CONDENSED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Revenues	$ 551,587	$ 445,647	$ 1,072,920	$ 869,795
Operating expenses:
Cost of revenues (exclusive of depreciation and amortization)	355,915	289,175	700,604	566,809
Selling, general and administrative expenses	111,762	93,273	213,548	182,914
Depreciation and amortization expense	11,028	8,962	21,228	17,138
Income from operations	72,882	54,237	137,540	102,934
Interest and other income, net	1,190	1,052	4,266	501
Foreign exchange (loss)/gain	(3,562)	1,830	(7,046)	1,583
Income before provision for/(benefit from) income taxes	70,510	57,119	134,760	105,018
Provision for/(benefit from) income taxes	11,733	6,864	15,229	(9,655)
Net income	58,777	50,255	119,531	114,673
Foreign currency translation adjustments, net of tax	2,620	(15,834)	5,563	(12,525)
Unrealized gain/(loss) on cash-flow hedging instruments, net of tax	1,537	(2,076)	4,637	(2,007)
Comprehensive income	$ 62,934	$ 32,345	$ 129,731	$ 100,141
Net income per share:
Basic (in dollars per share)	$ 1.07	$ 0.94	$ 2.19	$ 2.15
Diluted (in dollars per share)	$ 1.02	$ 0.89	$ 2.08	$ 2.03
Shares used in calculation of net income per share:
Basic (in shares)	54,681,959	53,517,124	54,464,753	53,299,038
Diluted (in shares)	57,614,284	56,587,009	57,426,563	56,414,986