QUARTERLY FINANCIAL DATA (Quaterly financial data) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 313,525		$ 298,293		$ 283,832		$ 264,482		$ 260,253		$ 236,049		$ 217,781		$ 200,045		$ 1,160,132		$ 914,128		$ 730,027
Cost of revenues (exclusive of depreciation and amortization)	198,226		190,797		180,782		167,381		158,291		148,479		134,256		125,887		737,186		566,913		456,530
Selling, general and administrative expenses	71,432		67,491		64,241		61,494		64,414		55,431		55,976		46,938		264,658		222,759		163,666
Depreciation and amortization expense	6,237		5,925		6,123		5,102		4,899		4,393		3,903		4,200		23,387		17,395		17,483
Other operating expenses, net	247		178		606		174		884		(30)		40		200		1,205		1,094		3,924
Total segment operating profit:	37,383		33,902		32,080		30,331		31,765		27,776		23,606		22,820		133,696		105,967		86,183
Interest and other income, net	1,432		1,067		1,138		1,211		1,409		865		1,299		1,158		4,848		4,731		4,769
Foreign exchange loss	(6,765)		(1,728)		(2,295)		(1,290)		1,559		32		(465)		(5,754)		(12,078)		(4,628)		(2,075)
Income before provision for income taxes	32,050		33,241		30,923		30,252		34,733		28,673		24,440		18,224		126,466		106,070		86,953
Provision for income taxes	7,287		7,067		6,493		6,353		7,095		5,800		5,209		3,510		27,200		21,614		17,312
Net income	24,763		26,174		24,430		23,899		27,638		22,873		19,231		14,714		99,266		84,456		69,641
Comprehensive income	$ 19,554		$ 26,532		$ 22,044		$ 28,598		$ 21,939		$ 14,532		$ 22,905		$ 11,984		$ 96,728		$ 71,360		$ 49,390
Basic (in dollars per share)	$ 0.49	[1]	$ 0.51	[1]	$ 0.49	[1]	$ 0.48	[1]	$ 0.56	[1]	$ 0.47	[1]	$ 0.40	[1]	$ 0.31	[1]	$ 1.97	[1]	$ 1.73	[1]	$ 1.48
Diluted (in dollars per share)	$ 0.46	[1]	$ 0.49	[1]	$ 0.46	[1]	$ 0.45	[1]	$ 0.52	[1]	$ 0.44	[1]	$ 0.37	[1]	$ 0.29	[1]	$ 1.87	[1]	$ 1.62	[1]	$ 1.40

[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.