ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Narrative (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for credit losses		$ 321		$ 369	$ 302
Minimum qualifying balance		3
Nonperforming assets, net of valuation allowance		36		49
Mortgage loans collateralized by OREO		$ 181		$ 177
High loan to value criteria (exceeds)		90.00%		90.00%
Allowance for loan and lease losses
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for credit losses		$ 305	[1]	$ 355	305
Allowance for loan and lease losses | Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for credit losses		248	[1]	$ 242	$ 266
Scenario, Adjustment | Allowance for loan and lease losses | Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for credit losses	[1]	$ (50)

[1] Includes an increase of approximately $50 million to commercial and corresponding decrease to retail for the impact of the enhancement to the assessment of qualitative risks, factors and events that may not be measured in the modeled results.