Derivatives (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid	$ 66,870	$ 166,589
Net derivative asset position, Collateral received	0	0
Net derivative asset position, Fair value of derivatives	(20,480)	(146,330)
Net derivative asset position, net	46,390	20,259
Net derivative liability position, Collateral paid	0	0
Net derivative liability position, Collateral received	0	0
Net derivative liability position, Fair value of derivatives	0	0
Net derivative liability position, net	$ 0	$ 0