CONDENSED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Revenues	$ 521,333	$ 424,148
Operating expenses:
Cost of revenues (exclusive of depreciation and amortization)	344,689	277,634
Selling, general and administrative expenses	101,786	89,641
Depreciation and amortization expense	10,200	8,176
Income from operations	64,658	48,697
Interest and other income/(loss), net	3,076	(551)
Foreign exchange loss	(3,484)	(247)
Income before provision for/(benefit from) income taxes	64,250	47,899
Provision for/(benefit from) income taxes	3,496	(16,519)
Net income	60,754	64,418
Foreign currency translation adjustments, net of tax	2,943	3,309
Unrealized gain on cash-flow hedging instruments, net of tax	3,100	69
Comprehensive income	$ 66,797	$ 67,796
Net income per share:
Basic (in dollars per share)	$ 1.12	$ 1.21
Diluted (in dollars per share)	$ 1.06	$ 1.15
Shares used in calculation of net income per share:
Basic (in shares)	54,245,133	53,078,529
Diluted (in shares)	57,236,427	56,240,539