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	Mar. 31, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated	$ 142	$ 129
Formula-based evaluation	1,116	1,127
Allowance for credit losses	1,258	1,256
Commercial Banking
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated	35	20
Formula-based evaluation	599	585
Allowance for credit losses	634	605
Retail
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated	107	109
Formula-based evaluation	517	542
Allowance for credit losses	$ 624	$ 651