Derivatives (Details 1) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid	$ (13,772)	$ 66,870
Net derivative asset position, Fair value of derivatives	61,247	(20,480)
Net derivative asset position, net	47,475	46,390
Net derivative liability position, Collateral paid	0	0
Net derivative liability position, Fair value of derivatives	0	0
Net derivative liability position, net	$ 0	$ 0