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, 2016 USD ($) contract	Sep. 30, 2015 USD ($) contract	Sep. 30, 2016 USD ($) contract	Sep. 30, 2015 USD ($) contract
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		$ 6	$ 1	$ 7	$ 3
Charge-offs Resulting from Modification		1	2	2	6
Commercial Banking
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		4	0	3	(1)
Charge-offs Resulting from Modification		0	1	0	1
Commercial
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		4	0	3	(1)
Charge-offs Resulting from Modification		0	1	0	1
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
Retail
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		2	1	4	4
Charge-offs Resulting from Modification		1	1	2	5
Residential mortgages
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
Home equity loans
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
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	0	1	2
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	1
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
Automobile
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		0	0	0	0
Charge-offs Resulting from Modification		0	1	1	2
Student
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		1	1	3	4
Charge-offs Resulting from Modification		0	0	0	0
Credit cards
Financing Receivable, Modifications [Line Items]
Net Change to ALLL Resulting from Modification		1	0	2	1
Charge-offs Resulting from Modification		0	0	0	0
Other retail
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
Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	668	643	1,884	2,052
Pre-Modification Outstanding Recorded Investment	[1]	$ 11	$ 9	$ 26	$ 32
Post-Modification Outstanding Recorded Investment	[1]	$ 12	$ 10	$ 27	$ 33
Interest Rate Reduction | Commercial Banking
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	3	4	11	19
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 1	$ 3
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 1	$ 3
Interest Rate Reduction | Commercial
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	3	4	11	18
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 1	$ 3
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 1	$ 3
Interest Rate Reduction | Commercial real estate
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	0	0	0	1
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Interest Rate Reduction | Retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	665	639	1,873	2,033
Pre-Modification Outstanding Recorded Investment	[1]	$ 11	$ 9	$ 25	$ 29
Post-Modification Outstanding Recorded Investment	[1]	$ 12	$ 10	$ 26	$ 30
Interest Rate Reduction | Residential mortgages
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	28	24	53	77
Pre-Modification Outstanding Recorded Investment	[1]	$ 3	$ 4	$ 7	$ 13
Post-Modification Outstanding Recorded Investment	[1]	$ 3	$ 4	$ 7	$ 13
Interest Rate Reduction | Home equity loans
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	36	30	65	77
Pre-Modification Outstanding Recorded Investment	[1]	$ 2	$ 2	$ 4	$ 4
Post-Modification Outstanding Recorded Investment	[1]	$ 2	$ 2	$ 4	$ 4
Interest Rate Reduction | Home equity lines of credit
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	20	1	33	1
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 2	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 2	$ 0	$ 3	$ 0
Interest Rate Reduction | Home equity loans serviced by others
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	7	2	13	24
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 1	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 1	$ 1
Interest Rate Reduction | Home equity lines of credit serviced by others
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	2	0	4	0
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Interest Rate Reduction | Automobile
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	26	33	77	71
Pre-Modification Outstanding Recorded Investment	[1]	$ 1	$ 0	$ 2	$ 1
Post-Modification Outstanding Recorded Investment	[1]	$ 1	$ 1	$ 2	$ 2
Interest Rate Reduction | Student
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
Interest Rate Reduction | Credit cards
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	544	547	1,625	1,781
Pre-Modification Outstanding Recorded Investment	[1]	$ 3	$ 3	$ 9	$ 10
Post-Modification Outstanding Recorded Investment	[1]	$ 3	$ 3	$ 9	$ 10
Interest Rate Reduction | Other retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[1]	2	2	3	2
Pre-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[1]	$ 0	$ 0	$ 0	$ 0
Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	106	135	252	309
Pre-Modification Outstanding Recorded Investment	[2]	$ 13	$ 22	$ 32	$ 54
Post-Modification Outstanding Recorded Investment	[2]	$ 13	$ 22	$ 32	$ 54
Maturity Extension | Commercial Banking
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	8	50	62	114
Pre-Modification Outstanding Recorded Investment	[2]	$ 1	$ 7	$ 9	$ 19
Post-Modification Outstanding Recorded Investment	[2]	$ 1	$ 7	$ 9	$ 19
Maturity Extension | Commercial
