Derivatives (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid	$ 61,992	$ 15,480	$ 33,364
Net derivative asset position, Fair value of derivatives	52,731	31,055	46,329
Net derivative asset position, net	114,723	46,535	79,693
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