Condensed Consolidated Statement of Shareholders' Equity - 9 months ended Sep. 30, 2018 - 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
Stock-based compensation expense	$ 3,771		3,771
Repurchase of Common Shares, shares	(437,490)	(437,490)
Repurchase of common shares	$ (11,085)				(11,085)
Deferral of executive and director compensation	184		184
Executive and director deferred compensation distributions shares		61,366
Executive and director deferred compensation distributions	(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)