Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenues, net	$ 762,310	$ 685,988	$ 628,492
Cost of revenues (exclusive of depreciation and amortization)	495,586	447,956	402,917
Gross profit	266,724	238,032	225,575
Operating expenses:
General and administrative expenses	102,567	88,648	77,293
Selling and marketing expenses	53,383	50,582	49,474
Depreciation and amortization	38,549	34,580	31,465
Total operating expenses	194,499	173,810	158,232
Income from operations	72,225	64,222	67,343
Foreign exchange gain, net	2,839	5,597	2,744
Interest expense	(1,889)	(1,343)	(1,338)
Other income, net	11,859	15,408	7,027
Income before income tax expense	85,034	83,884	75,776
Income tax expense	36,146	22,151	24,211
Net income	$ 48,888	$ 61,733	$ 51,565
Earnings per share:
Basic (in dollars per share)	$ 1.44	$ 1.84	$ 1.55
Diluted (in dollars per share)	$ 1.39	$ 1.79	$ 1.51
Weighted-average number of shares used in computing earnings per share:
Basic (in shares)	33,897,916	33,566,367	33,298,104
Diluted (in shares)	35,110,210	34,563,319	34,178,340