SELECTED QUARTERLY INFORMATION (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended									12 Months Ended
		Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues		$ 93,307		$ 110,085	$ 109,482	$ 103,081	$ 108,249	$ 111,255	$ 107,563	$ 104,688	$ 415,955	$ 431,755	$ 389,668
Gross Profit		48,005		57,005	56,234	53,544	54,028	55,713	53,447	50,476	214,788	213,664	192,191
Net income (loss)		$ 22,736	[1]	$ 17,667	$ 15,381	$ 14,200	$ (16,898)	$ 16,506	$ 13,902	$ 14,099	$ 69,984	$ 27,609	$ 48,898
Earnings per share
Basic		$ 0.78		$ 0.60	$ 0.52	$ 0.48	$ (0.57)	$ 0.55	$ 0.47	$ 0.48	$ 2.38	$ 0.93	$ 1.69
Diluted	[2]	$ 0.77		$ 0.59	$ 0.51	$ 0.46	$ (0.57)	$ 0.54	$ 0.46	$ 0.47	$ 2.32	$ 0.90	$ 1.65
Shares used in per share calculation
Basic		29,164		29,365	29,505	29,799	29,759	29,759	29,720	29,456	29,456	29,674	28,925
Diluted	[2]	29,651		29,998	30,183	30,552	29,759	30,614	30,454	30,248	30,147	30,545	29,619

[1] In December 2017 the U.S. government enacted comprehensive tax legislation commonly referred to as the Tax Cuts and Jobs Act (Refer to Note 11, Provision (Benefit) for Income Taxes, in the Notes to Consolidated Financial Statements).

[2] The Company includes the shares underlying performance-based awards in the calculation of diluted earnings per share if the performance conditions have been satisfied as of the end of the reporting period and excludes such shares when the necessary conditions have not been met. The Company has included in the 2018, 2017 and 2016 calculations those shares that were contingently issuable upon the satisfaction of the performance conditions as of the end of the respective periods.