INCOME TAXES - Effective tax rate (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (decrease) resulting from:
Tax expense calculated at statutory rate										$ 15,408	$ 13,726	$ 12,225
State income tax										102	140	102
Tax exempt interest income										(1,504)	(1,585)	(1,349)
Life Insurance										(553)	(485)	(464)
Impact of tax law rate change										3,857
Other										(857)	214	277
Total income tax expense	$ 6,313	$ 3,927	$ 3,181	$ 3,032	$ 3,322	$ 3,032	$ 2,845	$ 2,811		$ 16,453	$ 12,010	$ 10,791
Effective tax rate (as a percent)										37.40%	30.70%	30.90%
Statutory rate (as a percent)										35.00%	35.00%	35.00%
Tax adjustment to income tax expense from enacted tax law change										$ 3,900
Forecast
Increase (decrease) resulting from:
Statutory rate (as a percent)									21.00%