Consolidated Statements Of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]
Beginning of year at Dec. 31, 2015		$ 90,985	$ 8,870	$ 1,721,030
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options		1,818	(313)
Payment for vested restricted stock units and related taxes		1,671	(5,122)
Reclassification of restricted stock units from equity to liability award		0	(3,435)
Share-based compensation expense		142
Excess tax benefits related to share-based compensation		913
Net (loss) income	$ 73,044
Dividends paid to shareholders				(137,201)
End of year at Dec. 31, 2016	1,752,402	95,529	0	1,656,873
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options		2,102	0
Payment for vested restricted stock units and related taxes		0	0
Reclassification of restricted stock units from equity to liability award		168	0
Share-based compensation expense		60
Excess tax benefits related to share-based compensation		0
Net (loss) income	144,877
Dividends paid to shareholders				(137,886)
End of year at Dec. 31, 2017	1,761,387	97,523	0	1,663,864
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options		358	0
Payment for vested restricted stock units and related taxes		0	0
Reclassification of restricted stock units from equity to liability award		0	0
Share-based compensation expense		145
Excess tax benefits related to share-based compensation		0
Net (loss) income	(5,728)
Dividends paid to shareholders				(138,478)
End of year at Dec. 31, 2018	$ 1,617,684	$ 98,026	$ 0	$ 1,519,658