Consolidated Statement of Shareholders' Equity - USD ($) $ in Thousands	Total	Preferred Shares [Member]	Common Shares [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Treasury Shares [Member]
Shares Outstanding, Beginning Balance at Dec. 31, 2014		2,000	24,512,910
Stockholders' Equity, Beginning Balance at Dec. 31, 2014	$ 544,295	$ 48,163	$ 271	$ 238,560	$ 308,539	$ (51,238)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	51,763				51,763
Dividends declared to preferred shareholders	(4,875)				(4,875)
Stock options exercised - shares			72,640
Stock options exercised - value	1,035			(408)		1,443
Stock-based compensation expense	3,942			3,942
Deferral of executive and director compensation	406			406
Executive and director deferred compensation distributions - shares			63,494
Executive and director deferred compensation distributions - value	0			(1,261)		1,261
Shares Outstanding, Ending Balance at Dec. 31, 2015		2,000	24,649,044
Stockholders' Equity, Ending Balance at Dec. 31, 2015	596,566	$ 48,163	$ 271	241,239	355,427	(48,534)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	56,609				56,609
Dividends declared to preferred shareholders	(4,875)				(4,875)
Reversal of deferred tax asset related to stock options and executive deferred compensation distributions				269
Stock options exercised - shares			14,600
Stock options exercised - value	182			(108)		290
Stock-based compensation expense	5,315			5,315
Deferral of executive and director compensation	108			108
Executive and director deferred compensation distributions - shares			13,789
Executive and director deferred compensation distributions - value	0			(274)		274
Shares Outstanding, Ending Balance at Dec. 31, 2016		2,000	24,677,433
Stockholders' Equity, Ending Balance at Dec. 31, 2016	654,174	$ 48,163	$ 271	246,549	407,161	(47,970)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	72,081				72,081
Fair value over carrying value of preferred share redeemed	0	$ 2,257			(2,257)
Dividends declared to preferred shareholders	(3,656)				(3,656)
Common share issuance for conversion of convertible notes - shares		0	2,415,903
Common share issuance for conversion of convertible notes - value	57,500	$ 0	$ 24	57,476	0	0
Preferred shares redeemed - shares		(2,000)
Preferred shares redeemed - value	$ (50,420)	$ (50,420)
Stock options exercised - shares	(678,781)		678,781
Stock options exercised - value	$ 11,225			(2,255)		13,480
Stock-based compensation expense	6,044			6,044
Deferral of executive and director compensation	350			350
Executive and director deferred compensation distributions - shares			84,635
Executive and director deferred compensation distributions - value	0			(1,681)		1,681
Shares Outstanding, Ending Balance at Dec. 31, 2017		0	27,856,752
Stockholders' Equity, Ending Balance at Dec. 31, 2017	$ 747,298	$ 0	$ 295	$ 306,483	$ 473,329	$ (32,809)