ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Summary of Allowance for Credit Losses by Evaluation Method (Details) (Loans and Leases, USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated	$ 129	$ 131
Formula-based evaluation	1,127	1,129
Allowance for credit losses	1,256	1,260
Commercial Banking
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated	20	23
Formula-based evaluation	585	514
Allowance for credit losses	605	537
Retail
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated	109	108
Formula-based evaluation	542	615
Allowance for credit losses	$ 651	$ 723