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, 2015	$ 116,462	$ 696	$ 116,598	$ 119,395	$ (120,227)
Beginning Balance, Shares at Dec. 31, 2015		61,809,060			7,829,458
Net income	67,847			67,847
Employee stock purchase and option plans	281	$ 2	279
Employee stock purchase and option plans, Shares		158,350
Shares withheld for taxes on share-based payment awards	(1,934)	$ (2)	(1,932)
Shares withheld for taxes on share-based payment awards, Shares		(26,386)
Stock-based compensation	4,790	$ 2	4,788
Stock-based compensation, Shares		17,984
Shares repurchased under our publicly announced share repurchase programs	(53,285)				$ (53,285)
Shares repurchased under our publicly announced share repurchase programs, Shares		(3,157,904)			3,157,904
Ending Balance at Dec. 31, 2016	134,161	$ 698	119,733	187,242	$ (173,512)
Ending Balance, Shares at Dec. 31, 2016		58,801,104			10,987,362
Net income	95,128			95,128
Employee stock purchase and option plans	393	$ 2	391
Employee stock purchase and option plans, Shares		33,228
Shares withheld for taxes on share-based payment awards	(3,619)	$ (2)	(3,617)
Shares withheld for taxes on share-based payment awards, Shares		(58,470)
Stock-based compensation	5,187		5,187
Stock-based compensation, Shares		80,998
Ending Balance at Dec. 31, 2017	231,250	$ 698	121,694	282,370	$ (173,512)
Ending Balance, Shares at Dec. 31, 2017		58,856,860			10,987,362
Net income	134,572			134,572
Employee stock purchase and option plans	882	$ 1	881
Employee stock purchase and option plans, Shares		63,448
Shares withheld for taxes on share-based payment awards	(4,695)		(4,695)
Shares withheld for taxes on share-based payment awards, Shares		(13,028)
Stock-based compensation	6,345	$ 1	6,344
Stock-based compensation, Shares		103,694
Shares repurchased under our publicly announced share repurchase programs	(25,391)				$ (25,391)
Shares repurchased under our publicly announced share repurchase programs, Shares		(459,321)			459,321
Ending Balance at Dec. 31, 2018	$ 342,963	$ 700	$ 124,224	$ 416,942	$ (198,903)
Ending Balance, Shares at Dec. 31, 2018		58,551,653			11,446,683