Summarized Quarterly Results (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																	12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2010	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Quarterly Financial Data [Line Items]
Revenues	$ 125,538		$ 110,078		$ 103,800		$ 94,383		$ 95,127		$ 86,423		$ 80,176		$ 72,802			$ 433,799		$ 334,528		$ 221,824
Operating expenses:
Cost of revenues (exclusive of depreciation and amortization)	77,284		69,099		63,803		60,175		59,388		51,627		48,816		45,505			270,361		205,336		132,528
Selling, general and administrative expenses	26,377		21,153		20,711		17,627		18,510		15,822		16,805		13,793			85,868		64,930		47,635
Depreciation and amortization expense	3,208		3,040		2,423		2,211		1,806		2,083		1,959		1,690			10,882		7,538		6,242
Goodwill impairment loss	0								0				1,697				0			1,697
Other operating expenses, net	13		50		33		586		(4)				21		2			682		19		2,629
Income from operations	18,656		16,736		16,830		13,784		15,427		16,891		10,878		11,812			66,006		55,008		32,790
Interest and other income, net	519		486		460		476		422		385		428		187			1,941		1,422
Foreign exchange gain/(loss)	(135)		(635)		(1,394)		80		(500)		(2,301)		(703)		(134)			(2,084)		(3,638)		(2,181)
Income before provision for income taxes	19,040		16,587		15,896		14,340		15,349		14,975		10,603		11,865			65,863		52,792		31,095
Provision for income taxes	4,041		2,522		2,575		2,241		2,965		1,025		2,326		2,123			11,379		8,439		2,787
Net income	14,999		14,065		13,321		12,099		12,384		13,950		8,277		9,742			54,484		44,353		28,308
Comprehensive income	15,640		16,769		10,857		13,711		11,535		12,130		8,445		10,993			56,977		43,103		27,911
Common Stock
Quarterly Financial Data [Line Items]
Basic net income per share	$ 0.35	[1]	$ 0.33	[1]	$ 0.31	[1]	$ 0.30	[1]	$ 0.32	[1]	$ 0.36	[1]	$ (0.41)	[1]	$ 0.19	[1]		$ 1.27	[1]	$ 0.69	[1]	$ 0.84
Diluted net income per share	$ 0.32	[1]	$ 0.30	[1]	$ 0.29	[1]	$ 0.27	[1]	$ 0.29	[1]	$ 0.33	[1]	$ (0.41)	[1]	$ 0.18	[1]		$ 1.17	[1]	$ 0.63	[1]	$ 0.79
Operating expenses:
Net income																		$ 54,484		$ 44,353		$ 28,308

[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.