ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Troubled Debt Restructuring (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) contract	Dec. 31, 2018 USD ($) contract	Dec. 31, 2017 USD ($) contract
Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	3,678	2,641	2,724
Recorded Investment | $	$ 53	$ 32	$ 36
Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	182	339	386
Recorded Investment | $	$ 26	$ 88	$ 66
Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	2,632	2,334	2,562
Recorded Investment | $	$ 279	$ 308	$ 161
Commercial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	3	7	7
Recorded Investment | $	$ 0	$ 1	$ 1
Commercial | Interest Rate Reduction | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	3	7	7
Recorded Investment | $	$ 0	$ 1	$ 1
Commercial | Interest Rate Reduction | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0
Commercial | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	27	52	46
Recorded Investment | $	$ 5	$ 53	$ 22
Commercial | Maturity Extension | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	26	49	45
Recorded Investment | $	$ 5	$ 22	$ 22
Commercial | Maturity Extension | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	1	3	1
Recorded Investment | $	$ 0	$ 31	$ 0
Commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	56	55	16
Recorded Investment | $	$ 210	$ 231	$ 71
Commercial | Other | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	56	53	15
Recorded Investment | $	$ 210	$ 200	$ 71
Commercial | Other | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	2	1
Recorded Investment | $	$ 0	$ 31	$ 0
Retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	3,675	2,634	2,717
Recorded Investment | $	$ 53	$ 31	$ 35
Retail | Interest Rate Reduction | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	60	35	71
Recorded Investment | $	$ 12	$ 4	$ 10
Retail | Interest Rate Reduction | Home equity loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	31	43	82
Recorded Investment | $	$ 2	$ 4	$ 6
Retail | Interest Rate Reduction | Home equity lines of credit
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	163	76	50
Recorded Investment | $	$ 18	$ 7	$ 3
Retail | Interest Rate Reduction | Home equity loans serviced by others
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	2	4	15
Recorded Investment | $	$ 0	$ 0	$ 1
Retail | Interest Rate Reduction | Home equity lines of credit serviced by others
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	5	5
Recorded Investment | $	$ 0	$ 0	$ 0
Retail | Interest Rate Reduction | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	160	158	130
Recorded Investment | $	$ 3	$ 3	$ 2
Retail | Interest Rate Reduction | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0
Retail | Interest Rate Reduction | Credit cards
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	3,259	2,312	2,363
Recorded Investment | $	$ 18	$ 13	$ 13
Retail | Interest Rate Reduction | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	1	1
Recorded Investment | $	$ 0	$ 0	$ 0
Retail | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	155	287	340
Recorded Investment | $	$ 21	$ 35	$ 44
Retail | Maturity Extension | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	62	61	73
Recorded Investment | $	$ 10	$ 8	$ 13
Retail | Maturity Extension | Home equity loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	1	1
Recorded Investment | $	$ 0	$ 0	$ 0
Retail | Maturity Extension | Home equity lines of credit
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	72	178	235
Recorded Investment | $	$ 11	$ 26	$ 30
Retail | Maturity Extension | Home equity loans serviced by others
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0
Retail | Maturity Extension | Home equity lines of credit serviced by others
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	1	2
Recorded Investment | $	$ 0	$ 0	$ 0
Retail | Maturity Extension | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	21	46	29
Recorded Investment | $	$ 0	$ 1	$ 1
Retail | Maturity Extension | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0
Retail | Maturity Extension | Credit cards
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0
Retail | Maturity Extension | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0
Retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	2,576	2,279	2,546
Recorded Investment | $	$ 69	$ 77	$ 90
Retail | Other | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	120	142	171
Recorded Investment | $	$ 17	$ 17	$ 19
Retail | Other | Home equity loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	82	134	232
Recorded Investment | $	$ 4	$ 5	$ 13
Retail | Other | Home equity lines of credit
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	350	413	395
Recorded Investment | $	$ 22	$ 29	$ 27
Retail | Other | Home equity loans serviced by others
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	14	23	52
Recorded Investment | $	$ 0	$ 1	$ 2
Retail | Other | Home equity lines of credit serviced by others
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	8	14	26
Recorded Investment | $	$ 0	$ 1	$ 2
Retail | Other | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	1,250	1,189	1,336
Recorded Investment | $	$ 17	$ 17	$ 20
Retail | Other | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	272	355	329
Recorded Investment | $	$ 7	$ 7	$ 7
Retail | Other | Credit cards
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	304	0	0
Recorded Investment | $	$ 1	$ 0	$ 0
Retail | Other | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	176	9	5
Recorded Investment | $	$ 1	$ 0	$ 0