CONDENSED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Revenues	$ 298,293	$ 236,049	$ 846,607	$ 653,875
Operating expenses:
Cost of revenues (exclusive of depreciation and amortization)	190,797	148,479	538,960	408,622
Selling, general and administrative expenses	67,491	55,431	193,226	158,345
Depreciation and amortization expense	5,925	4,393	17,150	12,496
Other operating expenses/(income), net	178	(30)	958	210
Income from operations	33,902	27,776	96,313	74,202
Interest and other income, net	1,067	865	3,416	3,322
Foreign exchange (loss)/gain	(1,728)	32	(5,313)	(6,187)
Income before provision for income taxes	33,241	28,673	94,416	71,337
Provision for income taxes	7,067	5,800	19,913	14,519
Net income	26,174	22,873	74,503	56,818
Foreign currency translation adjustments	358	(8,341)	2,671	(7,397)
Comprehensive income	$ 26,532	$ 14,532	$ 77,174	$ 49,421
Net income per share:
Basic (in dollars per share)	$ 0.51	$ 0.47	$ 1.48	$ 1.17
Diluted (in dollars per share)	$ 0.49	$ 0.44	$ 1.40	$ 1.10
Shares used in calculation of net income per share:
Basic (in shares)	51,131	49,043	50,172	48,506
Diluted (in shares)	53,864	52,344	53,159	51,755