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, 2014	$ 366,906	$ 25	$ 223,713	$ (2,189)	$ 145,357
Beginning Balance, Shares at Dec. 31, 2014		24,871,806
Stock compensation, including options and long-term incentives	2,983		2,983
Stock compensation, including options and long-term incentives, Shares		37,311
Director deferred value for annual deferral elections	895		895
Exercise of stock options, including tax benefits	2,533		2,533
Exercise of stock options, including tax benefits, Shares		116,088
Shares issued for long-term incentive awards, net of shares withheld for taxes, Shares		116,594
Shares issued for long-term incentive awards, net of shares withheld for taxes	(3,118)		(3,118)
Deferred tax adjustment for long-term incentive plan	2,674		2,674
Purchase of shares by Deferred Compensation Trust	(2,227)			(2,227)
Sale of shares by Deferred Compensation Trust	2,227		913	1,314
Net income	55,016				55,016
Ending Balance at Dec. 31, 2015	427,889	$ 25	230,593	(3,102)	200,373
Ending 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 value for annual deferral elections	1,037		1,037
Exercise of stock options, including tax benefits	3,173		3,173
Exercise of stock options, including tax benefits, 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	483,078	$ 25	237,846	(3,190)	248,397
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 value for annual deferral elections	952		952
Director deferred shares for annual deferral elections		40,142
Exercise of stock options, including tax benefits	$ 4,480	$ 1	4,479
Exercise of stock options, including tax benefits, Shares	141,500	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	91,156				91,156
Ending Balance at Dec. 31, 2017	$ 582,547	$ 26	$ 246,454	$ (3,486)	$ 339,553
Ending Balance, Shares at Dec. 31, 2017		25,551,617