Derivatives (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010	Jun. 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	$ 43,191	$ 166,589	$ 0
Net derivative liability position, collateral paid	0	0	0
Net derivative asset position, collateral received	0	0	(17,675)
Net derivative liability position, collateral received	0	0	(7,467)
Net derivative asset position, fair value of derivatives	40,476	(146,330)	25,059
Net derivative liability position, fair value of derivatives	0	0	0
Net derivative asset position, net	83,667	20,259	7,384
Net derivative liability position, net	$ 0	$ 0	$ (7,467)