Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Deferred Compensation Trust [Member]	Retained Earnings [Member]
Beginning Balance at Dec. 31, 2017	$ 582,494	$ 26	$ 246,454	$ (3,486)	$ 339,500
Beginning Balance, Shares at Dec. 31, 2017		25,551,617
Stock compensation, including options and long-term incentives	1,058		1,058
Exercise of stock options less shares withheld for taxes	4,018		4,018
Exercise of stock options less shares withheld for taxes, Shares		97,370
Shares issued for long-term incentive awards, net of shares withheld for taxes, Shares		31,428
Shares issued for long-term incentive awards, net of shares withheld for taxes	(1,302)		(1,302)
Purchase of shares by Deferred Compensation Trust	(290)		5	(295)
Sale of shares by Deferred Compensation Trust	291			291
Net income	21,125				21,125
Ending Balance at Mar. 31, 2018	607,394	$ 26	250,233	(3,490)	360,625
Ending Balance, Shares at Mar. 31, 2018		25,680,415
Beginning Balance at Dec. 31, 2017	582,494	$ 26	246,454	(3,486)	339,500
Beginning Balance, Shares at Dec. 31, 2017		25,551,617
Net income	79,601
Ending Balance at Sep. 30, 2018	669,291	$ 26	253,366	(3,202)	419,101
Ending Balance, Shares at Sep. 30, 2018		25,693,651
Beginning Balance at Mar. 31, 2018	607,394	$ 26	250,233	(3,490)	360,625
Beginning Balance, Shares at Mar. 31, 2018		25,680,415
Stock compensation, including options and long-term incentives	1,061		1,061
Director deferred share activity	1,109		1,109
Director deferred share activity , Shares		5,184
Exercise of stock options less shares withheld for taxes	147		147
Exercise of stock options less shares withheld for taxes, Shares		6,333
Shares issued for long-term incentive awards, net of shares withheld for taxes, Shares		635
Shares issued for long-term incentive awards, net of shares withheld for taxes	(18)		(18)
Purchase of shares by Deferred Compensation Trust	(152)		4	(156)
Sale of shares by Deferred Compensation Trust	152			152
Net income	30,281				30,281
Ending Balance at Jun. 30, 2018	639,974	$ 26	252,536	(3,494)	390,906
Ending Balance, Shares at Jun. 30, 2018		25,692,567
Stock compensation, including options and long-term incentives	1,197		1,197
Shares issued for long-term incentive awards, net of shares withheld for taxes, Shares		1,084
Shares issued for long-term incentive awards, net of shares withheld for taxes	(76)		(76)
Purchase of shares by Deferred Compensation Trust	(349)		(291)	(58)
Sale of shares by Deferred Compensation Trust	350			350
Net income	28,195				28,195
Ending Balance at Sep. 30, 2018	669,291	$ 26	253,366	(3,202)	419,101
Ending Balance, Shares at Sep. 30, 2018		25,693,651
Beginning Balance at Dec. 31, 2018	695,864	$ 26	254,738	(3,381)	444,481
Beginning Balance, Shares at Dec. 31, 2018		25,693,651
Stock compensation, including options and long-term incentives	998		998
Director deferred share activity , Shares		45,075
Exercise of stock options less shares withheld for taxes	1,798		1,798
Exercise of stock options less shares withheld for taxes, Shares		68,169
Shares issued for long-term incentive awards, net of shares withheld for taxes, Shares		83,281
Shares issued for long-term incentive awards, net of shares withheld for taxes	(3,268)		(3,268)
Purchase of shares by Deferred Compensation Trust	(149)		309	(458)
Sale of shares by Deferred Compensation Trust	148			148
Net income	22,259				22,259
Ending Balance at Mar. 31, 2019	717,650	$ 26	254,575	(3,691)	466,740
Ending Balance, Shares at Mar. 31, 2019		25,890,176
Beginning Balance at Dec. 31, 2018	695,864	$ 26	254,738	(3,381)	444,481
Beginning Balance, Shares at Dec. 31, 2018		25,693,651
Net income	92,300
Ending Balance at Sep. 30, 2019	792,371	$ 26	259,388	(3,824)	536,781
Ending Balance, Shares at Sep. 30, 2019		25,936,012
Beginning Balance at Mar. 31, 2019	717,650	$ 26	254,575	(3,691)	466,740
Beginning Balance, Shares at Mar. 31, 2019		25,890,176
Stock compensation, including options and long-term incentives	1,342		1,342
Director deferred share activity	1,117		1,117
Director deferred share activity , Shares		4,155
Exercise of stock options less shares withheld for taxes	356		356
Exercise of stock options less shares withheld for taxes, Shares		10,832
Shares issued for long-term incentive awards, net of shares withheld for taxes, Shares		501
Shares issued for long-term incentive awards, net of shares withheld for taxes	(36)		(36)
Purchase of shares by Deferred Compensation Trust			229	(229)
Net income	37,073				37,073
Ending Balance at Jun. 30, 2019	757,502	$ 26	257,583	(3,920)	503,813
Ending Balance, Shares at Jun. 30, 2019		25,905,664
Stock compensation, including options and long-term incentives	1,295		1,295
Exercise of stock options less shares withheld for taxes	773		773
Exercise of stock options less shares withheld for taxes, Shares		28,170
Shares issued for long-term incentive awards, net of shares withheld for taxes, Shares		2,178
Shares issued for long-term incentive awards, net of shares withheld for taxes	(167)		(167)
Sale of shares by Deferred Compensation Trust			(96)	96
Net income	32,968				32,968
Ending Balance at Sep. 30, 2019	$ 792,371	$ 26	$ 259,388	$ (3,824)	$ 536,781
Ending Balance, Shares at Sep. 30, 2019		25,936,012