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, 2017 USD ($) contract	Sep. 30, 2016 USD ($) contract	Sep. 30, 2017 USD ($) contract	Sep. 30, 2016 USD ($) contract
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		$ 0	$ 6	$ 3	$ 7
Charge-offs Resulting from Modification		$ 2	$ 1	$ 4	$ 2
Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	744	668	2,133	1,884
Pre-Modification Outstanding Recorded Investment	[1]	$ 9	$ 11	$ 27	$ 26
Post-Modification Outstanding Recorded Investment	[1]	$ 9	$ 12	$ 28	$ 27
Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	127	106	316	252
Pre-Modification Outstanding Recorded Investment	[2]	$ 20	$ 13	$ 57	$ 32
Post-Modification Outstanding Recorded Investment	[2]	$ 20	$ 13	$ 57	$ 32
Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	685	603	1,941	1,939
Pre-Modification Outstanding Recorded Investment	[3]	$ 51	$ 30	$ 134	$ 123
Post-Modification Outstanding Recorded Investment	[3]	51	29	131	120
Commercial
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	4	1	3
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0	$ 0
Commercial | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	3	3	7	11
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 2	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 2	$ 1
Commercial | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	18	8	36	62
Pre-Modification Outstanding Recorded Investment	[2]	$ 8	$ 1	$ 22	$ 9
Post-Modification Outstanding Recorded Investment	[2]	$ 7	$ 1	$ 21	$ 9
Commercial | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	8	4	13	13
Pre-Modification Outstanding Recorded Investment	[3]	$ 28	$ 6	$ 64	$ 47
Post-Modification Outstanding Recorded Investment	[3]	30	5	65	46
Commercial | Commercial
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	4	1	3
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0	$ 0
Commercial | Commercial | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	3	3	7	11
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 2	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 2	$ 1
Commercial | Commercial | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	17	8	35	62
Pre-Modification Outstanding Recorded Investment	[2]	$ 8	$ 1	$ 22	$ 9
Post-Modification Outstanding Recorded Investment	[2]	$ 7	$ 1	$ 21	$ 9
Commercial | Commercial | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	7	4	12	13
Pre-Modification Outstanding Recorded Investment	[3]	$ 28	$ 6	$ 64	$ 47
Post-Modification Outstanding Recorded Investment	[3]	30	5	65	46
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]	1	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]	1	0	1	0
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[3]	0	0	0	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	0
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[3]	0	0	0	0
Retail
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	2	2	4
Charge-offs Resulting from Modification		$ 2	$ 1	$ 4	$ 2
Retail | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	741	665	2,126	1,873
Pre-Modification Outstanding Recorded Investment	[1]	$ 8	$ 11	$ 25	$ 25
Post-Modification Outstanding Recorded Investment	[1]	$ 8	$ 12	$ 26	$ 26
Retail | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	109	98	280	190
Pre-Modification Outstanding Recorded Investment	[2]	$ 12	$ 12	$ 35	$ 23
Post-Modification Outstanding Recorded Investment	[2]	$ 13	$ 12	$ 36	$ 23
Retail | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	677	599	1,928	1,926
Pre-Modification Outstanding Recorded Investment	[3]	$ 23	$ 24	$ 70	$ 76
Post-Modification Outstanding Recorded Investment	[3]	21	24	66	74
Retail | Residential mortgages
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		(1)	0	(1)	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]	13	28	56	53
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 3	$ 6	$ 7
Post-Modification Outstanding Recorded Investment	[1]	$ 2	$ 3	$ 7	$ 7
Retail | Residential mortgages | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	15	33	50	49
Pre-Modification Outstanding Recorded Investment	[2]	$ 1	$ 6	$ 9	$ 9
Post-Modification Outstanding Recorded Investment	[2]	$ 2	$ 5	$ 10	$ 8
Retail | Residential mortgages | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	38	55	122	186
Pre-Modification Outstanding Recorded Investment	[3]	$ 3	$ 5	$ 13	$ 20
Post-Modification Outstanding Recorded Investment	[3]	3	5	13	20
Retail | Home equity loans
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 | Home equity loans | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	25	36	68	65
Pre-Modification Outstanding Recorded Investment	[1]	$ 2	$ 2	$ 4	$ 4
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 2	$ 4	$ 4
Retail | Home equity loans | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	2	1	39
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 4
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 1	$ 0	$ 5
Retail | Home equity loans | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	49	52	192	233
Pre-Modification Outstanding Recorded Investment	[3]	$ 3	$ 3	$ 11	$ 14
Post-Modification Outstanding Recorded Investment	[3]	3	3	11	14
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		$ 1	$ 1	$ 1	$ 1
Retail | Home equity lines of credit | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	11	20	41	33
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1	$ 2	$ 2
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 2	$ 2	$ 3
Retail | Home equity lines of credit | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	86	56	204	83
Pre-Modification Outstanding Recorded Investment	[2]	$ 11	$ 6	$ 26	$ 9
Post-Modification Outstanding Recorded Investment	[2]	$ 11	$ 6	$ 26	$ 9
Retail | Home equity lines of credit | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	110	94	295	218
Pre-Modification Outstanding Recorded Investment	[3]	$ 6	$ 8	$ 20	$ 16
Post-Modification Outstanding Recorded Investment	[3]	7	7	20	15
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]	3	7	14	13
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 1	$ 1	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 1	$ 1	$ 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]	11	17	41	51
Pre-Modification Outstanding Recorded Investment	[3]	$ 1	$ 1	$ 2	$ 2
Post-Modification Outstanding Recorded Investment	[3]	0	1	1	2
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]	0	2	3	4
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	1	2	5
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 1
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 1
Retail | Home equity lines of credit serviced by others | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	8	6	21	19
Pre-Modification Outstanding Recorded Investment	[3]	$ 1	$ 0	$ 2	$ 1
Post-Modification Outstanding Recorded Investment	[3]	0	1	1	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	$ 0	$ 3	$ 1
Retail | Automobile | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	28	26	93	77
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1	$ 2	$ 2
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1	$ 2	$ 2
Retail | Automobile | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	8	6	23	14
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Retail | Automobile | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	392	264	1,017	803
Pre-Modification Outstanding Recorded Investment	[3]	$ 7	$ 5	$ 18	$ 15
Post-Modification Outstanding Recorded Investment	[3]	6	5	16	14
Retail | Education
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	1	1	3
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]	67	108	235	405
Pre-Modification Outstanding Recorded Investment	[3]	$ 2	$ 2	$ 4	$ 8
Post-Modification Outstanding Recorded Investment	[3]	2	2	4	8
Retail | Credit cards
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		1	1	3	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]	661	544	1,850	1,625
Pre-Modification Outstanding Recorded Investment	[1]	$ 3	$ 3	$ 10	$ 9
Post-Modification Outstanding Recorded Investment	[1]	$ 3	$ 3	$ 10	$ 9
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	(1)	0
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	2	1	3
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]	2	3	5	11
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.