CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenues	$ 1,450,448	$ 1,160,132	$ 914,128
Operating expenses:
Cost of revenues (exclusive of depreciation and amortization)	921,352	737,186	566,913
Selling, general and administrative expenses	324,855	264,658	222,759
Depreciation and amortization expense	28,562	23,387	17,395
Other operating expenses, net	2,733	1,205	1,094
Income from operations	172,946	133,696	105,967
Interest and other income, net	4,601	4,848	4,731
Foreign exchange loss	(3,242)	(12,078)	(4,628)
Income before provision for income taxes	174,305	126,466	106,070
Provision for income taxes	101,545	27,200	21,614
Net income	72,760	99,266	84,456
Foreign currency translation adjustments	20,065	(2,538)	(13,096)
Comprehensive income	$ 92,825	$ 96,728	$ 71,360
Net income per share:
Basic (in dollars per share)	$ 1.40	$ 1.97	$ 1.73
Diluted (in dollars per share)	$ 1.32	$ 1.87	$ 1.62
Shares used in calculation of net income per share:
Basic (in shares)	52,077,011	50,309,362	48,720,844
Diluted (in shares)	54,984,173	53,215,392	51,985,873