SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ 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
SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED)
Revenues	$ 65,355	$ 64,968	$ 60,335	$ 61,354	$ 55,343	$ 61,040	$ 65,835	$ 63,675	$ 252,012	$ 245,893	$ 232,434
Gross profit	20,551	19,546	17,881	17,701	16,728	17,907	19,864	18,505	75,679	73,004	68,772
Net income	$ 95	$ 3,057	$ 2,227	$ 2,657	$ 709	$ 2,821	$ 3,193	$ 2,355	$ 8,036	$ 9,078	$ 11,074
Basic earnings per share (in dollars per share)	$ 0.01	$ 0.33	$ 0.24	$ 0.29	$ 0.08	$ 0.30	$ 0.34	$ 0.25	$ 0.88	$ 1.01	$ 1.20
Diluted earnings per share (in dollars per share)	$ 0.01	$ 0.33	$ 0.24	$ 0.29	$ 0.08	$ 0.30	$ 0.34	$ 0.25	$ 0.87	$ 1.00	$ 1.20