EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income for common shareholders									$ 27,571	$ 27,208	$ 24,136
Weighted-average shares
Basic weighted-average shares outstanding									22,457	22,049	22,462
Dilutive effect of outstanding stock options and unvested restricted stock awards									116	186	213
Diluted weighted-average shares outstanding									22,573	22,235	22,675
Earnings per share:
Basic	$ 0.08	$ 0.46	$ 0.39	$ 0.31	$ 0.34	$ 0.31	$ 0.29	$ 0.29	$ 1.23	$ 1.23	$ 1.07
Diluted	$ 0.08	$ 0.45	$ 0.39	$ 0.31	$ 0.34	$ 0.31	$ 0.29	$ 0.29	$ 1.22	$ 1.22	$ 1.06