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	159,791			159,791
Stock options exercised, shares		354,620
Stock options exercised	8,254		481		7,773
Stock-based compensation expense	4,915		4,915
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 Sep. 30, 2020		28,745,649
Stockholders' Equity, Ending Balance at Sep. 30, 2020	1,174,740	$ 301	336,623	868,370	(30,554)
Shares Outstanding, Beginning Balance at Jun. 30, 2020		28,543,629
Stockholders' Equity, Beginning Balance at Jun. 30, 2020	1,094,405	$ 301	334,261	794,833	(34,990)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	73,537			73,537
Stock options exercised, shares		202,020
Stock options exercised	4,609		173		4,436
Stock-based compensation expense	2,189		2,189
Shares Outstanding, Ending Balance at Sep. 30, 2020		28,745,649
Stockholders' Equity, Ending Balance at Sep. 30, 2020	1,174,740	$ 301	336,623	868,370	(30,554)
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	283,485			283,485
Stock options exercised, shares		452,100
Stock options exercised	11,241		1,227		10,014
Stock-based compensation expense	$ 6,199		6,199
Repurchase of common shares, shares	200,000	(242,500)
Repurchase of common shares	$ (15,494)				(15,494)
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 Sep. 30, 2021		29,099,630
Stockholders' Equity, Ending Balance at Sep. 30, 2021	1,544,467	$ 301	345,092	1,231,938	(32,864)
Shares Outstanding, Beginning Balance at Jun. 30, 2021		29,318,130
Stockholders' Equity, Beginning Balance at Jun. 30, 2021	1,466,544	$ 301	343,301	1,140,926	(17,984)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	91,012			91,012
Stock options exercised, shares		24,000
Stock options exercised	611		(3)		614
Stock-based compensation expense	$ 1,794		1,794
Repurchase of common shares, shares	200,000	(242,500)
Repurchase of common shares	$ (15,494)				(15,494)
Shares Outstanding, Ending Balance at Sep. 30, 2021		29,099,630
Stockholders' Equity, Ending Balance at Sep. 30, 2021	$ 1,544,467	$ 301	$ 345,092	$ 1,231,938	$ (32,864)