CONDENSED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Revenues	$ 348,977	$ 283,832	$ 673,628	$ 548,314
Operating expenses:
Cost of revenues (exclusive of depreciation and amortization)	220,132	180,782	427,862	348,163
Selling, general and administrative expenses	80,419	64,241	158,872	125,735
Depreciation and amortization expense	7,020	6,123	13,692	11,225
Other operating expenses, net	724	606	1,554	780
Income from operations	40,682	32,080	71,648	62,411
Interest and other income, net	802	1,138	1,386	2,349
Foreign exchange gain (loss)	1,562	(2,295)	(1,393)	(3,585)
Income before provision for income taxes	43,046	30,923	71,641	61,175
Provision for income taxes	5,687	6,493	10,641	12,846
Net income	37,359	24,430	61,000	48,329
Foreign currency translation adjustments	4,551	(2,386)	10,937	2,313
Comprehensive income	$ 41,910	$ 22,044	$ 71,937	$ 50,642
Net income per share:
Basic (in dollars per share)	$ 0.72	$ 0.49	$ 1.19	$ 0.97
Diluted (in dollars per share)	$ 0.68	$ 0.46	$ 1.12	$ 0.92
Shares used in calculation of net income per share:
Basic (in shares)	51,899	50,211	51,431	49,688
Diluted (in shares)	54,848	53,271	54,371	52,803