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	31,746			31,746
Stock options exercised, shares		132,300
Stock options exercised	3,182		290		2,892
Stock-based compensation expense	973		973
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 Mar. 31, 2020		28,523,329
Stockholders' Equity, Ending Balance at Mar. 31, 2020	1,037,681	$ 301	332,490	740,325	(35,435)
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	84,866			84,866
Stock options exercised, shares		295,600
Stock options exercised	7,256		766		6,490
Stock-based compensation expense	$ 2,264		2,264
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 Mar. 31, 2021		29,185,630
Stockholders' Equity, Ending Balance at Mar. 31, 2021	$ 1,353,422	$ 301	$ 340,696	$ 1,033,319	$ (20,894)