ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Unfunded Commitments (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded commitments related to TDRs	$ 25	$ 30
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR balance included in impaired loans	296	304
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR balance included in impaired loans	$ 696	$ 723