ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Recorded Investment in Retail Loans by Delinquency Status (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 1,128	$ 1,456
Total	111,425	110,617
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	418	501
Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	130	163
Financing Receivables 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	580	792
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	55,574	55,195
Past Due	936	1,090
Total	58,281	58,586
Retail | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	17,092	16,714
Past Due	165	184
Total	17,346	17,045
Retail | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,144	1,212
Past Due	67	78
Total	1,298	1,392
Retail | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	12,575	12,756
Past Due	272	289
Total	13,190	13,483
Retail | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	449	477
Past Due	30	36
Total	504	542
Retail | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	107	116
Past Due	12	12
Total	136	149
Retail | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	11,540	11,596
Past Due	260	335
Total	12,794	13,204
Retail | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	8,162	7,898
Past Due	50	76
Total	8,324	8,134
Retail | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,728	1,747
Past Due	37	38
Total	1,808	1,848
Retail | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,777	2,679
Past Due	43	42
Total	2,881	2,789
Retail | Financing Receivables, 1 to 29 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,771	2,301
Retail | Financing Receivables, 1 to 29 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	89	147
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	87	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	343	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	25	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	994	1,273
Retail | Financing Receivables, 1 to 29 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	112	160
Retail | Financing Receivables, 1 to 29 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	43	63
Retail | Financing Receivables, 1 to 29 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	61	68
Retail | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	343	433
Retail | Financing Receivables, 30 to 59 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	35	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	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	10	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	4	4
Retail | Financing Receivables, 30 to 59 Days Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	173	220
Retail | Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	22	23
Retail | Financing Receivables, 30 to 59 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	11	12
Retail | Financing Receivables, 30 to 59 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	21	20
Retail | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	106	138
Retail | Financing Receivables 60 To 89 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	9	18
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5	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	19	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	1	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	37	55
Retail | Financing Receivables 60 To 89 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	15	12
Retail | Financing Receivables 60 To 89 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	8	9
Retail | Financing Receivables 60 To 89 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	11	12
Retail | Financing Receivables 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	487	519
Retail | Financing Receivables 90 Days or More Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	121	120
Retail | Financing Receivables 90 Days or More Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	50	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	198	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	19	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	50	60
Retail | Financing Receivables 90 Days or More Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13	41
Retail | Financing Receivables 90 Days or More Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	18	17
Retail | Financing Receivables 90 Days or More Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 11	$ 10