CONDENSED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Revenues	$ 445,647	$ 348,977	$ 869,795	$ 673,628
Operating expenses:
Cost of revenues (exclusive of depreciation and amortization)	289,175	220,132	566,809	427,862
Selling, general and administrative expenses	91,843	80,419	179,620	158,872
Depreciation and amortization expense	8,962	7,020	17,138	13,692
Other operating expenses, net	1,430	724	3,294	1,554
Income from operations	54,237	40,682	102,934	71,648
Interest and other income, net	1,052	802	501	1,386
Foreign exchange gain/(loss)	1,830	1,562	1,583	(1,393)
Income before provision for/(benefit from) income taxes	57,119	43,046	105,018	71,641
Provision for/(benefit from) income taxes	6,864	5,687	(9,655)	10,641
Net income	50,255	37,359	114,673	61,000
Foreign currency translation adjustments, net of tax	(15,834)	4,551	(12,525)	10,937
Unrealized net loss on cash-flow hedging instruments, net of tax	(2,076)	0	(2,007)	0
Comprehensive income	$ 32,345	$ 41,910	$ 100,141	$ 71,937
Net income per share:
Basic (in dollars per share)	$ 0.94	$ 0.72	$ 2.15	$ 1.19
Diluted (in dollars per share)	$ 0.89	$ 0.68	$ 2.03	$ 1.12
Shares used in calculation of net income per share:
Basic (in shares)	53,517,124	51,899,416	53,299,038	51,431,353
Diluted (in shares)	56,587,009	54,848,619	56,414,986	54,371,284