ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Performance Indicators for Nonperforming Assets (Details)	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total nonperforming loans and leases as a percentage of total loans and leases	0.78%	0.79%
Total nonperforming assets as a percentage of total assets	0.59%	0.60%
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total nonperforming loans and leases as a percentage of total loans and leases	0.25%	0.24%
Total nonperforming assets as a percentage of total assets	0.18%	0.17%
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total nonperforming loans and leases as a percentage of total loans and leases	0.53%	0.55%
Total nonperforming assets as a percentage of total assets	0.41%	0.43%