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, 2018	$ 342,963	$ 1,400	$ 123,524	$ 416,942	$ (198,903)
Beginning Balance, Shares at Dec. 31, 2018		117,103,306			22,893,366
Net income	31,555			31,555
Employee stock plans	302		302
Employee stock plans, Shares		48,944
Shares withheld for taxes on awards	(5,727)		(5,727)
Shares withheld for taxes on awards, Shares		(148,020)
Stock-based compensation	2,794	$ 1	2,793
Stock-based compensation, Shares		320,718
Repurchases of common stock	(8,730)				$ (8,730)
Repurchases of common stock, Shares		(249,978)			249,978
Ending Balance at Mar. 31, 2019	363,157	$ 1,401	120,892	448,497	$ (207,633)
Ending Balance, Shares at Mar. 31, 2019		117,074,970			23,143,344
Beginning Balance at Dec. 31, 2018	342,963	$ 1,400	123,524	416,942	$ (198,903)
Beginning Balance, Shares at Dec. 31, 2018		117,103,306			22,893,366
Net income	109,241
Ending Balance at Sep. 30, 2019	423,550	$ 1,402	122,141	526,183	$ (226,176)
Ending Balance, Shares at Sep. 30, 2019		116,707,946			23,643,606
Beginning Balance at Mar. 31, 2019	363,157	$ 1,401	120,892	448,497	$ (207,633)
Beginning Balance, Shares at Mar. 31, 2019		117,074,970			23,143,344
Net income	35,710			35,710
Employee stock plans	257		257
Employee stock plans, Shares		29,810
Shares withheld for taxes on awards	(1,254)		(1,254)
Shares withheld for taxes on awards, Shares		(38,348)
Stock-based compensation	2,125		2,125
Stock-based compensation, Shares		64,278
Repurchases of common stock	(8,462)				$ (8,462)
Repurchases of common stock, Shares		(250,302)			250,302
Ending Balance at Jun. 30, 2019	391,533	$ 1,401	122,020	484,207	$ (216,095)
Ending Balance, Shares at Jun. 30, 2019		116,880,408			23,393,646
Net income	41,976			41,976
Employee stock plans	159	$ 1	158
Employee stock plans, Shares		52,324
Shares withheld for taxes on awards	(963)		(963)
Shares withheld for taxes on awards, Shares		(9,444)
Stock-based compensation	926		926
Stock-based compensation, Shares		34,618
Repurchases of common stock	(10,081)				$ (10,081)
Repurchases of common stock, Shares		(249,960)			249,960
Ending Balance at Sep. 30, 2019	423,550	$ 1,402	122,141	526,183	$ (226,176)
Ending Balance, Shares at Sep. 30, 2019		116,707,946			23,643,606
Beginning Balance at Dec. 31, 2019	449,175	$ 1,404	123,294	561,680	$ (237,203)
Beginning Balance, Shares at Dec. 31, 2019		116,481,442			23,893,484
Net income	42,402			42,402
Employee stock plans	299		299
Employee stock plans, Shares		32,772
Shares withheld for taxes on awards	(3,856)		(3,856)
Shares withheld for taxes on awards, Shares		(76,284)
Stock-based compensation	2,775		2,775
Stock-based compensation, Shares		152,408
Repurchases of common stock	(39,072)				$ (39,072)
Repurchases of common stock, Shares		(884,018)			884,018
Ending Balance at Mar. 31, 2020	451,723	$ 1,404	122,512	604,082	$ (276,275)
Ending Balance, Shares at Mar. 31, 2020		115,706,320			24,777,502
Beginning Balance at Dec. 31, 2019	449,175	$ 1,404	123,294	561,680	$ (237,203)
Beginning Balance, Shares at Dec. 31, 2019		116,481,442			23,893,484
Net income	132,331
Ending Balance at Sep. 30, 2020	544,064	$ 1,406	124,923	694,010	$ (276,275)
Ending Balance, Shares at Sep. 30, 2020		115,785,673			24,777,502
Beginning Balance at Mar. 31, 2020	451,723	$ 1,404	122,512	604,082	$ (276,275)
Beginning Balance, Shares at Mar. 31, 2020		115,706,320			24,777,502
Net income	47,218			47,218
Employee stock plans	391		391
Employee stock plans, Shares		16,412
Shares withheld for taxes on awards	(1,199)		(1,199)
Shares withheld for taxes on awards, Shares		(24,786)
Stock-based compensation	1,528	$ 1	1,527
Stock-based compensation, Shares		58,060
Ending Balance at Jun. 30, 2020	499,661	$ 1,405	123,231	651,300	$ (276,275)
Ending Balance, Shares at Jun. 30, 2020		115,756,006			24,777,502
Net income	42,710			42,710
Employee stock plans	390		390
Employee stock plans, Shares		6,559
Shares withheld for taxes on awards	(313)		(313)
Shares withheld for taxes on awards, Shares		(4,486)
Stock-based compensation	1,616	$ 1	1,615
Stock-based compensation, Shares		27,594
Ending Balance at Sep. 30, 2020	$ 544,064	$ 1,406	$ 124,923	$ 694,010	$ (276,275)
Ending Balance, Shares at Sep. 30, 2020		115,785,673			24,777,502