ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Unfunded Commitments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded commitments tied to TDRs	$ 35	$ 39
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR balance included in impaired loans	202	129
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR balance included in impaired loans	$ 748	$ 761