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, 2017		27,856,752
Stockholders' Equity, Beginning Balance at Dec. 31, 2017	$ 747,298	$ 295	$ 306,483	$ 473,329	$ (32,809)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	107,663			107,663
Common share issuance for conversion of convertible notes - shares		628,515
Common share issuance for conversion of convertible notes - value	20,309	$ 6	20,303
Treasury Stock, Shares, Acquired		(1,069,043)
Treasury Stock, Value, Acquired, Cost Method	(25,709)				(25,709)
Stock options exercised - shares		38,628
Stock options exercised - value	538		(254)		792
Stock-based compensation expense	5,974		5,974
Deferral of Executive and Director Compensation	185		185
Deferred Compensation Arrangement with Individual, Shares Issued		61,366
Deferred Compensation Arrangement with Individual, Distribution Paid	(955)		(2,174)		1,219
Shares Outstanding, Ending Balance at Dec. 31, 2018		27,516,218
Stockholders' Equity, Ending Balance at Dec. 31, 2018	855,303	$ 301	330,517	580,992	(56,507)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	127,587			127,587
Treasury Stock, Shares, Acquired		(201,088)
Treasury Stock, Value, Acquired, Cost Method	(5,150)				(5,150)
Stock options exercised - shares		954,370
Stock options exercised - value	19,644		(1,204)		20,848
Stock-based compensation expense	5,846		5,846
Deferral of Executive and Director Compensation	247		247
Deferred Compensation Arrangement with Individual, Shares Issued		116,956
Deferred Compensation Arrangement with Individual, Distribution Paid	0		(2,545)		2,545
Shares Outstanding, Ending Balance at Dec. 31, 2019		28,386,456
Stockholders' Equity, Ending Balance at Dec. 31, 2019	1,003,477	$ 301	332,861	708,579	(38,264)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 239,874			239,874
Treasury Stock, Shares, Acquired	100,000	(80,000)
Treasury Stock, Value, Acquired, Cost Method	$ (1,912)				(1,912)
Stock options exercised - shares	422,820	422,820
Stock options exercised - value	$ 9,906		636		9,270
Stock-based compensation expense	7,138		7,138
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 Dec. 31, 2020		28,813,849
Stockholders' Equity, Ending Balance at Dec. 31, 2020	$ 1,258,698	$ 301	$ 339,001	$ 948,453	$ (29,057)