QUARTERLY FINANCIAL DATA (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 Information Disclosure [Abstract]
Revenues	$ 202,184	$ 192,764	$ 174,695	$ 160,384	$ 157,585	$ 140,150	$ 133,184	$ 124,198	$ 730,027	$ 555,117	$ 433,799
Cost of revenues (exclusive of depreciation and amortization)	121,465	122,509	110,102	102,454	97,627	88,539	83,547	77,937	456,530	347,650	270,361
Selling, general and administrative expenses	49,761	42,875	38,671	32,359	32,980	27,893	28,541	27,083	163,666	116,497	85,868
Depreciation and amortization expense	2,833	5,510	5,451	3,689	3,743	3,906	3,854	3,617	17,483	15,120	10,882
Goodwill impairment loss	2,241	0	0	0	0	0	0	0	2,241	0	0
Other operating (income)/ expenses, net	1,869	35	1,995	25	43	(418)	(293)	25	3,924	(643)	682
Total segment operating profit:	24,015	21,835	18,476	21,857	23,192	20,230	17,535	15,536	86,183	76,493	66,006
Interest and other income, net	1,368	1,261	1,164	976	832	846	769	630	4,769	3,077	1,941
Change in fair value of contingent consideration	(1,924)	0	0	0	0	0	0	0	(1,924)	0	0
Foreign exchange loss	1,123	(718)	(1,239)	(1,241)	(712)	(720)	(869)	(499)	(2,075)	(2,800)	(2,084)
Income before provision for income taxes	24,582	22,378	18,401	21,592	23,312	20,356	17,435	15,667	86,953	76,770	65,863
Provision for income taxes	6,159	3,338	3,587	4,228	4,553	3,919	3,317	2,987	17,312	14,776	11,379
Net income	18,423	19,040	14,814	17,364	18,759	16,437	14,118	12,680	69,641	61,994	54,484
Comprehensive income	$ 7,115	$ 10,780	$ 17,708	$ 13,787	$ 18,361	$ 19,412	$ 13,073	$ 10,337	$ 49,390	$ 61,183	$ 56,977
Basic (in dollars per share)	$ 0.39	$ 0.40	$ 0.31	$ 0.37	$ 0.40	$ 0.36	$ 0.31	$ 0.28	$ 1.48	$ 1.35	$ 1.27
Diluted (in dollars per share)	$ 0.37	$ 0.38	$ 0.30	$ 0.35	$ 0.38	$ 0.34	$ 0.29	$ 0.27	$ 1.40	$ 1.28	$ 1.17