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	47,969			47,969
Stock options exercised, shares		185,280
Stock options exercised	3,277		(761)		4,038
Stock-based compensation expense	2,594		2,594
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 Jun. 30, 2019		27,617,366
Stockholders' Equity, Ending Balance at Jun. 30, 2019	904,240	$ 301	330,052	628,961	(55,074)
Shares Outstanding, Beginning Balance at Mar. 31, 2019		27,568,826
Stockholders' Equity, Beginning Balance at Mar. 31, 2019	871,462	$ 301	328,580	598,715	(56,134)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	30,246			30,246
Stock options exercised, shares		48,540
Stock options exercised	849		(211)		1,060
Stock-based compensation expense	1,682		1,682
Deferral of executive and director compensation	1		1
Shares Outstanding, Ending Balance at Jun. 30, 2019		27,617,366
Stockholders' Equity, Ending Balance at Jun. 30, 2019	904,240	$ 301	330,052	628,961	(55,074)
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	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 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)