Reconciliation of Segment Revenues and Operating Profit to Consolidated Income from Operations (Detail) (USD $) In Thousands, 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	Sep. 30, 2010
Segment Reporting Information [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
Stock-based compensation expense										(6,826)	(2,866)	(2,939)
Depreciation and amortization	(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 (Note 3)	0				0		(1,697)		0		(1,697)
Legal settlement													(2,600)
Provision for bad debts										(662)	(727)	(202)
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										1,941	1,422	486
Foreign exchange 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
Operating Segments
Segment Reporting Information [Line Items]
Revenues										433,722	334,287	221,509
Income from operations										87,455	71,703	48,086
Unallocated Amount to Segment
Segment Reporting Information [Line Items]
Revenues										77	241	315
Other revenue										77	241	315
Stock-based compensation expense										(6,826)	(2,866)	(2,939)
Stock charge (Note 2)										(640)
Non-corporate taxes										(2,346)	(2,722)	(2,344)
Professional fees										(2,850)	(2,802)	(1,229)
Depreciation and amortization										(1,100)	(810)	(1,021)
Bank charges										(1,136)	(793)	(548)
Goodwill impairment loss (Note 3)											(1,697)
Legal settlement												(2,608)
Provision for bad debts												(265)
Other corporate expenses										$ (6,628)	$ (5,246)	$ (4,657)