Natural Gas and Oil Properties (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Cash flow hedges impact on ceiling value	$ 262,500,000
Ceiling test impairment, amount net of tax			558,300,000
Discounted future net cash flows to proved natural gas and oil reserves	10.00%
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
Henry Hub Natural Gas [Member]
Full cost ceiling test, Price	$ 4.12	4.38	$ 3.87