ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Additional Impaired Loan Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	$ 41	$ 35	$ 35
Average Recorded Investment	1,031	1,130	1,102
Commercial | Commercial Banking
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	10	4	5
Average Recorded Investment	344	417	351
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	9	4	5
Average Recorded Investment	312	380	295
Commercial | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	1	0	0
Average Recorded Investment	32	37	53
Commercial | Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	0
Average Recorded Investment	0	0	3
Retail | Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	31	31	30
Average Recorded Investment	687	713	751
Retail | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	5	4	4
Average Recorded Investment	146	136	161
Retail | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	6	6	7
Average Recorded Investment	107	121	144
Retail | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	7	6	6
Average Recorded Investment	181	176	178
Retail | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	3	3	3
Average Recorded Investment	42	49	60
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	1	0
Average Recorded Investment	20	18	14
Retail | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	8	9	7
Average Recorded Investment	154	173	150
Retail | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	1	2	2
Average Recorded Investment	21	22	23
Retail | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Interest Income Recognized	0	0	1
Average Recorded Investment	$ 7	$ 9	$ 12