Disclosure About Oil and Gas Producing Activities (Unaudited) (Details 5) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Results of Operations, Oil and Gas Producing Activities Net Income (Excluding Corporate Overhead and Interest Costs) [Abstract]
Oil and gas revenue	$ 809,974	$ 1,101,796	$ 979,386
Production expenses	(184,229)	(205,363)	(191,978)
Depletion and depreciation expense	(388,985)	(346,394)	(241,796)
Write-downs of proved oil and gas properties	(2,972,464)	0	0
Income taxes	662,698	(197,464)	(193,692)
Total	$ (2,073,006)	$ 352,575	$ 351,920