ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Performance Indicators for Nonperforming Assets (Details)	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans and leases as a percentage of total loans and leases	0.68%	0.79%
Nonperforming assets as a percentage of total assets	0.52%	0.60%
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans and leases as a percentage of total loans and leases	0.17%	0.24%
Nonperforming assets as a percentage of total assets	0.13%	0.17%
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans and leases as a percentage of total loans and leases	0.51%	0.55%
Nonperforming assets as a percentage of total assets	0.39%	0.43%