CONDENSED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Revenues	$ 377,523	$ 298,293	$ 1,051,151	$ 846,607
Operating expenses:
Cost of revenues (exclusive of depreciation and amortization)	239,369	190,797	667,231	538,960
Selling, general and administrative expenses	81,190	67,491	240,062	193,226
Depreciation and amortization expense	7,174	5,925	20,866	17,150
Other operating expenses, net	542	178	2,096	958
Income from operations	49,248	33,902	120,896	96,313
Interest and other income, net	1,416	1,067	2,802	3,416
Foreign exchange loss	(77)	(1,728)	(1,470)	(5,313)
Income before provision for income taxes	50,587	33,241	122,228	94,416
Provision for income taxes	7,953	7,067	18,594	19,913
Net income	42,634	26,174	103,634	74,503
Foreign currency translation adjustments	5,703	358	16,640	2,671
Comprehensive income	$ 48,337	$ 26,532	$ 120,274	$ 77,174
Net income per share:
Basic (in dollars per share)	$ 0.81	$ 0.51	$ 2.00	$ 1.48
Diluted (in dollars per share)	$ 0.77	$ 0.49	$ 1.90	$ 1.40
Shares used in calculation of net income per share:
Basic (in shares)	52,545	51,131	51,807	50,172
Diluted (in shares)	55,229	53,864	54,662	53,159