Condensed Consolidated Statement of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Shares [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Treasury Shares [Member]
Shares Outstanding, Beginning Balance at Dec. 31, 2019		28,386,456
Stockholders' Equity, Beginning Balance at Dec. 31, 2019	$ 1,003,477	$ 301	$ 332,861	$ 708,579	$ (38,264)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	86,254			86,254
Stock options exercised, shares		152,600
Stock options exercised	3,645		308		3,337
Stock-based compensation expense	2,726		2,726
Repurchase of common shares, shares		(80,000)
Repurchase of common shares	(1,912)				(1,912)
Deferral of executive and director compensation	215		215
Deferred Compensation Arrangement with Individual, Shares Issued		84,573
Deferred Compensation Arrangement with Individual, Distribution Paid	0		(1,849)		1,849
Shares Outstanding, Ending Balance at Jun. 30, 2020		28,543,629
Stockholders' Equity, Ending Balance at Jun. 30, 2020	1,094,405	$ 301	334,261	794,833	(34,990)
Shares Outstanding, Beginning Balance at Mar. 31, 2020		28,523,329
Stockholders' Equity, Beginning Balance at Mar. 31, 2020	1,037,681	$ 301	332,490	740,325	(35,435)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	54,508			54,508
Stock options exercised, shares		20,300
Stock options exercised	463		18		445
Stock-based compensation expense	1,753		1,753
Shares Outstanding, Ending Balance at Jun. 30, 2020		28,543,629
Stockholders' Equity, Ending Balance at Jun. 30, 2020	1,094,405	$ 301	334,261	794,833	(34,990)
Shares Outstanding, Beginning Balance at Dec. 31, 2020		28,813,849
Stockholders' Equity, Beginning Balance at Dec. 31, 2020	1,258,698	$ 301	339,001	948,453	(29,057)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	192,473			192,473
Stock options exercised, shares		428,100
Stock options exercised	10,630		1,230		9,400
Stock-based compensation expense	$ 4,405		4,405
Repurchase of common shares, shares	0.0
Deferral of executive and director compensation	$ 338		338
Deferred Compensation Arrangement with Individual, Shares Issued		76,181
Deferred Compensation Arrangement with Individual, Distribution Paid	0		(1,673)		1,673
Shares Outstanding, Ending Balance at Jun. 30, 2021		29,318,130
Stockholders' Equity, Ending Balance at Jun. 30, 2021	1,466,544	$ 301	343,301	1,140,926	(17,984)
Shares Outstanding, Beginning Balance at Mar. 31, 2021		29,185,630
Stockholders' Equity, Beginning Balance at Mar. 31, 2021	1,353,422	$ 301	340,696	1,033,319	(20,894)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	107,607			107,607
Stock options exercised, shares		132,500
Stock options exercised	3,374		464		2,910
Stock-based compensation expense	2,141		2,141
Shares Outstanding, Ending Balance at Jun. 30, 2021		29,318,130
Stockholders' Equity, Ending Balance at Jun. 30, 2021	$ 1,466,544	$ 301	$ 343,301	$ 1,140,926	$ (17,984)