89 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	18	12
Retail | Financing Receivables, 60 to 89 Days Past Due | Total retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	142	138
Retail | Financing Receivables, 90 Days or More Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	133	120
Retail | Financing Receivables, 90 Days or More Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	38	54
Retail | Financing Receivables, 90 Days or More Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	195	188
Retail | Financing Receivables, 90 Days or More Past Due | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13	22
Retail | Financing Receivables, 90 Days or More Past Due | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	7	7
Retail | Financing Receivables, 90 Days or More Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	72	60
Retail | Financing Receivables, 90 Days or More Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	11	41
Retail | Financing Receivables, 90 Days or More Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	20	17
Retail | Financing Receivables, 90 Days or More Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	15	10
Retail | Financing Receivables, 90 Days or More Past Due | Total retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 504	$ 519