ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - TDRs and Unfunded Commitments (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded commitments tied to TDRs	$ 39	$ 42
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR balance	129	120
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR balance	$ 761	$ 799