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, 2018 USD ($) contract	Jun. 30, 2017 USD ($) contract	Jun. 30, 2018 USD ($) contract	Jun. 30, 2017 USD ($) contract
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		$ 1	$ 2	$ 2	$ 3
Charge-offs Resulting from Modification		$ 1	$ 1	$ 2	$ 2
Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	646	719	1,314	1,389
Pre-Modification Outstanding Recorded Investment	[1]	$ 8	$ 9	$ 15	$ 18
Post-Modification Outstanding Recorded Investment	[1]	$ 9	$ 9	$ 16	$ 19
Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	92	110	165	190
Pre-Modification Outstanding Recorded Investment	[2]	$ 9	$ 27	$ 17	$ 37
Post-Modification Outstanding Recorded Investment	[2]	$ 9	$ 27	$ 17	$ 37
Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	657	578	1,248	1,121
Pre-Modification Outstanding Recorded Investment	[3]	$ 111	$ 55	$ 206	$ 83
Post-Modification Outstanding Recorded Investment	[3]	111	53	206	80
Commercial
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	1	0	1
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0	$ 0
Commercial | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	4	2	5	4
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 1	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 1	$ 1
Commercial | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	4	11	11	18
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 13	$ 1	$ 14
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 13	$ 1	$ 14
Commercial | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	19	4	37	5
Pre-Modification Outstanding Recorded Investment	[3]	$ 90	$ 32	$ 164	$ 36
Post-Modification Outstanding Recorded Investment	[3]	90	31	165	35
Commercial | Commercial
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	1	0	1
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0	$ 0
Commercial | Commercial | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	4	2	5	4
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 1	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 1	$ 1
Commercial | Commercial | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	4	11	10	18
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 13	$ 1	$ 14
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 13	$ 1	$ 14
Commercial | Commercial | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	17	4	35	4
Pre-Modification Outstanding Recorded Investment	[3]	$ 59	$ 32	$ 133	$ 32
Post-Modification Outstanding Recorded Investment	[3]	59	31	134	31
Commercial | Commercial real estate
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	0	0
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0	$ 0
Commercial | Commercial real estate | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	0	0	0	0
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Commercial | Commercial real estate | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0	1	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Commercial | Commercial real estate | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	2	0	2	0
Pre-Modification Outstanding Recorded Investment	[3]	$ 31	$ 0	$ 31	$ 0
Post-Modification Outstanding Recorded Investment	[3]	31	0	31	0
Commercial | Leases
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	0	0
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0	$ 0
Commercial | Leases | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	0	0	0	0
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Commercial | Leases | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0	0	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Commercial | Leases | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	0	0	0	1
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0	$ 0	$ 4
Post-Modification Outstanding Recorded Investment	[3]	0	0	0	4
Retail
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		1	1	2	2
Charge-offs Resulting from Modification		$ 1	$ 1	$ 2	$ 2
Retail | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	642	717	1,309	1,385
Pre-Modification Outstanding Recorded Investment	[1]	$ 7	$ 9	$ 14	$ 17
Post-Modification Outstanding Recorded Investment	[1]	$ 8	$ 9	$ 15	$ 18
Retail | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	88	99	154	172
Pre-Modification Outstanding Recorded Investment	[2]	$ 9	$ 14	$ 16	$ 23
Post-Modification Outstanding Recorded Investment	[2]	$ 9	$ 14	$ 16	$ 23
Retail | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	638	574	1,211	1,116
Pre-Modification Outstanding Recorded Investment	[3]	$ 21	$ 23	$ 42	$ 47
Post-Modification Outstanding Recorded Investment	[3]	21	22	41	45
Retail | Residential mortgages
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	0	0
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0	$ 0
Retail | Residential mortgages | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	16	25	23	43
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 4	$ 2	$ 5
Post-Modification Outstanding Recorded Investment	[1]	$ 2	$ 3	$ 3	$ 5
Retail | Residential mortgages | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	23	25	30	36
Pre-Modification Outstanding Recorded Investment	[2]	$ 3	$ 5	$ 4	$ 8
Post-Modification Outstanding Recorded Investment	[2]	$ 3	$ 5	$ 4	$ 8
Retail | Residential mortgages | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	33	44	86	92
Pre-Modification Outstanding Recorded Investment	[3]	$ 4	$ 6	$ 10	$ 10
Post-Modification Outstanding Recorded Investment	[3]	5	6	11	10
Retail | Home equity loans
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	0	0
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0	$ 0
Retail | Home equity loans | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	11	22	22	43
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1	$ 2	$ 2
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 2	$ 2	$ 3
Retail | Home equity loans | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	1	0	1	1
