ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Troubled Debt Restructuring (Details) $ in Millions		12 Months Ended
		Dec. 31, 2017 USD ($) contract	Dec. 31, 2016 USD ($) contract	Dec. 31, 2015 USD ($) contract
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		$ 1	$ 8	$ 4
Charge-offs Resulting from Modification		$ 5	$ 4	$ 7
Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	2,724	2,585	2,834
Pre-Modification Outstanding Recorded Investment	[1]	$ 34	$ 37	$ 73
Post-Modification Outstanding Recorded Investment	[1]	$ 36	$ 37	$ 73
Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	386	348	421
Pre-Modification Outstanding Recorded Investment	[2]	$ 65	$ 54	$ 66
Post-Modification Outstanding Recorded Investment	[2]	$ 66	$ 55	$ 65
Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	2,562	2,501	3,261
Pre-Modification Outstanding Recorded Investment	[3]	$ 164	$ 150	$ 165
Post-Modification Outstanding Recorded Investment	[3]	161	146	159
Commercial
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		(1)	3	(1)
Charge-offs Resulting from Modification		0	0	1
Commercial | Commercial
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		(1)	3	(1)
Charge-offs Resulting from Modification		0	0	1
Commercial | Commercial real estate
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	0
Charge-offs Resulting from Modification		0	0	0
Commercial | Leases
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	0
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0
Commercial | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	7	13	26
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1	$ 19
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1	$ 19
Commercial | Interest Rate Reduction | Commercial
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	7	12	25
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1	$ 19
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1	$ 19
Commercial | Interest Rate Reduction | Commercial real estate
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	0	1	1
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0
Commercial | Interest Rate Reduction | Leases
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	0	0	0
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0
Commercial | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	46	82	161
Pre-Modification Outstanding Recorded Investment	[2]	$ 22	$ 25	$ 22
Post-Modification Outstanding Recorded Investment	[2]	$ 22	$ 26	$ 22
Commercial | Maturity Extension | Commercial
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	45	81	160
Pre-Modification Outstanding Recorded Investment	[2]	$ 22	$ 20	$ 22
Post-Modification Outstanding Recorded Investment	[2]	$ 22	$ 21	$ 22
Commercial | Maturity Extension | Commercial real estate
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	1	1	1
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 5	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 5	$ 0
Commercial | Maturity Extension | Leases
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0
Commercial | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	16	14	17
Pre-Modification Outstanding Recorded Investment	[3]	$ 70	$ 48	$ 38
Post-Modification Outstanding Recorded Investment	[3]	$ 71	$ 48	$ 38
Commercial | Other | Commercial
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	15	14	16
Pre-Modification Outstanding Recorded Investment	[3]	$ 70	$ 48	$ 34
Post-Modification Outstanding Recorded Investment	[3]	$ 71	$ 48	$ 34
Commercial | Other | Commercial real estate
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	1	0	1
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0	$ 4
Post-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0	$ 4
Commercial | Other | Leases
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	0	0	0
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[3]	0	0	0
Retail
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		2	5	5
Charge-offs Resulting from Modification		5	4	6
Retail | Residential mortgages
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		(1)	(1)	(1)
Charge-offs Resulting from Modification		0	0	0
Retail | Home equity loans
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	(1)	0
Charge-offs Resulting from Modification		0	0	1
Retail | Home equity lines of credit
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	0
Charge-offs Resulting from Modification		1	1	2
Retail | Home equity loans serviced by others
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	0
Charge-offs Resulting from Modification		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
Charge-offs Resulting from Modification		0	0	0
Retail | Automobile
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	0
Charge-offs Resulting from Modification		4	3	2
Retail | Education
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		2	4	4
Charge-offs Resulting from Modification		0	0	0
Retail | Credit cards
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		3	3	2
Charge-offs Resulting from Modification		0	0	0
Retail | Other retail
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		(2)	0	0
Charge-offs Resulting from Modification		$ 0	$ 0	$ 0
Retail | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	2,717	2,572	2,808
Pre-Modification Outstanding Recorded Investment	[1]	$ 33	$ 36	$ 54
Post-Modification Outstanding Recorded Investment	[1]	$ 35	$ 36	$ 54
Retail | Interest Rate Reduction | Residential mortgages
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	71	71	153
Pre-Modification Outstanding Recorded Investment	[1]	$ 9	$ 10	$ 31
Post-Modification Outstanding Recorded Investment	[1]	$ 10	$ 10	$ 31
Retail | Interest Rate Reduction | Home equity loans
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	82	97	96
