Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Deferred Compensation Trust [Member]	Retained Earnings [Member]
Beginning Balance at Dec. 31, 2015	$ 427,863	$ 25	$ 230,593	$ (3,102)	$ 200,347
Beginning Balance, Shares at Dec. 31, 2015		25,141,799
Stock compensation, including options and long-term incentives	3,562		3,562
Stock compensation, including options and long-term incentives, Shares		12,358
Director deferred share activity	1,037		1,037
Exercise of stock options	3,173		3,173
Exercise of stock options, Shares		124,065
Shares issued for long-term incentive awards, net of shares withheld for taxes, Shares		44,479
Shares issued for long-term incentive awards, net of shares withheld for taxes	(651)		(651)
Deferred tax adjustment for long-term incentive plan	44		44
Purchase of shares by Deferred Compensation Trust	(203)		88	(291)
Sale of shares by Deferred Compensation Trust	203			203
Net income	48,024				48,024
Ending Balance at Dec. 31, 2016	483,052	$ 25	237,846	(3,190)	248,371
Ending Balance, Shares at Dec. 31, 2016		25,322,701
Stock compensation, including options and long-term incentives	4,131		4,131
Stock compensation, including options and long-term incentives, Shares		4,840
Director deferred share activity	952		952
Director deferred share activity , Shares		40,142
Exercise of stock options	4,480	$ 1	4,479
Exercise of stock options, Shares		141,500
Shares issued for long-term incentive awards, net of shares withheld for taxes, Shares		42,434
Shares issued for long-term incentive awards, net of shares withheld for taxes	(1,250)		(1,250)
Purchase of shares by Deferred Compensation Trust	(95)		296	(391)
Sale of shares by Deferred Compensation Trust	95			95
Net income	91,129				91,129
Ending Balance at Dec. 31, 2017	582,494	$ 26	246,454	(3,486)	339,500
Ending Balance, Shares at Dec. 31, 2017		25,551,617
Stock compensation, including options and long-term incentives	4,509		4,509
Stock compensation, including options and long-term incentives, Shares		5,184
Director deferred share activity	1,111		1,111
Exercise of stock options	$ 4,165		4,165
Exercise of stock options, Shares	105,860	103,703
Shares issued for long-term incentive awards, net of shares withheld for taxes, Shares		33,147
Shares issued for long-term incentive awards, net of shares withheld for taxes	$ (1,396)		(1,396)
Purchase of shares by Deferred Compensation Trust	(805)		(105)	(700)
Sale of shares by Deferred Compensation Trust	805			805
Net income	104,981				104,981
Ending Balance at Dec. 31, 2018	$ 695,864	$ 26	$ 254,738	$ (3,381)	$ 444,481
Ending Balance, Shares at Dec. 31, 2018		25,693,651