Derivatives (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid	$ 151,939	$ 66,870	$ 43,191
Net derivative asset position, Fair value of derivatives	(106,629)	(20,480)	40,476
Net derivative asset position, net	45,310	46,390	83,667
Net derivative liability position, Collateral paid	0	0	0
Net derivative liability position, Fair value of derivatives	0	0	0
Net derivative liability position, net	$ 0	$ 0	$ 0