Derivatives (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 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	$ 33,364	$ (13,772)	$ 151,939
Net derivative asset position, Fair value of derivatives	46,329	61,247	(106,629)
Net derivative asset position, net	79,693	47,475	45,310
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