Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) (Details 4) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves
Future cash inflows	$ 23,954,048	$ 19,676,630
Future Costs
Production	(5,113,902)	(4,305,292)
Development	(3,230,577)	(2,855,407)
Future net cash flows before income taxes	15,609,569	12,515,931
Future income tax expense	(5,040,104)	(3,923,264)
Total future net cash flows before 10% discount	10,569,465	8,592,667
10% annual discount	(6,054,568)	(5,113,541)
Standardized measure of discounted future net cash flows	$ 4,514,897	$ 3,479,126