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, 2017	$ 231,250	$ 698	$ 121,694	$ 282,370	$ (173,512)
Beginning Balance, Shares at Dec. 31, 2017		58,856,860			10,987,362
Net income	37,110			37,110
Employee stock plans	195		195
Employee stock plans, Shares		26,832
Shares withheld for taxes on awards	(3,782)		(3,782)
Shares withheld for taxes on awards, Shares		(13,028)
Stock-based compensation	2,296	$ 1	2,295
Stock-based compensation, Shares		80,988
Repurchases of common stock	(5,211)				$ (5,211)
Repurchases of common stock, Shares		(100,044)			100,044
Ending Balance at Mar. 31, 2018	261,858	$ 699	120,402	319,480	$ (178,723)
Ending Balance, Shares at Mar. 31, 2018		58,851,608			11,087,406
Beginning Balance at Dec. 31, 2017	231,250	$ 698	121,694	282,370	$ (173,512)
Beginning Balance, Shares at Dec. 31, 2017		58,856,860			10,987,362
Net income	79,930
Ending Balance at Jun. 30, 2018	297,999	$ 700	121,541	362,299	$ (186,541)
Ending Balance, Shares at Jun. 30, 2018		58,726,399			11,237,406
Beginning Balance at Mar. 31, 2018	261,858	$ 699	120,402	319,480	$ (178,723)
Beginning Balance, Shares at Mar. 31, 2018		58,851,608			11,087,406
Net income	42,819			42,819
Employee stock plans	210	$ 1	209
Employee stock plans, Shares		22,549
Shares withheld for taxes on awards	(420)		(420)
Shares withheld for taxes on awards, Shares		(7,493)
Stock-based compensation	1,350	$ 0	1,350
Stock-based compensation, Shares		9,735
Repurchases of common stock	(7,818)				$ (7,818)
Repurchases of common stock, Shares		(150,000)			150,000
Ending Balance at Jun. 30, 2018	297,999	$ 700	121,541	362,299	$ (186,541)
Ending Balance, Shares at Jun. 30, 2018		58,726,399			11,237,406
Beginning Balance at Dec. 31, 2018	342,963	$ 700	124,224	416,942	$ (198,903)
Beginning Balance, Shares at Dec. 31, 2018		58,551,653			11,446,683
Net income	31,555			31,555
Employee stock plans	302		302
Employee stock plans, Shares		24,472
Shares withheld for taxes on awards	(5,727)		(5,727)
Shares withheld for taxes on awards, Shares		(74,010)
Stock-based compensation	2,794	$ 1	2,793
Stock-based compensation, Shares		160,359
Repurchases of common stock	(8,730)				$ (8,730)
Repurchases of common stock, Shares		(124,989)			124,989
Ending Balance at Mar. 31, 2019	363,157	$ 701	121,592	448,497	$ (207,633)
Ending Balance, Shares at Mar. 31, 2019		58,537,485			11,571,672
Beginning Balance at Dec. 31, 2018	342,963	$ 700	124,224	416,942	$ (198,903)
Beginning Balance, Shares at Dec. 31, 2018		58,551,653			11,446,683
Net income	67,265
Ending Balance at Jun. 30, 2019	391,533	$ 701	122,720	484,207	$ (216,095)
Ending Balance, Shares at Jun. 30, 2019		58,440,204			11,696,823
Beginning Balance at Mar. 31, 2019	363,157	$ 701	121,592	448,497	$ (207,633)
Beginning Balance, Shares at Mar. 31, 2019		58,537,485			11,571,672
Net income	35,710			35,710
Employee stock plans	257		257
Employee stock plans, Shares		14,905
Shares withheld for taxes on awards	(1,254)		(1,254)
Shares withheld for taxes on awards, Shares		(19,174)
Stock-based compensation	2,125	$ 0	2,125
Stock-based compensation, Shares		32,139
Repurchases of common stock	(8,462)				$ (8,462)
Repurchases of common stock, Shares		(125,151)			125,151
Ending Balance at Jun. 30, 2019	$ 391,533	$ 701	$ 122,720	$ 484,207	$ (216,095)
Ending Balance, Shares at Jun. 30, 2019		58,440,204			11,696,823