CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Income Statement [Abstract]
Revenues	$ 504,931		$ 468,186		$ 445,647		$ 424,148		$ 399,297		$ 377,523		$ 348,977		$ 324,651		$ 1,842,912		$ 1,450,448		$ 1,160,132
Operating expenses:
Cost of revenues (exclusive of depreciation and amortization)	319,031		301,081		289,175		277,634		254,121		239,369		220,132		207,730		1,186,921		921,352		737,186
Selling, general and administrative expenses	97,447		93,226		93,273		89,641		85,430		81,732		81,143		79,283		373,587		327,588		265,863
Depreciation and amortization expense	10,183		9,319		8,962		8,176		7,696		7,174		7,020		6,672		36,640		28,562		23,387
Income from operations	78,270		64,560		54,237		48,697		52,050		49,248		40,682		30,966		245,764		172,946		133,696
Interest and other income, net	1,080		1,941		1,052		(551)		1,799		1,416		802		584		3,522		4,601		4,848
Foreign exchange gain/(loss)	(582)		(514)		1,830		(247)		(1,772)		(77)		1,562		(2,955)		487		(3,242)		(12,078)
Income before provision for income taxes	78,768		65,987		57,119		47,899		52,077		50,587		43,046		28,595		249,773		174,305		126,466
Provision for income taxes	18,803		369		6,864		(16,519)		82,951		7,953		5,687		4,954		9,517		101,545		27,200
Net income	59,965		65,618		50,255		64,418		(30,874)		42,634		37,359		23,641		240,256		72,760		99,266
Foreign currency translation adjustments, net of tax																	(21,338)		20,065		(2,538)
Unrealized loss on cash-flow hedging instruments, net of tax																	(2,553)		0		0
Comprehensive income	$ 52,798		$ 63,426		$ 32,345		$ 67,796		$ (27,449)		$ 48,337		$ 41,910		$ 30,027		$ 216,365		$ 92,825		$ 96,728
Net income per share:
Basic (in dollars per share)	$ 1.11	[1]	$ 1.22	[1]	$ 0.94	[1]	$ 1.21	[1]	$ (0.58)	[1],[2]	$ 0.81	[1]	$ 0.72	[1]	$ 0.46	[1]	$ 4.48	[1]	$ 1.40	[1]	$ 1.97
Diluted (in dollars per share)	$ 1.05	[1]	$ 1.15	[1]	$ 0.89	[1]	$ 1.15	[1]	$ (0.58)	[1],[2]	$ 0.77	[1]	$ 0.68	[1]	$ 0.44	[1]	$ 4.24	[1]	$ 1.32	[1]	$ 1.87
Shares used in calculation of net income per share:
Basic (in shares)																	53,623		52,077		50,309
Diluted (in shares)																	56,673		54,984		53,215

[1] (1)Earnings per share amounts for each quarter may not necessarily total to the yearly earnings per share due to the weighting of shares outstanding on a quarterly and year to date basis.

[2] (2)Due to the net loss during the three months ended December 31, 2017, zero incremental shares are included in the calculation of diluted loss per share because of their antidilutive effect.