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, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total	$ 117,880	$ 116,660
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	392	493
Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	273	177
Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	418	587
Retail
Financing Receivable, Past Due [Line Items]
Current	58,403	56,883
Total	61,147	59,877
Retail | Residential mortgages
Financing Receivable, Past Due [Line Items]
Current	19,375	18,664
Total	19,699	18,978
Retail | Home equity loans
Financing Receivable, Past Due [Line Items]
Current	772	945
Total	876	1,073
Retail | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Current	11,593	12,042
Total	12,148	12,710
Retail | Home equity loans serviced by others
Financing Receivable, Past Due [Line Items]
Current	275	355
Total	318	399
Retail | Home equity lines of credit serviced by others
Financing Receivable, Past Due [Line Items]
Current	56	79
Total	81	104
Retail | Automobile
Financing Receivable, Past Due [Line Items]
Current	10,849	10,729
Total	12,070	12,106
Retail | Education
Financing Receivable, Past Due [Line Items]
Current	9,513	8,694
Total	9,729	8,900
Retail | Credit cards
Financing Receivable, Past Due [Line Items]
Current	2,028	1,894
Total	2,133	1,991
Retail | Other retail
Financing Receivable, Past Due [Line Items]
Current	3,942	3,481
Total	4,093	3,616
Retail | Financing Receivables, 1 to 29 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	1,783	1,955
Retail | Financing Receivables, 1 to 29 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	129	131
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity loans
Financing Receivable, Past Due [Line Items]
Past Due	62	75
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Past Due	331	386
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity loans serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	23	21
Retail | Financing Receivables, 1 to 29 Days Past Due | Home equity lines of credit serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	11	15
Retail | Financing Receivables, 1 to 29 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	932	1,039
Retail | Financing Receivables, 1 to 29 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	162	159
Retail | Financing Receivables, 1 to 29 Days Past Due | Credit cards
Financing Receivable, Past Due [Line Items]
Past Due	53	53
Retail | Financing Receivables, 1 to 29 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	80	76
Retail | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	368	393
Retail | Financing Receivables, 30 to 59 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	28	37
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity loans
Financing Receivable, Past Due [Line Items]
Past Due	10	12
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Past Due	52	65
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity loans serviced by others
Financing Receivable, 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, Past Due [Line Items]
Past Due	2	2
Retail | Financing Receivables, 30 to 59 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	194	207
Retail | Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	27	23
Retail | Financing Receivables, 30 to 59 Days Past Due | Credit cards
Financing Receivable, Past Due [Line Items]
Past Due	20	14
Retail | Financing Receivables, 30 to 59 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	30	26
Retail | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	253	142
Retail | Financing Receivables 60 To 89 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	78	13
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity loans
Financing Receivable, Past Due [Line Items]
Past Due	25	3
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Past Due	27	22
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity loans serviced by others
Financing Receivable, 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, Past Due [Line Items]
Past Due	1	1
Retail | Financing Receivables 60 To 89 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	71	59
Retail | Financing Receivables 60 To 89 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	16	13
Retail | Financing Receivables 60 To 89 Days Past Due | Credit cards
Financing Receivable, Past Due [Line Items]
Past Due	11	10
Retail | Financing Receivables 60 To 89 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	21	18
Retail | Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	340	504
Retail | Financing Receivables 90 Days or More Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	89	133
Retail | Financing Receivables 90 Days or More Past Due | Home equity loans
Financing Receivable, Past Due [Line Items]
Past Due	7	38
Retail | Financing Receivables 90 Days or More Past Due | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Past Due	145	195
Retail | Financing Receivables 90 Days or More Past Due | Home equity loans serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	12	13
Retail | Financing Receivables 90 Days or More Past Due | Home equity lines of credit serviced by others
Financing Receivable, Past Due [Line Items]
Past Due	11	7
Retail | Financing Receivables 90 Days or More Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	24	72
Retail | Financing Receivables 90 Days or More Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	11	11
Retail | Financing Receivables 90 Days or More Past Due | Credit cards
Financing Receivable, Past Due [Line Items]
Past Due	21	20
Retail | Financing Receivables 90 Days or More Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	$ 20	$ 15