ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Performance Indicators for Nonperforming Assets (Details)	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans and leases as a percentage of total loans and leases	0.66%	0.68%
Nonperforming assets as a percentage of total assets	0.49%	0.52%
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans and leases as a percentage of total loans and leases	0.19%	0.17%
Nonperforming assets as a percentage of total assets	0.13%	0.13%
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans and leases as a percentage of total loans and leases	0.47%	0.51%
Nonperforming assets as a percentage of total assets	0.36%	0.39%