Condensed Consolidated Statement of Shareholders' Equity - 3 months ended Mar. 31, 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	18,063			18,063
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	1,039		1,039
Deferral of executive and director compensation	185		185
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 Mar. 31, 2018		28,570,853
Stockholders' Equity, Ending Balance at Mar. 31, 2018	$ 786,365	$ 301	$ 325,780	$ 491,392	$ (31,108)