Natural Gas and Oil Producing Activities (Standardized Measures of Discounted Future Net Cash Flows Relating to Proved Natural Gas and Oil Reserves) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Natural Gas And Oil Producing Activities Abstract
Future cash inflows	$ 22,012,205	$ 19,620,254	$ 12,533,868
Future production costs	(8,080,207)	(6,826,915)	(4,488,884)
Future development costs	(3,425,185)	(3,025,433)	(2,367,206)
Future income tax expense	(3,366,175)	(3,143,571)	(1,569,242)
Future net cash flows	7,140,638	6,624,335	4,108,536
10% annual discount for estimated timing of cash flows	(3,689,838)	(3,610,585)	(2,306,718)
Standardized measure of discounted future net cash flows	$ 3,450,800	$ 3,013,750	$ 1,801,818
Average market prices per MMBtu for natural gas	4.12	4.38	3.87
Average market prices per barrel for oil	92.71	75.96	57.65