QUARTERLY FINANCIAL DATA (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 260,253		$ 236,049		$ 217,781		$ 200,045		$ 202,184		$ 192,764		$ 174,695		$ 160,384		$ 914,128		$ 730,027		$ 555,117
Cost of revenues (exclusive of depreciation and amortization)	158,291		148,479		134,256		125,887		121,465		122,509		110,102		102,454		566,913		456,530		347,650
Selling, general and administrative expenses	64,414		55,431		55,976		46,938		49,761		42,875		38,671		32,359		222,759		163,666		116,497
Depreciation and amortization expense	4,899		4,393		3,903		4,200		2,833		5,510		5,451		3,689		17,395		17,483		15,120
Goodwill impairment loss	0		0		0		0		2,241		0		0		0		0		2,241		0
Other operating (income)/expenses, net	884		(30)		40		200		1,869		35		1,995		25		1,094		3,924		(643)
Total segment operating profit:	31,765		27,776		23,606		22,820		24,015		21,835		18,476		21,857		105,967		86,183		76,493
Interest and other income, net	1,409		865		1,299		1,158		1,368		1,261		1,164		976		4,731		4,769		3,077
Change in fair value of contingent consideration	0		0		0		0		(1,924)		0		0		0		0		(1,924)		0
Foreign exchange loss	1,559		32		(465)		(5,754)		1,123		(718)		(1,239)		(1,241)		(4,628)		(2,075)		(2,800)
Income before provision for income taxes	34,733		28,673		24,440		18,224		24,582		22,378		18,401		21,592		106,070		86,953		76,770
Provision for income taxes	7,095		5,800		5,209		3,510		6,159		3,338		3,587		4,228		21,614		17,312		14,776
Net income	27,638		22,873		19,231		14,714		18,423		19,040		14,814		17,364		84,456		69,641		61,994
Comprehensive income	$ 21,939		$ 14,532		$ 22,905		$ 11,984		$ 7,115		$ 10,780		$ 17,708		$ 13,787		$ 71,360		$ 49,390		$ 61,183
Basic (in dollars per share)	$ 0.56	[1]	$ 0.47	[1]	$ 0.40	[1]	$ 0.31	[1]	$ 0.39	[2]	$ 0.40	[2]	$ 0.31	[2]	$ 0.37	[2]	$ 1.73	[1]	$ 1.48	[2]	$ 1.35
Diluted (in dollars per share)	$ 0.52	[1]	$ 0.44	[1]	$ 0.37	[1]	$ 0.29	[1]	$ 0.37	[2]	$ 0.38	[2]	$ 0.30	[2]	$ 0.35	[2]	$ 1.62	[1]	$ 1.40	[2]	$ 1.28

[1] (1)Earnings per share amounts for each quarter may not necessarily total to the yearly earnings per share due to the weighting of shares outstanding on a quarterly and year to date basis.

[2] (1)Earnings per share amounts for each quarter may not necessarily total to the yearly earnings per share due to the weighting of shares outstanding on a quarterly and year to date basis.