CONDENSED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Revenues	$ 588,103	$ 468,186	$ 1,661,023	$ 1,337,981
Operating expenses:
Cost of revenues (exclusive of depreciation and amortization)	377,525	301,081	1,078,129	867,890
Selling, general and administrative expenses	118,886	93,226	332,434	276,140
Depreciation and amortization expense	11,127	9,319	32,355	26,457
Income from operations	80,565	64,560	218,105	167,494
Interest and other income, net	2,509	1,941	6,775	2,442
Foreign exchange (loss)/gain	(3,105)	(514)	(10,151)	1,069
Income before provision for/(benefit from) income taxes	79,969	65,987	214,729	171,005
Provision for/(benefit from) income taxes	12,967	369	28,196	(9,286)
Net income	67,002	65,618	186,533	180,291
Foreign currency translation adjustments, net of tax	(10,114)	(2,118)	(4,551)	(14,643)
Unrealized (loss)/gain on cash-flow hedging instruments, net of tax	(2,163)	(74)	2,474	(2,081)
Comprehensive income	$ 54,725	$ 63,426	$ 184,456	$ 163,567
Net income per share:
Basic (in dollars per share)	$ 1.22	$ 1.22	$ 3.42	$ 3.37
Diluted (in dollars per share)	$ 1.16	$ 1.15	$ 3.24	$ 3.19
Shares used in calculation of net income per share:
Basic (in shares)	54,877,666	53,851,865	54,603,903	53,485,339
Diluted (in shares)	57,844,355	56,962,867	57,567,339	56,599,638