ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Financing Receivable, Troubled Debt Restructuring [Line Items]
Nonperforming assets, net of valuation allowance	$ 40		$ 40		$ 34
Mortgage loans collateralized by OREO	164		164		$ 172
Larger balance commercial loans minimum balance (greater than)	3		3
Net change to ALLL from modification	3	$ 1	7	$ 3
Charge-offs from modification of loans	1	2	$ 3	3
High loan to value criteria (exceeds)			90.00%
Commercial | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDR's that defaulted within 12 months of modification date	0	32	$ 1	52
Retail | Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDR's that defaulted within 12 months of modification date	$ 9	$ 10	$ 28	$ 30