Natural Gas and Oil Producing Activities (Unaudited) (Narrative) (Details) (USD $)	3 Months Ended		12 Months Ended			72 Months Ended	108 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012
Cash flow hedges impact on ceiling value			$ 257,600,000				$ 257,600,000
Impairment Of Oil And Gas Properties Net Of Tax	185,700,000	496,400,000	510,400,000
Discount rate of natural gas and oil reserves			10.00%
Acquisition Costs, Period Cost			193,258,000	232,615,000	123,736,000	45,081,000	594,690,000	[1]
Average market prices per MMBtu for natural gas			2.76	4.12	4.38
Average Market Price Per Barrel of Oil			91.21	92.71	75.96
West Texas Intermediate Oil [Member]
Full cost ceiling test, Price			91.21	92.71	75.96
Henry Hub Natural Gas [Member]
Full cost ceiling test, Price			$ 2.76	$ 4.12	$ 4.38

[1] Property acquisition costs include $14.8 million, exploration costs include $22.5 million and capitalized interest includes $3.1 million related to our exploration program in Canada.