CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Income Statement [Abstract]
Revenues	$ 632,775		$ 588,103		$ 551,587		$ 521,333		$ 504,931		$ 468,186		$ 445,647		$ 424,148		$ 2,293,798		$ 1,842,912		$ 1,450,448
Operating expenses:
Cost of revenues (exclusive of depreciation and amortization)	410,069		377,525		355,915		344,689		319,031		301,081		289,175		277,634		1,488,198		1,186,921		921,352
Selling, general and administrative expenses	124,999		118,886		111,762		101,786		97,447		93,226		93,273		89,641		457,433		373,587		327,588
Depreciation and amortization expense	12,962		11,127		11,028		10,200		10,183		9,319		8,962		8,176		45,317		36,640		28,562
Income from operations	84,745		80,565		72,882		64,658		78,270		64,560		54,237		48,697		302,850		245,764		172,946
Interest and other income, net	1,950		2,509		1,190		3,076		1,080		1,941		1,052		(551)		8,725		3,522		4,601
Foreign exchange (loss)/gain	(1,898)		(3,105)		(3,562)		(3,484)		(582)		(514)		1,830		(247)		(12,049)		487		(3,242)
Income before provision for income taxes	84,797		79,969		70,510		64,250		78,768		65,987		57,119		47,899		299,526		249,773		174,305
Provision for income taxes	10,273		12,967		11,733		3,496		18,803		369		6,864		(16,519)		38,469		9,517		101,545
Net income	74,524		67,002		58,777		60,754		59,965		65,618		50,255		64,418		261,057		240,256		72,760
Foreign currency translation adjustments, net of tax																	6,295		(21,338)		20,065
Unrealized gain/(loss) on cash-flow hedging instruments, net of tax																	3,845		(2,553)		0
Comprehensive income	$ 86,741		$ 54,725		$ 62,934		$ 66,797		$ 52,798		$ 63,426		$ 32,345		$ 67,796		$ 271,197		$ 216,365		$ 92,825
Net income per share:
Basic (in dollars per share)	$ 1.35	[1]	$ 1.22	[1]	$ 1.07	[1]	$ 1.12	[1]	$ 1.11	[2]	$ 1.22	[2]	$ 0.94	[2]	$ 1.21	[2]	$ 4.77	[1]	$ 4.48	[2]	$ 1.40
Diluted (in dollars per share)	$ 1.29	[1]	$ 1.16	[1]	$ 1.02	[1]	$ 1.06	[1]	$ 1.05	[2]	$ 1.15	[2]	$ 0.89	[2]	$ 1.15	[2]	$ 4.53	[1]	$ 4.24	[2]	$ 1.32
Shares used in calculation of net income per share:
Basic (in shares)																	54,719,414		53,622,989		52,077,011
Diluted (in shares)																	57,667,789		56,672,676		54,984,173

[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] (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.