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	$ 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		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
Beginning Balance at Dec. 31, 2019	449,175	$ 702	123,996	561,680	$ (237,203)
Beginning Balance, Shares at Dec. 31, 2019		58,240,721			11,946,742
Net income	42,402			42,402
Employee stock plans	299		299
Employee stock plans, Shares		16,386
Shares withheld for taxes on awards	(3,856)		(3,856)
Shares withheld for taxes on awards, Shares		(38,142)
Stock-based compensation	2,775		2,775
Stock-based compensation, Shares		76,204
Repurchases of common stock	(39,072)				$ (39,072)
Repurchases of common stock, Shares		(442,009)			442,009
Ending Balance at Mar. 31, 2020	451,723	$ 702	123,214	604,082	$ (276,275)
Ending Balance, Shares at Mar. 31, 2020		57,853,160			12,388,751
Beginning Balance at Dec. 31, 2019	449,175	$ 702	123,996	561,680	$ (237,203)
Beginning Balance, Shares at Dec. 31, 2019		58,240,721			11,946,742
Net income	89,620
Ending Balance at Jun. 30, 2020	499,661	$ 703	123,933	651,300	$ (276,275)
Ending Balance, Shares at Jun. 30, 2020		57,878,003			12,388,751
Beginning Balance at Mar. 31, 2020	451,723	$ 702	123,214	604,082	$ (276,275)
Beginning Balance, Shares at Mar. 31, 2020		57,853,160			12,388,751
Net income	47,218			47,218
Employee stock plans	391		391
Employee stock plans, Shares		8,206
Shares withheld for taxes on awards	(1,199)		(1,199)
Shares withheld for taxes on awards, Shares		(12,393)
Stock-based compensation	1,528	$ 1	1,527
Stock-based compensation, Shares		29,030
Ending Balance at Jun. 30, 2020	$ 499,661	$ 703	$ 123,933	$ 651,300	$ (276,275)
Ending Balance, Shares at Jun. 30, 2020		57,878,003			12,388,751