Supplemental Oil And Natural Gas Disclosures (Unaudited) (Changes in Standardized Measure) (Details 5) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Principal Sources of Change in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Abstract]
Beginning of year	$ 7,007,605	$ 4,917,927	$ 2,457,385
Sales of oil and natural gas produced, net of production costs	(1,673,253)	(1,597,288)	(1,177,322)
Net changes in sales prices	(584,526)	4,646,086	2,062,181
Previously estimated development costs incurred	376,199	354,228	193,947
Revisions of previous estimates, including revised estimates of development costs, reserves and rates of production	(797,975)	(1,673,283)	(285,158)
Accretion of discount	875,383	729,234	307,546
Acquisition of minerals in place	767,267	29,737	3,671,439
Sales of minerals in place	(1,805,309)	0	(1,474,443)
Net change in income taxes	56,322	(1,177,114)	(1,756,344)
End of year	6,414,380	7,007,605	4,917,927
Tertiary Recovery [Member]
Principal Sources of Change in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Abstract]
Extensions, Discoveries, Additions and Improved Recovery, Less Related Costs	1,901,109	15,708	623,622
Non-tertiary
Principal Sources of Change in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Abstract]
Extensions, Discoveries, Additions and Improved Recovery, Less Related Costs	$ 291,558	$ 762,370	$ 295,074