Derivatives (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, collateral paid	$ 0	$ 166,589	$ 161,274
Net derivative liability position, collateral paid	0	0	0
Net derivative asset position, collateral received	(60,247)	0	0
Net derivative liability position, collateral received	0	0	0
Net derivative asset position, fair value of derivatives	139,882	(146,330)	(102,662)
Net derivative liability position, fair value of derivatives	0	0	0
Net derivative asset position, net	79,635	20,259	58,612
Net derivative liability position, net	$ 0	$ 0	$ 0