LOANS - Additional Information (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) loan_type loan	Dec. 31, 2016 USD ($) loan	Dec. 31, 2015 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of types of loans | loan_type	3
Net deferred loan origination fees and unamortized premium and discount	$ 13,600,000	$ 17,400,000
Non-accretable discount on acquired credit impaired loans	1,400,000	3,000,000
Accrued interest receivable on loans	$ 8,800,000	7,400,000
Nonaccrual status, days past due	90 days
Total nonaccrual loans	$ 57,300,000	76,800,000
Interest foregone on non-accrual loans	5,400,000	4,700,000	$ 806,000
Allowance allocated to impaired loans	5,667,000	1,775,000
Pre-modification outstanding recorded investment	9,066,000	23,396,000
Recorded investment	$ 2,281,000	$ 21,272,000
Number of loans paid in full | loan	5	0
Total amount, after restructuring, that was repaid	$ 6,700,000
Amount charged off during year	0	$ 0
Allowance for loan losses, troubled debt restructurings	810,000	0
Amount of loans at end of period, gross	8,000,000
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Overdrafts	48,000	$ 73,000
Pre-modification outstanding recorded investment	208,000
Recorded investment	$ 208,000
Reserved based energy loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total funded loans	0.60%	0.50%
Loans	$ 19,200,000
Energy related service industry loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total funded loans	1.10%	2.60%
Loans	$ 17,500,000