Natural Gas and Oil Producing Activities (Unaudited) (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, 2012	Dec. 31, 2011	Dec. 31, 2010
Natural Gas And Oil Properties [Abstract]
Future cash inflows	$ 9,570,652	$ 22,012,205	$ 19,620,254
Future production costs	(4,737,297)	(8,080,207)	(6,826,915)
Future development costs	(711,050)	(3,425,185)	(3,025,433)
Future income tax expense	(745,251)	(3,366,175)	(3,143,571)
Future net cash flows	3,377,054	7,140,638	6,624,335
10% annual discount for estimated timing of cash flows	(1,326,389)	(3,689,838)	(3,610,585)
Standardized measure of discounted future net cash flows	$ 2,050,665	$ 3,450,800	$ 3,013,750