ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Troubled Debt Restructuring (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) contract	Sep. 30, 2018 USD ($) contract	Sep. 30, 2019 USD ($) contract	Sep. 30, 2018 USD ($) contract
Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	928	718	2,649	2,032
Recorded Investment | $	$ 14	$ 9	$ 37	$ 25
Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	34	97	130	262
Recorded Investment | $	$ 4	$ 13	$ 17	$ 30
Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	809	549	1,975	1,801
Recorded Investment | $	$ 31	$ 18	$ 154	$ 246
Commercial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	2	1	3	6
Recorded Investment | $	$ 0	$ 0	$ 0	$ 1
Commercial | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	6	13	19	24
Recorded Investment | $	$ 1	$ 1	$ 2	$ 2
Commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	6	1	24	42
Recorded Investment | $	$ 15	$ 0	$ 102	$ 187
Commercial | Commercial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	2	1	3	6
Recorded Investment | $	$ 0	$ 0	$ 0	$ 1
Commercial | Commercial | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	6	13	18	23
Recorded Investment | $	$ 1	$ 1	$ 2	$ 2
Commercial | Commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	6	1	24	40
Recorded Investment | $	$ 15	$ 0	$ 102	$ 156
Commercial | Commercial real estate | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Commercial | Commercial real estate | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	1	1
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Commercial | Commercial real estate | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0	2
Recorded Investment | $	$ 0	$ 0	$ 0	$ 31
Retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	926	717	2,646	2,026
Recorded Investment | $	$ 14	$ 9	$ 37	$ 24
Retail | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	28	84	111	238
Recorded Investment | $	$ 3	$ 12	$ 15	$ 28
Retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	803	548	1,951	1,759
Recorded Investment | $	$ 16	$ 18	$ 52	$ 59
Retail | Residential mortgages | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	12	9	25	32
Recorded Investment | $	$ 2	$ 1	$ 6	$ 4
Retail | Residential mortgages | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	8	17	29	47
Recorded Investment | $	$ 2	$ 2	$ 5	$ 6
Retail | Residential mortgages | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	25	31	87	117
Recorded Investment | $	$ 4	$ 3	$ 13	$ 14
Retail | Home equity loans | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	11	10	24	32
Recorded Investment | $	$ 0	$ 1	$ 1	$ 3
Retail | Home equity loans | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0	1
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Home equity loans | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	19	40	64	106
Recorded Investment | $	$ 1	$ 2	$ 3	$ 5
Retail | Home equity lines of credit | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	51	27	123	55
Recorded Investment | $	$ 6	$ 3	$ 14	$ 5
Retail | Home equity lines of credit | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	16	58	66	147
Recorded Investment | $	$ 1	$ 10	$ 10	$ 21
Retail | Home equity lines of credit | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	95	104	277	310
Recorded Investment | $	$ 5	$ 7	$ 17	$ 21
Retail | Home equity loans serviced by others | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	1	2	1	3
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Home equity loans serviced by others | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Home equity loans serviced by others | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	4	5	11	20
Recorded Investment | $	$ 0	$ 1	$ 1	$ 1
Retail | Home equity lines of credit serviced by others | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	1	0	5
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Home equity lines of credit serviced by others | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0	1
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Home equity lines of credit serviced by others | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	2	8	6	13
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Automobile | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	46	45	111	122
Recorded Investment | $	$ 1	$ 0	$ 2	$ 2
Retail | Automobile | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	4	9	16	42
Recorded Investment | $	$ 0	$ 0	$ 0	$ 1
Retail | Automobile | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	309	315	933	893
Recorded Investment | $	$ 4	$ 4	$ 13	$ 13
Retail | Education | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Education | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Education | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	131	45	211	296
Recorded Investment | $	$ 2	$ 1	$ 5	$ 5
Retail | Credit cards | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	805	623	2,362	1,776
Recorded Investment | $	$ 5	$ 4	$ 14	$ 10
Retail | Credit cards | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Credit cards | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	163	0	304	0
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Other retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0	1
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Other retail | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Other retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	55	0	58	4
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0