Pre-Modification Outstanding Recorded Investment	[1]	$ 5	$ 6	$ 5
Post-Modification Outstanding Recorded Investment	[1]	$ 6	$ 6	$ 5
Retail | Interest Rate Reduction | Home equity lines of credit
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	50	49	4
Pre-Modification Outstanding Recorded Investment	[1]	$ 3	$ 4	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 3	$ 4	$ 1
Retail | Interest Rate Reduction | Home equity loans serviced by others
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	15	18	29
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1	$ 2
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1	$ 2
Retail | Interest Rate Reduction | Home equity lines of credit serviced by others
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	5	8	2
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0
Retail | Interest Rate Reduction | Automobile
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	130	138	108
Pre-Modification Outstanding Recorded Investment	[1]	$ 2	$ 3	$ 2
Post-Modification Outstanding Recorded Investment	[1]	$ 2	$ 3	$ 2
Retail | Interest Rate Reduction | Education
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	0	0	0
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0
Retail | Interest Rate Reduction | Credit cards
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	2,363	2,187	2,413
Pre-Modification Outstanding Recorded Investment	[1]	$ 13	$ 12	$ 13
Post-Modification Outstanding Recorded Investment	[1]	$ 13	$ 12	$ 13
Retail | Interest Rate Reduction | Other retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	1	4	3
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0
Retail | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	340	266	260
Pre-Modification Outstanding Recorded Investment	[2]	$ 43	$ 29	$ 44
Post-Modification Outstanding Recorded Investment	[2]	$ 44	$ 29	$ 43
Retail | Maturity Extension | Residential mortgages
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	73	60	40
Pre-Modification Outstanding Recorded Investment	[2]	$ 12	$ 10	$ 7
Post-Modification Outstanding Recorded Investment	[2]	$ 13	$ 10	$ 6
Retail | Maturity Extension | Home equity loans
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	1	39	191
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 4	$ 35
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 5	$ 35
Retail | Maturity Extension | Home equity lines of credit
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	235	121	23
Pre-Modification Outstanding Recorded Investment	[2]	$ 30	$ 13	$ 2
Post-Modification Outstanding Recorded Investment	[2]	$ 30	$ 12	$ 2
Retail | Maturity Extension | Home equity loans serviced by others
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0
Retail | Maturity Extension | Home equity lines of credit serviced by others
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	2	5	1
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 1	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 1	$ 0
Retail | Maturity Extension | Automobile
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	29	41	5
Pre-Modification Outstanding Recorded Investment	[2]	$ 1	$ 1	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 1	$ 1	$ 0
Retail | Maturity Extension | Education
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0
Retail | Maturity Extension | Credit cards
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0
Retail | Maturity Extension | Other retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0
Retail | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	2,546	2,487	3,244
Pre-Modification Outstanding Recorded Investment	[3]	$ 94	$ 102	$ 127
Post-Modification Outstanding Recorded Investment	[3]	$ 90	$ 98	$ 121
Retail | Other | Residential mortgages
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	171	247	275
Pre-Modification Outstanding Recorded Investment	[3]	$ 19	$ 26	$ 33
Post-Modification Outstanding Recorded Investment	[3]	$ 19	$ 26	$ 33
Retail | Other | Home equity loans
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	232	279	448
Pre-Modification Outstanding Recorded Investment	[3]	$ 13	$ 18	$ 28
Post-Modification Outstanding Recorded Investment	[3]	$ 13	$ 17	$ 28
Retail | Other | Home equity lines of credit
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	395	304	320
Pre-Modification Outstanding Recorded Investment	[3]	$ 27	$ 23	$ 21
Post-Modification Outstanding Recorded Investment	[3]	$ 27	$ 22	$ 19
Retail | Other | Home equity loans serviced by others
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	52	60	124
Pre-Modification Outstanding Recorded Investment	[3]	$ 2	$ 2	$ 6
Post-Modification Outstanding Recorded Investment	[3]	$ 2	$ 2	$ 5
Retail | Other | Home equity lines of credit serviced by others
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	26	24	41
Pre-Modification Outstanding Recorded Investment	[3]	$ 2	$ 1	$ 3
Post-Modification Outstanding Recorded Investment	[3]	$ 2	$ 1	$ 2
Retail | Other | Automobile
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	1,336	1,081	812
Pre-Modification Outstanding Recorded Investment	[3]	$ 24	$ 20	$ 14
Post-Modification Outstanding Recorded Investment	[3]	$ 20	$ 18	$ 12
Retail | Other | Education
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	329	479	1,204
Pre-Modification Outstanding Recorded Investment	[3]	$ 7	$ 12	$ 22
Post-Modification Outstanding Recorded Investment	[3]	$ 7	$ 12	$ 22
Retail | Other | Credit cards
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	0	0	0
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0	$ 0
Retail | Other | Other retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	5	13	20
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[3]	$ 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.