QUARTERLY FINANCIAL DATA (Quaterly Financial Data) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 399,297		$ 377,523		$ 348,977		$ 324,651		$ 313,525		$ 298,293		$ 283,832		$ 264,482		$ 1,450,448	$ 1,160,132	$ 914,128
Cost of revenues (exclusive of depreciation and amortization)	254,121		239,369		220,132		207,730		198,226		190,797		180,782		167,381		921,352	737,186	566,913
Selling, general and administrative expenses	84,793		81,190		80,419		78,453		71,432		67,491		64,241		61,494		324,855	264,658	222,759
Depreciation and amortization expense	7,696		7,174		7,020		6,672		6,237		5,925		6,123		5,102		28,562	23,387	17,395
Other operating expenses, net	637		542		724		830		247		178		606		174		2,733	1,205	1,094
Total segment operating profit:	52,050		49,248		40,682		30,966		37,383		33,902		32,080		30,331		172,946	133,696	105,967
Interest and other income, net	1,799		1,416		802		584		1,432		1,067		1,138		1,211		4,601	4,848	4,731
Foreign exchange loss	(1,772)		(77)		1,562		(2,955)		(6,765)		(1,728)		(2,295)		(1,290)		(3,242)	(12,078)	(4,628)
Income before provision for income taxes	52,077		50,587		43,046		28,595		32,050		33,241		30,923		30,252		174,305	126,466	106,070
Provision for income taxes	82,951		7,953		5,687		4,954		7,287		7,067		6,493		6,353		101,545	27,200	21,614
Net income	(30,874)		42,634		37,359		23,641		24,763		26,174		24,430		23,899		72,760	99,266	84,456
Comprehensive income/(loss)	$ (27,449)		$ 48,337		$ 41,910		$ 30,027		$ 19,554		$ 26,532		$ 22,044		$ 28,598		$ 92,825	$ 96,728	$ 71,360
Basic net income/(loss) per share	$ (0.58)	[1],[2]	$ 0.81	[1]	$ 0.72	[1]	$ 0.46	[1]	$ 0.49	[1]	$ 0.51	[1]	$ 0.49	[1]	$ 0.48	[1]	$ 1.40	$ 1.97	$ 1.73
Diluted net income/(loss) per share	$ (0.58)	[1],[2]	$ 0.77	[1]	$ 0.68	[1]	$ 0.44	[1]	$ 0.46	[1]	$ 0.49	[1]	$ 0.46	[1]	$ 0.45	[1]	$ 1.32	$ 1.87	$ 1.62
Incremental shares (in shares)	0																2,907,162	2,906,030	3,265,029

[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] (2)Due to the net loss during the three months ended December 31, 2017, zero incremental shares are included in the calculation of diluted earnings per share because of their antidilutive effect.