Derivatives (Details 1) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid	$ 15,480	$ (13,772)
Net derivative asset position, Fair value of derivatives	31,055	61,247
Net derivative asset position, net	46,535	47,475
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