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, 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	75,256			75,256
Proceeds from issuance of common shares for conversion of convertible notes	20,309	$ 6	20,303
Common shares issued for conversion of convertible notes		628,515
Stock options exercised, shares		24,220
Stock options exercised	426		(56)		482
APIC, Share-based Payment Arrangement, Increase for Cost Recognition	3,771		3,771
Repurchase of common shares, shares		(437,490)
Repurchase of common shares	(11,085)				(11,085)
Deferral of executive and director compensation	184		184
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 Sep. 30, 2018		28,133,363
Stockholders' Equity, Ending Balance at Sep. 30, 2018	835,204	$ 301	328,511	548,585	(42,193)
Shares Outstanding, Beginning Balance at Jun. 30, 2018		28,570,853
Stockholders' Equity, Beginning Balance at Jun. 30, 2018	815,966	$ 301	327,470	519,303	(31,108)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	29,282			29,282
APIC, Share-based Payment Arrangement, Increase for Cost Recognition	1,041		1,041
Repurchase of common shares, shares		(437,490)
Repurchase of common shares	(11,085)				(11,085)
Shares Outstanding, Ending Balance at Sep. 30, 2018		28,133,363
Stockholders' Equity, Ending Balance at Sep. 30, 2018	835,204	$ 301	328,511	548,585	(42,193)
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
APIC, Share-based Payment Arrangement, Increase for Cost Recognition	$ 4,086		4,086
Repurchase of common shares, shares	(200,000)	(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
APIC, Share-based Payment Arrangement, Increase for Cost Recognition	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)