QUARTERLY FINANCIAL DATA (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
QUARTERLY FINANCIAL DATA (unaudited)
Revenue	$ 59,360	$ 53,090	$ 45,777	$ 42,899	$ 50,406	$ 45,739	$ 39,980	$ 33,013	$ 201,126	$ 169,138	$ 154,917
Gross profit	23,056	21,620	18,928	15,382	18,306	18,589	17,326	12,076	78,986	66,297	53,867
Operating income	15,354	15,512	12,409	9,656	10,814	12,393	10,960	6,471	52,931	40,638	32,365
Net income	$ 9,207	$ 10,328	$ 7,210	$ 5,406	$ 6,112	$ 8,662	$ 6,362	$ 4,523	$ 32,151	$ 25,659	$ 18,370
Earnings per share - diluted (in dollars per share)	$ 0.48	$ 0.56	$ 0.39	$ 0.29	$ 0.33	$ 0.47	$ 0.35	$ 0.25	$ 1.72	$ 1.40	$ 1.01
Weighted average common shares outstanding used in the diluted earnings per share calculation	19,281	18,533	18,483	18,407	18,332	18,270	18,264	18,254	18,676	18,281	18,223
Dividends paid per share (in dollars per share)	$ 0.18	$ 0.18	$ 0.18		$ 0.36	$ 0.18	$ 0.18	$ 0.18	$ 0.54	$ 0.90