CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning balance at Dec. 31, 2009	$ 218,809	$ 63,690	$ 155,204	$ (85)
Beginning balance, shares at Dec. 31, 2009		35,226,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	12,460		12,460
Other comprehensive income, net of tax	945			945
Tax benefit attributable to appreciation of common stock options exercised	399	399
Stock-based compensation expense	1,294	1,294
Issuance of common stock under Employee Stock Purchase Plans	378	378
Issuance of common stock under Employee Stock Purchase Plans, shares		31,000
Options exercised	1,330	1,330
Options exercised, shares		239,000
Shares surrendered in exchange for the exercise of stock options	0
Shares surrendered in exchange for the exercise of stock options, shares	0
Ending balance at Dec. 31, 2010	235,615	67,091	167,664	860
Ending balance, shares at Dec. 31, 2010		35,496,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	23,044		23,044
Other comprehensive income, net of tax	(710)			(710)
Tax benefit attributable to appreciation of common stock options exercised	3,122	3,122
Stock-based compensation expense	1,644	1,644
Issuance of common stock, net of offering costs	87,700	87,700
Issuance of common stock, net of offering costs, shares		5,520,000
Issuance of common stock under Employee Stock Purchase Plans	430	430
Issuance of common stock under Employee Stock Purchase Plans, shares		31,000
Options exercised	8,449	8,449
Options exercised, shares		1,099,000
Shares surrendered in exchange for payment of payroll tax liabilities	(953)	(953)
Shares surrendered in exchange for payment of payroll tax liabilities, shares		(60,000)
Shares surrendered in exchange for the exercise of stock options	(1,252)	(1,252)
Shares surrendered in exchange for the exercise of stock options, shares	(78,000)	(78,000)
Ending balance at Dec. 31, 2011	357,089	166,231	190,708	150
Ending balance, shares at Dec. 31, 2011		42,008,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	19,710		19,710
Other comprehensive income, net of tax	(1,332)			(1,332)
Tax benefit attributable to appreciation of common stock options exercised	877	877
Stock-based compensation expense	1,917	1,917
Issuance of common stock under Employee Stock Purchase Plans	430	430
Issuance of common stock under Employee Stock Purchase Plans, shares		33,000
Options exercised	5,156	5,156
Options exercised, shares	610,000	610,000
Shares surrendered in exchange for payment of payroll tax liabilities	(439)	(439)
Shares surrendered in exchange for payment of payroll tax liabilities, shares		(31,000)
Shares surrendered in exchange for the exercise of stock options	(1,831)	(1,831)
Shares surrendered in exchange for the exercise of stock options, shares	(131,000)	(131,000)
Ending balance at Dec. 31, 2012	$ 381,577	$ 172,341	$ 210,418	$ (1,182)
Ending balance, shares at Dec. 31, 2012		42,489,000