Condensed 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, 2019	$ 815,226	$ 26	$ 260,871	$ (3,871)	$ 558,200
Beginning Balance, Shares at Dec. 31, 2019		25,936,532
Stock compensation, including options and long-term incentives	1,317		1,317
Exercise of stock options less shares withheld for taxes	2,137		2,137
Exercise of stock options less shares withheld for taxes, Shares		69,640
Shares issued for long-term incentive awards, net of shares withheld for taxes, Shares		57,176
Shares issued for long-term incentive awards, net of shares withheld for taxes	(3,404)		(3,404)
Purchase of shares by Deferred Compensation Trust			1,146	(1,146)
Sale of shares by Deferred Compensation Trust			(59)	59
Net income	28,111				28,111
Ending Balance at Mar. 31, 2020	843,387	$ 26	262,008	(4,958)	586,311
Ending Balance, Shares at Mar. 31, 2020		26,063,348
Beginning Balance at Dec. 31, 2020	961,288	$ 26	267,666	(2,944)	696,540
Beginning Balance, Shares at Dec. 31, 2020		26,236,570
Stock compensation, including options and long-term incentives	1,711		1,711
Exercise of stock options less shares withheld for taxes	3,678		3,678
Exercise of stock options less shares withheld for taxes, Shares		46,741
Shares issued for long-term incentive awards, net of shares withheld for taxes, Shares		50,381
Shares issued for long-term incentive awards, net of shares withheld for taxes	(6,350)		(6,350)
Purchase of shares by Deferred Compensation Trust			742	(742)
Sale of shares by Deferred Compensation Trust			(17)	17
Net income	37,291				37,291
Ending Balance at Mar. 31, 2021	$ 997,618	$ 26	$ 267,430	$ (3,669)	$ 733,831
Ending Balance, Shares at Mar. 31, 2021		26,333,692