ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Performance Indicators for Nonperforming Assets (Details) - USD ($) $ in Millions		Jun. 30, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans and leases as a percentage of total loans and leases	[1]	1.01%	1.07%
Nonperforming assets as a percentage of total assets	[1]	0.75%	0.80%
Loans accruing and 90 days or more past due		$ 26	$ 9
Commercial Banking
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans and leases as a percentage of total loans and leases	[1]	0.34%	0.15%
Nonperforming assets as a percentage of total assets	[1]	0.24%	0.11%
Loans accruing and 90 days or more past due		$ 4	$ 1
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans and leases as a percentage of total loans and leases	[1]	0.67%	0.92%
Nonperforming assets as a percentage of total assets	[1]	0.51%	0.69%
Loans accruing and 90 days or more past due		$ 22	$ 8

[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.