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Retail | Home equity loans | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	34	42	66	144
Pre-Modification Outstanding Recorded Investment	[3]	$ 1	$ 2	$ 3	$ 8
Post-Modification Outstanding Recorded Investment	[3]	1	2	3	8
Retail | Home equity lines of credit
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	0	0
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0	$ 0
Retail | Home equity lines of credit | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	13	14	28	30
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 2	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 2	$ 1
Retail | Home equity lines of credit | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	47	67	89	118
Pre-Modification Outstanding Recorded Investment	[2]	$ 6	$ 9	$ 11	$ 15
Post-Modification Outstanding Recorded Investment	[2]	$ 6	$ 9	$ 11	$ 15
Retail | Home equity lines of credit | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	113	112	206	187
Pre-Modification Outstanding Recorded Investment	[3]	$ 8	$ 8	$ 15	$ 14
Post-Modification Outstanding Recorded Investment	[3]	7	7	14	13
Retail | Home equity loans serviced by others
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	0	0
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0	$ 0
Retail | Home equity loans serviced by others | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	0	5	1	11
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 1
Retail | Home equity loans serviced by others | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0	0	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Retail | Home equity loans serviced by others | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	8	16	15	30
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0	$ 0	$ 1
Post-Modification Outstanding Recorded Investment	[3]	0	0	0	1
Retail | Home equity lines of credit serviced by others
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	0	0
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0	$ 0
Retail | Home equity lines of credit serviced by others | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	2	2	4	3
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Retail | Home equity lines of credit serviced by others | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	1	0	1	2
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Retail | Home equity lines of credit serviced by others | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	2	2	5	13
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0	$ 0	$ 1
Post-Modification Outstanding Recorded Investment	[3]	0	0	0	1
Retail | Automobile
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	0	0
Charge-offs Resulting from Modification		$ 1	$ 1	$ 2	$ 2
Retail | Automobile | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	41	25	77	65
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 2	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 2	$ 1
Retail | Automobile | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	16	7	33	15
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 1	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 1	$ 0
Retail | Automobile | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	309	349	578	625
Pre-Modification Outstanding Recorded Investment	[3]	$ 5	$ 6	$ 10	$ 11
Post-Modification Outstanding Recorded Investment	[3]	5	6	9	10
Retail | Education
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	1	0	1
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0	$ 0
Retail | Education | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	0	0	0	0
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Retail | Education | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0	0	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Retail | Education | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	139	7	251	22
Pre-Modification Outstanding Recorded Investment	[3]	$ 3	$ 1	$ 4	$ 2
Post-Modification Outstanding Recorded Investment	[3]	3	1	4	2
Retail | Credit cards
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		1	1	2	2
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0	$ 0
Retail | Credit cards | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	559	624	1,153	1,189
Pre-Modification Outstanding Recorded Investment	[1]	$ 3	$ 4	$ 6	$ 7
Post-Modification Outstanding Recorded Investment	[1]	$ 3	$ 4	$ 6	$ 7
Retail | Credit cards | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0	0	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Retail | Credit cards | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	0	0	0	0
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[3]	0	0	0	0
Retail | Other retail
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	(1)	0	(1)
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0	$ 0
Retail | Other retail | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	0	0	1	1
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Retail | Other retail | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0	0	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Retail | Other retail | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	0	2	4	3
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0	$ 0	$ 0

[1] Includes modifications that consist of multiple concessions, one of which is an interest rate reduction.

[2] Includes modifications that consist of multiple concessions, one of which is a maturity extension (unless one of the other concessions was an interest rate reduction).

[3] Includes modifications other than interest rate reductions or maturity extensions, such as lowering scheduled payments for a specified period of time, principal forgiveness, and capitalizing arrearages. Also included are the following: deferrals, trial modifications, certain bankruptcies, loans in forbearance and prepayment plans. Modifications can include the deferral of accrued interest resulting in post modification balances being higher than pre-modification.