SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED)
Revenues	$ 92,998	$ 94,653	$ 86,661	$ 92,382	$ 87,925	$ 96,633	$ 92,630	$ 93,896	$ 366,694	$ 371,084	$ 310,611
Gross profit	25,933	28,140	26,460	28,042	26,470	30,030	28,422	27,662	108,575	112,584	91,403
Net income	$ 2,699	$ 4,013	$ 2,896	$ 4,035	$ 3,489	$ 4,618	$ 4,445	$ 4,470	$ 13,643	$ 17,022	$ 15,925
Basic earnings per share	$ 0.28	$ 0.42	$ 0.30	$ 0.43	$ 0.37	$ 0.49	$ 0.47	$ 0.48	$ 1.44	$ 1.81	$ 1.72
Diluted earnings per share	$ 0.28	$ 0.42	$ 0.30	$ 0.42	$ 0.37	$ 0.49	$ 0.47	$ 0.48	$ 1.43	$ 1.80	$ 1.70