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, 2019	815,226	$ 26	260,871	(3,871)	558,200
Beginning Balance, Shares at Dec. 31, 2019		25,936,532
Net income	98,104
Ending Balance at Sep. 30, 2020	918,466	$ 26	267,075	(4,939)	656,304
Ending Balance, Shares at Sep. 30, 2020		26,156,220
Beginning Balance at Mar. 31, 2020	843,387	$ 26	262,008	(4,958)	586,311
Beginning Balance, Shares at Mar. 31, 2020		26,063,348
Stock compensation, including options and long-term incentives	1,640		1,640
Director deferred share activity	1,230		1,230
Director deferred share activity , Shares		71,681
Exercise of stock options less shares withheld for taxes	454		454
Exercise of stock options less shares withheld for taxes, Shares		12,800
Shares issued for long-term incentive awards, net of shares withheld for taxes, Shares		1,002
Shares issued for long-term incentive awards, net of shares withheld for taxes	(75)		(75)
Purchase of shares by Deferred Compensation Trust			128	(128)
Sale of shares by Deferred Compensation Trust			(121)	121
Net income	28,454				28,454
Ending Balance at Jun. 30, 2020	875,090	$ 26	265,264	(4,965)	614,765
Ending Balance, Shares at Jun. 30, 2020		26,148,831
Stock compensation, including options and long-term incentives	1,670		1,670
Exercise of stock options less shares withheld for taxes	287		287
Exercise of stock options less shares withheld for taxes, Shares		6,190
Shares issued for long-term incentive awards, net of shares withheld for taxes, Shares		1,199
Shares issued for long-term incentive awards, net of shares withheld for taxes	(120)		(120)
Sale of shares by Deferred Compensation Trust			(26)	26
Net income	41,539				41,539
Ending Balance at Sep. 30, 2020	918,466	$ 26	267,075	(4,939)	656,304
Ending Balance, Shares at Sep. 30, 2020		26,156,220
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
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
Net income	179,474
Ending Balance at Sep. 30, 2021	1,144,726	$ 26	272,545	(3,859)	876,014
Ending Balance, Shares at Sep. 30, 2021		26,336,589
Beginning Balance at Mar. 31, 2021	997,618	$ 26	267,430	(3,669)	733,831
Beginning Balance, Shares at Mar. 31, 2021		26,333,692
Stock compensation, including options and long-term incentives	1,810		1,810
Director deferred share activity	1,256		1,256
Director deferred share activity , Shares		1,404
Purchase of shares by Deferred Compensation Trust			112	(112)
Net income	62,474				62,474
Ending Balance at Jun. 30, 2021	1,063,158	$ 26	270,608	(3,781)	796,305
Ending Balance, Shares at Jun. 30, 2021		26,335,096
Stock compensation, including options and long-term incentives	1,878		1,878
Director deferred share activity	202		202
Director deferred share activity , Shares		294
Shares issued for long-term incentive awards, net of shares withheld for taxes, Shares		1,199
Shares issued for long-term incentive awards, net of shares withheld for taxes	(221)		(221)
Purchase of shares by Deferred Compensation Trust			98	(98)
Sale of shares by Deferred Compensation Trust			(20)	20
Net income	79,709				79,709
Ending Balance at Sep. 30, 2021	$ 1,144,726	$ 26	$ 272,545	$ (3,859)	$ 876,014
Ending Balance, Shares at Sep. 30, 2021		26,336,589