Derivatives (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid	$ 27,101	$ (13,772)	$ 88,246
Net derivative asset position, Fair value of derivatives	38,352	61,247	(73,424)
Net derivative asset position, net	65,453	47,475	14,822
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