ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Recorded Investment in Retail Loans by Delinquency Status (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 1,190	$ 1,456
Total	114,720	110,617
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	450	501
Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	142	163
Financing Receivables 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	598	792
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	56,323	55,195
Past Due	967	1,090
Total	59,315	58,586
Retail | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	18,173	16,714
Past Due	184	184
Total	18,493	17,045
Retail | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	999	1,212
Past Due	53	78
Total	1,131	1,392
Retail | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	12,214	12,756
Past Due	266	289
Total	12,863	13,483
Retail | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	382	477
Past Due	23	36
Total	429	542
Retail | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	87	116
Past Due	10	12
Total	114	149
Retail | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	10,849	11,596
Past Due	297	335
Total	12,255	13,204
Retail | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	8,514	7,898
Past Due	47	76
Total	8,712	8,134
Retail | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,814	1,747
Past Due	40	38
Total	1,911	1,848
Retail | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	3,291	2,679
Past Due	47	42
Total	3,407	2,789
Retail | Financing Receivables, 1 to 29 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,025	2,301
Retail | Financing Receivables, 1 to 29 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	136	147
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	79	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	383	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	24	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	17	21
Retail | Financing Receivables, 1 to 29 Days Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,109	1,273
Retail | Financing Receivables, 1 to 29 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	151	160
Retail | Financing Receivables, 1 to 29 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	57	63
Retail | Financing Receivables, 1 to 29 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	69	68
Retail | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	358	433
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	11	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	55	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	6	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	189	220
Retail | Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	24	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	20	20
Retail | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	132	138
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	25	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	2	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	51	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	9	9
Retail | Financing Receivables 60 To 89 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	15	12
Retail | Financing Receivables 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	477	519
Retail | Financing Receivables 90 Days or More Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	134	120
Retail | Financing Receivables 90 Days or More Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	39	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	186	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	15	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	57	60
Retail | Financing Receivables 90 Days or More Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	10	41
Retail | Financing Receivables 90 Days or More Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	17	17
Retail | Financing Receivables 90 Days or More Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 12	$ 10