ALLOWANCE FOR LOAN LOSSES AND CREDIT QUALITY INFORMATION - Additional Information (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) SecurityLoan loan	Sep. 30, 2017 USD ($) loan	Dec. 31, 2017 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs			$ 8,700,000	$ 6,500,000
Percentage of average loans annualized, charged-offs			0.24%	0.19%
Interest income on impaired loans	$ 600,000	$ 300,000	$ 1,300,000	$ 1,000,000
Troubled debt restructuring related reserves	$ 2,000,000		$ 2,000,000		$ 1,000,000
Usual sustained repayment performance period			6 months
TRD defaulted | loan			5	4
Subsequent default, loan amount			$ 500,000	$ 3,700,000
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of residential loans in the process of foreclosure | SecurityLoan			27		33
Total loans outstanding, residential loans			$ 2,100,000		$ 2,900,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of residential loans in the process of foreclosure | SecurityLoan			8		8
Total loans outstanding, residential loans			$ 5,600,000		$ 6,000,000
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period for impairment loans			90 days
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase (decrease) in allowance for loan losses (less than)			$ (700,000)
Troubled debt restructurings charged off (less than)			$ 100,000	$ 0
Total Residential and Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impairment loans, charge off period			90 days