ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Minimum qualifying balance	$ 3
Nonperforming assets, net of valuation allowance	45	$ 34
Mortgage loans collateralized by residential real estate property	152	172
Net change to ALLL resulting from modifications	9	3	$ 1
Charge-offs resulting from modification	$ 7	5	5
High loan to value criteria (exceeds)	90.00%
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance Defaulted	$ 1	63	9
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance Defaulted	$ 37	$ 40	$ 41