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, 2018 USD ($) contract	Sep. 30, 2017 USD ($) contract	Sep. 30, 2018 USD ($) contract	Sep. 30, 2017 USD ($) contract
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		$ 1	$ 0	$ 3	$ 3
Charge-offs Resulting from Modification		$ 2	$ 2	$ 3	$ 4
Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	718	744	2,032	2,133
Pre-Modification Outstanding Recorded Investment	[1]	$ 9	$ 9	$ 24	$ 27
Post-Modification Outstanding Recorded Investment	[1]	$ 9	$ 9	$ 25	$ 28
Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	97	127	262	316
Pre-Modification Outstanding Recorded Investment	[2]	$ 13	$ 20	$ 30	$ 57
Post-Modification Outstanding Recorded Investment	[2]	$ 13	$ 20	$ 30	$ 57
Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	549	685	1,801	1,941
Pre-Modification Outstanding Recorded Investment	[3]	$ 20	$ 51	$ 248	$ 134
Post-Modification Outstanding Recorded Investment	[3]	18	51	246	131
Commercial
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	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]	1	3	6	7
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 1	$ 1	$ 2
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 1	$ 1	$ 2
Commercial | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	13	18	24	36
Pre-Modification Outstanding Recorded Investment	[2]	$ 1	$ 8	$ 2	$ 22
Post-Modification Outstanding Recorded Investment	[2]	$ 1	$ 7	$ 2	$ 21
Commercial | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	1	8	42	13
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 28	$ 186	$ 64
Post-Modification Outstanding Recorded Investment	[3]	0	30	187	65
Commercial | Commercial
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	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]	1	3	6	7
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 1	$ 1	$ 2
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 1	$ 1	$ 2
Commercial | Commercial | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	13	17	23	35
Pre-Modification Outstanding Recorded Investment	[2]	$ 1	$ 8	$ 2	$ 22
Post-Modification Outstanding Recorded Investment	[2]	$ 1	$ 7	$ 2	$ 21
Commercial | Commercial | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	1	7	40	12
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 28	$ 155	$ 64
Post-Modification Outstanding Recorded Investment	[3]	0	30	156	65
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	1	1	1
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]	0	1	2	1
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0	$ 31	$ 0
Post-Modification Outstanding Recorded Investment	[3]	0	0	31	0
Retail
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		1	0	3	2
Charge-offs Resulting from Modification		$ 2	$ 2	$ 3	$ 4
Retail | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	717	741	2,026	2,126
Pre-Modification Outstanding Recorded Investment	[1]	$ 9	$ 8	$ 23	$ 25
Post-Modification Outstanding Recorded Investment	[1]	$ 9	$ 8	$ 24	$ 26
Retail | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	84	109	238	280
Pre-Modification Outstanding Recorded Investment	[2]	$ 12	$ 12	$ 28	$ 35
Post-Modification Outstanding Recorded Investment	[2]	$ 12	$ 13	$ 28	$ 36
Retail | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	548	677	1,759	1,928
Pre-Modification Outstanding Recorded Investment	[3]	$ 20	$ 23	$ 62	$ 70
Post-Modification Outstanding Recorded Investment	[3]	18	21	59	66
Retail | Residential mortgages
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		(1)	(1)	(1)	(1)
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]	9	13	32	56
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1	$ 3	$ 6
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 2	$ 4	$ 7
Retail | Residential mortgages | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	17	15	47	50
Pre-Modification Outstanding Recorded Investment	[2]	$ 2	$ 1	$ 6	$ 9
Post-Modification Outstanding Recorded Investment	[2]	$ 2	$ 2	$ 6	$ 10
Retail | Residential mortgages | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	31	38	117	122
Pre-Modification Outstanding Recorded Investment	[3]	$ 4	$ 3	$ 14	$ 13
Post-Modification Outstanding Recorded Investment	[3]	3	3	14	13
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]	10	25	32	68
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 2	$ 2	$ 4
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1	$ 3	$ 4
Retail | Home equity loans | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	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]	40	49	106	192
Pre-Modification Outstanding Recorded Investment	[3]	$ 2	$ 3	$ 5	$ 11
Post-Modification Outstanding Recorded Investment	[3]	2	3	5	11
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	$ 1	$ 0	$ 1
Retail | Home equity lines of credit | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	27	11	55	41
Pre-Modification Outstanding Recorded Investment	[1]	$ 3	$ 1	$ 5	$ 2
Post-Modification Outstanding Recorded Investment	[1]	$ 3	$ 1	$ 5	$ 2
Retail | Home equity lines of credit | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	58	86	147	204
Pre-Modification Outstanding Recorded Investment	[2]	$ 10	$ 11	$ 21	$ 26
Post-Modification Outstanding Recorded Investment	[2]	$ 10	$ 11	$ 21	$ 26
Retail | Home equity lines of credit | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	104	110	310	295
Pre-Modification Outstanding Recorded Investment	[3]	$ 7	$ 6	$ 22	$ 20
Post-Modification Outstanding Recorded Investment	[3]	7	7	21	20
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]	2	3	3	14
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]	5	11	20	41
Pre-Modification Outstanding Recorded Investment	[3]	$ 1	$ 1	$ 1	$ 2
Post-Modification Outstanding Recorded Investment	[3]	1	0	1	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]	1	0	5	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]	0	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]	8	8	13	21
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 1	$ 0	$ 2
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		$ 2	$ 1	$ 3	$ 3
Retail | Automobile | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	45	28	122	93
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1	$ 3	$ 2
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 1	$ 2	$ 2
Retail | Automobile | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	9	8	42	23
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]	315	392	893	1,017
Pre-Modification Outstanding Recorded Investment	[3]	$ 5	$ 7	$ 15	$ 18
Post-Modification Outstanding Recorded Investment	[3]	4	6	13	16
Retail | Education
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		1	0	1	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]	45	67	296	235
Pre-Modification Outstanding Recorded Investment	[3]	$ 1	$ 2	$ 5	$ 4
Post-Modification Outstanding Recorded Investment	[3]	1	2	5	4
Retail | Credit cards
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		1	1	3	3
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]	623	661	1,776	1,850
Pre-Modification Outstanding Recorded Investment	[1]	$ 4	$ 3	$ 10	$ 10
Post-Modification Outstanding Recorded Investment	[1]	$ 4	$ 3	$ 10	$ 10
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	0	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	5
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.