Consolidated Statements Of Shareholders' Equity (USD $) In Thousands	Common Stock [Member]	Additional Paid In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]	Total
Beginning of year at Dec. 31, 2008	$ 71,428	$ 0	$ (876)	$ 1,423,499
Proceeds of stock options exercised	393				393
Share-based compensation expense	763	0			763
Tax benefit on sales of incentive stock options	5				5
Exercise of stock options		0
Change in other comprehensive loss, net of tax			279
Net income				403,072	403,072
Dividends paid to shareholders				(127,617)	(127,617)
End of year at Dec. 31, 2009	72,589	0	(597)	1,698,954	1,770,946
Proceeds of stock options exercised	816				733
Share-based compensation expense	651	161			812
Tax benefit on sales of incentive stock options	132				132
Exercise of stock options		(83)
Change in other comprehensive loss, net of tax			(143)
Net income				152,198	152,198
Dividends paid to shareholders				(129,863)	(129,863)
End of year at Dec. 31, 2010	74,188	78	(740)	1,721,289	1,794,815
Proceeds of stock options exercised	1,951				1,951
Share-based compensation expense	439	460			899
Tax benefit on sales of incentive stock options	56				56
Exercise of stock options		0
Change in other comprehensive loss, net of tax			740
Net income				191,164	191,164
Dividends paid to shareholders				(132,142)	(132,142)
End of year at Dec. 31, 2011	$ 76,634	$ 538	$ 0	$ 1,780,311	$ 1,857,483