ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Performance Indicators for Nonperforming Assets (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans accruing and 90 days or more past due	$ 32	$ 9
Nonperforming loans and leases as a percentage of total loans and leases	1.07%	1.07%	[1]
Nonperforming assets as a percentage of total assets	0.80%	0.80%	[1]
Commercial Banking
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans accruing and 90 days or more past due	$ 4	$ 1
Nonperforming loans and leases as a percentage of total loans and leases	0.37%	0.15%	[1]
Nonperforming assets as a percentage of total assets	0.26%	0.11%	[1]
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans accruing and 90 days or more past due	$ 28	$ 8
Nonperforming loans and leases as a percentage of total loans and leases	0.70%	0.92%	[1]
Nonperforming assets as a percentage of total assets	0.54%	0.69%	[1]

[1] December 31, 2015 ratios included loans accruing and 90 days or more past due of $1 million and $8 million for commercial and retail, respectively.