ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Troubled Debt Restructuring (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) contract	Jun. 30, 2018 USD ($) contract	Jun. 30, 2019 USD ($) contract	Jun. 30, 2018 USD ($) contract
Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1,040	646	1,721	1,314
Recorded Investment | $	$ 13	$ 9	$ 23	$ 16
Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	40	92	96	165
Recorded Investment | $	$ 4	$ 9	$ 13	$ 17
Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	629	657	1,166	1,248
Recorded Investment | $	$ 64	$ 111	$ 123	$ 206
Commercial | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1	4	1	5
Recorded Investment | $	$ 0	$ 1	$ 0	$ 1
Commercial | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	8	4	13	11
Recorded Investment | $	$ 0	$ 0	$ 1	$ 1
Commercial | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	6	19	18	37
Recorded Investment | $	$ 47	$ 90	$ 87	$ 165
Commercial | Commercial | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1	4	1	5
Recorded Investment | $	$ 0	$ 1	$ 0	$ 1
Commercial | Commercial | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	7	4	12	10
Recorded Investment | $	$ 0	$ 0	$ 1	$ 1
Commercial | Commercial | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	6	17	18	35
Recorded Investment | $	$ 47	$ 59	$ 87	$ 134
Commercial | Commercial real estate | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Commercial | Commercial real estate | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1	0	1	1
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Commercial | Commercial real estate | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	2	0	2
Recorded Investment | $	$ 0	$ 31	$ 0	$ 31
Retail | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1,039	642	1,720	1,309
Recorded Investment | $	$ 13	$ 8	$ 23	$ 15
Retail | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	32	88	83	154
Recorded Investment | $	$ 4	$ 9	$ 12	$ 16
Retail | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	623	638	1,148	1,211
Recorded Investment | $	$ 17	$ 21	$ 36	$ 41
Retail | Residential mortgages | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	9	16	13	23
Recorded Investment | $	$ 2	$ 2	$ 4	$ 3
Retail | Residential mortgages | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	10	23	21	30
Recorded Investment | $	$ 1	$ 3	$ 3	$ 4
Retail | Residential mortgages | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	32	33	62	86
Recorded Investment | $	$ 5	$ 5	$ 9	$ 11
Retail | Home equity loans | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	6	11	13	22
Recorded Investment | $	$ 1	$ 1	$ 1	$ 2
Retail | Home equity loans | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	1	0	1
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Home equity loans | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	18	34	45	66
Recorded Investment | $	$ 1	$ 1	$ 2	$ 3
Retail | Home equity lines of credit | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	43	13	72	28
Recorded Investment | $	$ 4	$ 1	$ 8	$ 2
Retail | Home equity lines of credit | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	15	47	50	89
Recorded Investment | $	$ 3	$ 6	$ 9	$ 11
Retail | Home equity lines of credit | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	77	113	182	206
Recorded Investment | $	$ 4	$ 7	$ 12	$ 14
Retail | Home equity loans serviced by others | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0	0	1
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Home equity loans serviced by others | Maturity Extension
Financing Receivable, Modifications [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, Modifications [Line Items]
Number of Contracts | contract	3	8	7	15
Recorded Investment | $	$ 1	$ 0	$ 1	$ 0
Retail | Home equity lines of credit serviced by others | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	2	0	4
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Home equity lines of credit serviced by others | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	1	0	1
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Home equity lines of credit serviced by others | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	2	2	4	5
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Automobile | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	40	41	65	77
Recorded Investment | $	$ 1	$ 1	$ 1	$ 2
Retail | Automobile | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	7	16	12	33
Recorded Investment | $	$ 0	$ 0	$ 0	$ 1
Retail | Automobile | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	335	309	624	578
Recorded Investment | $	$ 5	$ 5	$ 9	$ 9
Retail | Education | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Education | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Education | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	13	139	80	251
Recorded Investment | $	$ 1	$ 3	$ 3	$ 4
Retail | Credit cards | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	941	559	1,557	1,153
Recorded Investment | $	$ 5	$ 3	$ 9	$ 6
Retail | Credit cards | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Credit cards | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	141	0	141	0
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Other retail | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0	0	1
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Other retail | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0	0	0
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0
Retail | Other retail | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	2	0	3	4
Recorded Investment | $	$ 0	$ 0	$ 0	$ 0