QUARTERLY FINANCIAL DATA (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
QUARTERLY FINANCIAL DATA (unaudited)
Revenue	$ 157,174	$ 170,859	$ 66,024	$ 53,354	$ 59,360	$ 53,090	$ 45,777	$ 42,899	$ 447,411	$ 201,126	$ 169,138
Gross profit	46,213	52,206	25,247	22,120	23,056	21,620	18,928	15,382	145,786	78,986	66,297
Operating income	19,148	26,796	11,022	15,484	15,354	15,512	12,409	9,656	72,450	52,931	40,638
Net income	$ 8,677	$ 13,333	$ 6,865	$ 9,361	$ 9,207	$ 10,328	$ 7,210	$ 5,406	$ 38,236	$ 32,151	$ 25,659
Earnings per share - diluted (in dollars per share)	$ 0.40	$ 0.61	$ 0.32	$ 0.43	$ 0.48	$ 0.56	$ 0.39	$ 0.29	$ 1.77	$ 1.72	$ 1.40
Weighted average common shares outstanding used in the diluted earnings per share calculation	21,673	21,680	21,667	21,586	19,281	18,533	18,483	18,407	21,655	18,676	18,281
Dividends paid per share (in dollars per share)	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18		$ 0.72	$ 0.54