Summary Of Significant Accounting Policies (Net Income (Loss) Per Common Share) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Summary Of Significant Accounting Policies [Abstract]
Net income (loss)	$ 41,042	$ 63,054	$ 131,312	$ (315,894)	$ 302,533	$ 174,200	$ 156,673	$ (22,652)	$ (80,486)		$ 610,754		$ 806,989
Less: dividends declared-common shares and nonvested common shares									(103,848)		(108,477)		(116,856)
Undistributed earnings									(184,334)		502,277		690,133
Percentage allocated to common shareholders									99.30%	[1]	99.50%	[1]	99.70%	[1]
Net income (loss) available to common stockholders, total									(183,134)		499,882		687,729
Add: dividends declared-common shareholders									103,150		107,926		116,477
Numerator for basic and diluted earnings per common share									$ (79,984)		$ 607,808		$ 804,207
Denominator for basic earnings per weighted-average common shares									53,792		56,630		60,661
Effect of dilutive securities, Options									124		156		187
Denominator for diluted earnings per adjusted weighted-average common shares									53,916		56,786		60,847
Per common share net income (loss), Basic	$ 0.76	$ 1.16	$ 2.42	$ (5.81)	$ 5.53	$ 3.12	$ 2.70	$ (0.38)	$ (1.49)		$ 10.73		$ 13.26
Per common share net income (loss), Diluted	$ 0.76	$ 1.16	$ 2.41	$ (5.81)	$ 5.51	$ 3.11	$ 2.70	$ (0.38)	$ (1.49)		$ 10.70		$ 13.22
Basic weighted-average common shares outstanding									53,792		56,630		60,661
Basic weighted-average common shares outstanding and nonvested common shares expected to vest									54,145		56,902		60,873

[1] Some amounts may not reconcile due to rounding