Derivatives (Details 1) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid	$ 3,008	$ 79,646
Net derivative asset position, Fair value of derivatives	25,356	(10,981)
Net derivative asset position, net	28,364	68,665
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