ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Additional Impaired Loan Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	$ 35	$ 35	$ 47
Average Recorded Investment	1,130	1,102	1,283
Commercial | Commercial Banking
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	4	5	5
Average Recorded Investment	417	351	179
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	4	5	4
Average Recorded Investment	380	295	135
Commercial | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	1
Average Recorded Investment	37	53	44
Commercial | Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	0
Average Recorded Investment	0	3	0
Retail | Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	31	30	42
Average Recorded Investment	713	751	1,104
Retail | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	4	4	15
Average Recorded Investment	136	161	415
Retail | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	6	7	9
Average Recorded Investment	121	144	222
Retail | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	6	6	4
Average Recorded Investment	176	178	173
Retail | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	3	3	4
Average Recorded Investment	49	60	75
Retail | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	0
Average Recorded Investment	9	9	9
Retail | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	1	0	0
Average Recorded Investment	18	14	11
Retail | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	9	7	7
Average Recorded Investment	173	150	157
Retail | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	2	2	2
Average Recorded Investment	22	23	26
Retail | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	1	1
Average Recorded Investment	$ 9	$ 12	$ 16