ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Performance Indicators for Nonperforming Assets (Details)	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans and leases as a percentage of total loans and leases	0.97%	0.97%
Nonperforming assets as a percentage of total assets	0.73%	0.73%
Commercial Banking
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans and leases as a percentage of total loans and leases	0.38%	0.36%
Nonperforming assets as a percentage of total assets	0.27%	0.26%
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans and leases as a percentage of total loans and leases	0.59%	0.61%
Nonperforming assets as a percentage of total assets	0.46%	0.47%