Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings (Deficit) [Member]	Treasury Stock [Member]
Beginning Balance at Dec. 31, 2014	$ 113,385	$ 348	$ 116,740	$ 71,297	$ (75,000)
Beginning Balance, Shares at Dec. 31, 2014		32,020,123			2,780,429
Net income	48,098			48,098
Employee stock purchase and option plans	315	$ 1	314
Employee stock purchase and option plans, Shares		113,996
Shares withheld for taxes on share-based payment awards	(8,086)	$ (1)	(8,085)
Shares withheld for taxes on share-based payment awards, Shares		(115,453)
Stock-based compensation	4,861		4,861
Stock-based compensation, Shares		20,164
Excess tax benefits from stock compensation	3,117		3,117
Shares repurchased under our publicly announced share repurchase programs	(45,227)				$ (45,227)
Shares repurchased under our publicly announced share repurchase programs, Shares					1,134,300
Shares repurchased under our publicly announced share repurchase programs, Shares		(1,134,300)
Ending Balance at Dec. 31, 2015	116,463	$ 348	116,947	119,395	$ (120,227)
Ending Balance, Shares at Dec. 31, 2015		30,904,530			3,914,729
Net income	67,847			67,847
Employee stock purchase and option plans	280	$ 1	279
Employee stock purchase and option plans, Shares		79,175
Shares withheld for taxes on share-based payment awards	(1,933)	$ (1)	(1,932)
Shares withheld for taxes on share-based payment awards, Shares		(13,193)
Stock-based compensation	4,789	$ 1	4,788
Stock-based compensation, Shares		8,992
Shares repurchased under our publicly announced share repurchase programs	(53,285)				$ (53,285)
Shares repurchased under our publicly announced share repurchase programs, Shares					1,578,952
Shares repurchased under our publicly announced share repurchase programs, Shares		(1,578,952)
Ending Balance at Dec. 31, 2016	134,161	$ 349	120,082	187,242	$ (173,512)
Ending Balance, Shares at Dec. 31, 2016		29,400,552			5,493,681
Net income	95,128			95,128
Employee stock purchase and option plans	392	$ 1	391
Employee stock purchase and option plans, Shares		16,614
Shares withheld for taxes on share-based payment awards	(3,618)	$ (1)	(3,617)
Shares withheld for taxes on share-based payment awards, Shares		(29,235)
Stock-based compensation	5,187		5,187
Stock-based compensation, Shares		40,499
Ending Balance at Dec. 31, 2017	$ 231,250	$ 349	$ 122,043	$ 282,370	$ (173,512)
Ending Balance, Shares at Dec. 31, 2017		29,428,430			5,493,681