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, 2018		27,516,218
Stockholders' Equity, Beginning Balance at Dec. 31, 2018	$ 855,303	$ 301	$ 330,517	$ 580,992	$ (56,507)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	85,807			85,807
Stock options exercised, shares		710,830
Stock options exercised	14,348		(1,177)		15,525
Stock-based compensation expense	4,086		4,086
Repurchase of common shares, shares		(201,088)
Repurchase of common shares	(5,150)				(5,150)
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 Sep. 30, 2019		28,142,916
Stockholders' Equity, Ending Balance at Sep. 30, 2019	954,641	$ 301	331,128	666,799	(43,587)
Shares Outstanding, Beginning Balance at Jun. 30, 2019		27,617,366
Stockholders' Equity, Beginning Balance at Jun. 30, 2019	904,240	$ 301	330,052	628,961	(55,074)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	37,838			37,838
Stock options exercised, shares		525,550
Stock options exercised	11,071		(416)		11,487
Stock-based compensation expense	1,492		1,492
Shares Outstanding, Ending Balance at Sep. 30, 2019		28,142,916
Stockholders' Equity, Ending Balance at Sep. 30, 2019	954,641	$ 301	331,128	666,799	(43,587)
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	100,000	(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)