CONDENSED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Revenues	$ 468,186	$ 377,523	$ 1,337,981	$ 1,051,151
Operating expenses:
Cost of revenues (exclusive of depreciation and amortization)	301,081	239,369	867,890	667,231
Selling, general and administrative expenses	92,490	81,190	272,110	240,062
Depreciation and amortization expense	9,319	7,174	26,457	20,866
Other operating expenses, net	736	542	4,030	2,096
Income from operations	64,560	49,248	167,494	120,896
Interest and other income, net	1,941	1,416	2,442	2,802
Foreign exchange (loss)/gain	(514)	(77)	1,069	(1,470)
Income before provision for/(benefit from) income taxes	65,987	50,587	171,005	122,228
Provision for/(benefit from) income taxes	369	7,953	(9,286)	18,594
Net income	65,618	42,634	180,291	103,634
Foreign currency translation adjustments, net of tax	(2,118)	5,703	(14,643)	16,640
Unrealized net loss on cash-flow hedging instruments, net of tax	(74)	0	(2,081)	0
Comprehensive income	$ 63,426	$ 48,337	$ 163,567	$ 120,274
Net income per share:
Basic (in dollars per share)	$ 1.22	$ 0.81	$ 3.37	$ 2.00
Diluted (in dollars per share)	$ 1.15	$ 0.77	$ 3.19	$ 1.90
Shares used in calculation of net income per share:
Basic (in shares)	53,851,865	52,545,155	53,485,339	51,806,700
Diluted (in shares)	56,962,867	55,228,781	56,599,638	54,661,196