ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Recorded Investment in Retail Loans by Delinquency Status (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 1,036	$ 1,257
Total	116,838	116,660
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	392	493
Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	188	177
Financing Receivables 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	456	587
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	56,902	56,883
Past Due	946	1,039
Total	59,875	59,877
Retail | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	18,882	18,664
Past Due	181	183
Total	19,192	18,978
Retail | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	825	945
Past Due	40	53
Total	938	1,073
Retail | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	11,652	12,042
Past Due	242	282
Total	12,266	12,710
Retail | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	301	355
Past Due	21	23
Total	348	399
Retail | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	60	79
Past Due	14	10
Total	88	104
Retail | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	10,630	10,729
Past Due	287	338
Total	12,000	12,106
Retail | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	9,089	8,694
Past Due	50	47
Total	9,305	8,900
Retail | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,935	1,894
Past Due	44	44
Total	2,046	1,991
Retail | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	3,528	3,481
Past Due	67	59
Total	3,692	3,616
Retail | Financing Receivables, 1 to 29 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,027	1,955
Retail | Financing Receivables, 1 to 29 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	129	131
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	73	75
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	372	386
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	26	21
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	14	15
Retail | Financing Receivables, 1 to 29 Days Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,083	1,039
Retail | Financing Receivables, 1 to 29 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	166	159
Retail | Financing Receivables, 1 to 29 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	67	53
Retail | Financing Receivables, 1 to 29 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	97	76
Retail | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	379	393
Retail | Financing Receivables, 30 to 59 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	32	37
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	11	12
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	58	65
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	5	7
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	2
Retail | Financing Receivables, 30 to 59 Days Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	201	207
Retail | Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	25	23
Retail | Financing Receivables, 30 to 59 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	15	14
Retail | Financing Receivables, 30 to 59 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	30	26
Retail | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	165	142
Retail | Financing Receivables 60 To 89 Days Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	15	13
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5	3
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	31	22
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	3
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	67	59
Retail | Financing Receivables 60 To 89 Days Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13	13
Retail | Financing Receivables 60 To 89 Days Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	9	10
Retail | Financing Receivables 60 To 89 Days Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	21	18
Retail | Financing Receivables 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	402	504
Retail | Financing Receivables 90 Days or More Past Due | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	134	133
Retail | Financing Receivables 90 Days or More Past Due | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	24	38
Retail | Financing Receivables 90 Days or More Past Due | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	153	195
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	13
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	11	7
Retail | Financing Receivables 90 Days or More Past Due | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	19	72
Retail | Financing Receivables 90 Days or More Past Due | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	12	11
Retail | Financing Receivables 90 Days or More Past Due | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	20	20
Retail | Financing Receivables 90 Days or More Past Due | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 16	$ 15