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, 2013		$ 81,591	$ 411	$ 1,740,484
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options		6,824	577
Share-based compensation expense		142	3,970
Payment for vested restricted stock units and related taxes		0	0
Reclassification of restricted stock units from equity to liability award			0
Excess tax benefits related to share-based compensation	$ 148	148
Net income	177,949			177,949
Dividends paid to shareholders				(135,496)
End of year at Dec. 31, 2014	1,875,446	88,705	3,804	1,782,937
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options		2,111	0
Share-based compensation expense		142	5,066
Payment for vested restricted stock units and related taxes		0	0
Reclassification of restricted stock units from equity to liability award			0
Excess tax benefits related to share-based compensation	27	27
Net income	74,479			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
Share-based compensation expense		142	0
Payment for vested restricted stock units and related taxes		1,671	(5,122)
Reclassification of restricted stock units from equity to liability award			(3,435)
Excess tax benefits related to share-based compensation	913	913
Net income	73,044			73,044
Dividends paid to shareholders				(137,201)
End of year at Dec. 31, 2016	$ 1,752,402	$ 95,529	$ 0	$ 1,656,873