ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Troubled Debt Restructuring (Details) $ in Millions		3 Months Ended
		Mar. 31, 2018 USD ($) contract	Mar. 31, 2017 USD ($) contract
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		$ 1	$ 1
Charge-offs Resulting from Modification		$ 1	$ 1
Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	668	670
Pre-Modification Outstanding Recorded Investment	[1]	$ 7	$ 9
Post-Modification Outstanding Recorded Investment	[1]	$ 7	$ 10
Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	73	80
Pre-Modification Outstanding Recorded Investment	[2]	$ 8	$ 10
Post-Modification Outstanding Recorded Investment	[2]	$ 8	$ 10
Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	591	543
Pre-Modification Outstanding Recorded Investment	[3]	$ 95	$ 25
Post-Modification Outstanding Recorded Investment	[3]	95	24
Commercial
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0
Charge-offs Resulting from Modification		$ 0	$ 0
Commercial | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	1	2
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 1
Commercial | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	7	7
Pre-Modification Outstanding Recorded Investment	[2]	$ 1	$ 1
Post-Modification Outstanding Recorded Investment	[2]	$ 1	$ 1
Commercial | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	18	1
Pre-Modification Outstanding Recorded Investment	[3]	$ 74	$ 1
Post-Modification Outstanding Recorded Investment	[3]	75	1
Commercial | Commercial
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0
Charge-offs Resulting from Modification		$ 0	$ 0
Commercial | Commercial | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	1	2
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 1
Commercial | Commercial | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	6	7
Pre-Modification Outstanding Recorded Investment	[2]	$ 1	$ 1
Post-Modification Outstanding Recorded Investment	[2]	$ 1	$ 1
Commercial | Commercial | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	18	0
Pre-Modification Outstanding Recorded Investment	[3]	$ 74	$ 0
Post-Modification Outstanding Recorded Investment	[3]	75	0
Commercial | Commercial real estate
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0
Charge-offs Resulting from Modification		$ 0	$ 0
Commercial | Commercial real estate | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	0	0
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0
Commercial | Commercial real estate | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	1	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Commercial | Commercial real estate | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	0	0
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[3]	0	0
Commercial | Leases
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0
Charge-offs Resulting from Modification		$ 0	$ 0
Commercial | Leases | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	0	0
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0
Commercial | Leases | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Commercial | Leases | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	0	1
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 1
Post-Modification Outstanding Recorded Investment	[3]	0	1
Retail
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		1	1
Charge-offs Resulting from Modification		$ 1	$ 1
Retail | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	667	668
Pre-Modification Outstanding Recorded Investment	[1]	$ 7	$ 8
Post-Modification Outstanding Recorded Investment	[1]	$ 7	$ 9
Retail | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	66	73
Pre-Modification Outstanding Recorded Investment	[2]	$ 7	$ 9
Post-Modification Outstanding Recorded Investment	[2]	$ 7	$ 9
Retail | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	573	542
Pre-Modification Outstanding Recorded Investment	[3]	$ 21	$ 24
Post-Modification Outstanding Recorded Investment	[3]	20	23
Retail | Residential mortgages
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0
Charge-offs Resulting from Modification		$ 0	$ 0
Retail | Residential mortgages | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	7	18
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 2
Retail | Residential mortgages | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	7	11
Pre-Modification Outstanding Recorded Investment	[2]	$ 1	$ 3
Post-Modification Outstanding Recorded Investment	[2]	$ 1	$ 3
Retail | Residential mortgages | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	53	48
Pre-Modification Outstanding Recorded Investment	[3]	$ 6	$ 4
Post-Modification Outstanding Recorded Investment	[3]	6	4
Retail | Home equity loans
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0
Charge-offs Resulting from Modification		$ 0	$ 0
Retail | Home equity loans | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	11	21
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1
Retail | Home equity loans | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	1
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Retail | Home equity loans | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	32	102
Pre-Modification Outstanding Recorded Investment	[3]	$ 2	$ 6
Post-Modification Outstanding Recorded Investment	[3]	2	6
Retail | Home equity lines of credit
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0
Charge-offs Resulting from Modification		$ 0	$ 0
Retail | Home equity lines of credit | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	15	16
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1
Retail | Home equity lines of credit | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	42	51
Pre-Modification Outstanding Recorded Investment	[2]	$ 5	$ 6
Post-Modification Outstanding Recorded Investment	[2]	$ 5	$ 6
Retail | Home equity lines of credit | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	93	75
Pre-Modification Outstanding Recorded Investment	[3]	$ 7	$ 6
Post-Modification Outstanding Recorded Investment	[3]	7	6
Retail | Home equity loans serviced by others
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0
Charge-offs Resulting from Modification		$ 0	$ 0
Retail | Home equity loans serviced by others | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	1	6
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 1
Retail | Home equity loans serviced by others | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Retail | Home equity loans serviced by others | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	7	14
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 1
Post-Modification Outstanding Recorded Investment	[3]	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
Charge-offs Resulting from Modification		$ 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	1
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0
Retail | Home equity lines of credit serviced by others | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	2
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Retail | Home equity lines of credit serviced by others | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	3	11
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 1
Post-Modification Outstanding Recorded Investment	[3]	0	1
Retail | Automobile
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0
Charge-offs Resulting from Modification		$ 1	$ 1
Retail | Automobile | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	36	40
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1
Retail | Automobile | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	17	8
Pre-Modification Outstanding Recorded Investment	[2]	$ 1	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 1	$ 0
Retail | Automobile | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	269	276
Pre-Modification Outstanding Recorded Investment	[3]	$ 5	$ 5
Post-Modification Outstanding Recorded Investment	[3]	4	4
Retail | Education
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0
Charge-offs Resulting from Modification		$ 0	$ 0
Retail | Education | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	0	0
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0
Retail | Education | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Retail | Education | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	112	15
Pre-Modification Outstanding Recorded Investment	[3]	$ 1	$ 1
Post-Modification Outstanding Recorded Investment	[3]	1	1
Retail | Credit cards
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	1
Charge-offs Resulting from Modification		$ 0	$ 0
Retail | Credit cards | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	594	565
Pre-Modification Outstanding Recorded Investment	[1]	$ 3	$ 3
Post-Modification Outstanding Recorded Investment	[1]	$ 3	$ 3
Retail | Credit cards | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Retail | Credit cards | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	0	0
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[3]	0	0
Retail | Other retail
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		1	0
Charge-offs Resulting from Modification		$ 0	$ 0
Retail | Other retail | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	1	1
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0
Retail | Other retail | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	0	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0
Retail | Other retail | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	4	1
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[3]	$ 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.