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, 2014		$ 88,705	$ 3,804	$ 1,782,937
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options		2,111	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		142	5,066
Excess tax benefits related to share-based compensation	$ 27	27
Net income	74,479
Income available to common stockholders after assumed conversions, Diluted	74,479
Dividends paid to shareholders				(136,386)
End of year at Dec. 31, 2015	1,820,885	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	0
Excess tax benefits related to share-based compensation	913	913
Net income	73,044
Income available to common stockholders after assumed conversions, Diluted	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	0
Excess tax benefits related to share-based compensation	(8)	0
Net income	144,877
Income available to common stockholders after assumed conversions, Diluted	144,877
Dividends paid to shareholders				(137,886)
End of year at Dec. 31, 2017	$ 1,761,387	$ 97,523	$ 0	$ 1,663,864