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	8	50	62	114
Pre-Modification Outstanding Recorded Investment	[2]	$ 1	$ 7	$ 9	$ 19
Post-Modification Outstanding Recorded Investment	[2]	$ 1	$ 7	$ 9	$ 19
Maturity Extension | Commercial real estate
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
Maturity Extension | Retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	98	85	190	195
Pre-Modification Outstanding Recorded Investment	[2]	$ 12	$ 15	$ 23	$ 35
Post-Modification Outstanding Recorded Investment	[2]	$ 12	$ 15	$ 23	$ 35
Maturity Extension | Residential mortgages
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	33	8	49	27
Pre-Modification Outstanding Recorded Investment	[2]	$ 6	$ 1	$ 9	$ 5
Post-Modification Outstanding Recorded Investment	[2]	$ 5	$ 1	$ 8	$ 5
Maturity Extension | Home equity loans
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	2	72	39	158
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 14	$ 4	$ 30
Post-Modification Outstanding Recorded Investment	[2]	$ 1	$ 14	$ 5	$ 30
Maturity Extension | Home equity lines of credit
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	56	2	83	5
Pre-Modification Outstanding Recorded Investment	[2]	$ 6	$ 0	$ 9	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 6	$ 0	$ 9	$ 0
Maturity Extension | Home equity loans serviced by others
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
Maturity Extension | Home equity lines of credit serviced by others
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	1	0	5	0
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 1	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 1	$ 0
Maturity Extension | Automobile
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[2]	6	3	14	5
Pre-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	[2]	$ 0	$ 0	$ 0	$ 0
Maturity Extension | Student
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
Maturity Extension | Credit cards
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
Maturity Extension | Other retail
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
Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	603	604	1,939	2,549
Pre-Modification Outstanding Recorded Investment	[3]	$ 30	$ 56	$ 123	$ 129
Post-Modification Outstanding Recorded Investment	[3]	$ 29	$ 55	$ 120	$ 124
Other | Commercial Banking
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	4	8	13	13
Pre-Modification Outstanding Recorded Investment	[3]	$ 6	$ 28	$ 47	$ 34
Post-Modification Outstanding Recorded Investment	[3]	$ 5	$ 28	$ 46	$ 34
Other | Commercial
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	4	8	13	12
Pre-Modification Outstanding Recorded Investment	[3]	$ 6	$ 28	$ 47	$ 30
Post-Modification Outstanding Recorded Investment	[3]	$ 5	$ 28	$ 46	$ 30
Other | Commercial real estate
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
Other | Retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	599	596	1,926	2,536
Pre-Modification Outstanding Recorded Investment	[3]	$ 24	$ 28	$ 76	$ 95
Post-Modification Outstanding Recorded Investment	[3]	$ 24	$ 27	$ 74	$ 90
Other | Residential mortgages
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	55	78	186	184
Pre-Modification Outstanding Recorded Investment	[3]	$ 5	$ 9	$ 20	$ 19
Post-Modification Outstanding Recorded Investment	[3]	$ 5	$ 9	$ 20	$ 19
Other | Home equity loans
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	52	85	233	379
Pre-Modification Outstanding Recorded Investment	[3]	$ 3	$ 8	$ 14	$ 25
Post-Modification Outstanding Recorded Investment	[3]	$ 3	$ 7	$ 14	$ 24
Other | Home equity lines of credit
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	94	58	218	271
Pre-Modification Outstanding Recorded Investment	[3]	$ 8	$ 4	$ 16	$ 18
Post-Modification Outstanding Recorded Investment	[3]	$ 7	$ 3	$ 15	$ 15
Other | Home equity loans serviced by others
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	17	26	51	97
Pre-Modification Outstanding Recorded Investment	[3]	$ 1	$ 2	$ 2	$ 5
Post-Modification Outstanding Recorded Investment	[3]	$ 1	$ 1	$ 2	$ 4
Other | Home equity lines of credit serviced by others
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	6	12	19	34
Pre-Modification Outstanding Recorded Investment	[3]	$ 0	$ 1	$ 1	$ 2
Post-Modification Outstanding Recorded Investment	[3]	$ 1	$ 1	$ 1	$ 2
Other | Automobile
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	264	182	803	651
Pre-Modification Outstanding Recorded Investment	[3]	$ 5	$ 2	$ 15	$ 10
Post-Modification Outstanding Recorded Investment	[3]	$ 5	$ 3	$ 14	$ 9
Other | Student
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	108	151	405	901
Pre-Modification Outstanding Recorded Investment	[3]	$ 2	$ 2	$ 8	$ 16
Post-Modification Outstanding Recorded Investment	[3]	$ 2	$ 3	$ 8	$ 17
Other | Credit cards
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
Other | Other retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	[3]	3	4	11	19
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, capitalizing arrearages, and principal forgiveness. 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.