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, 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
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