Derivatives (Details 1) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid	$ 28,273	$ 3,008
Net derivative asset position, Fair value of derivatives	1,599	25,356
Net derivative asset position, net	29,872	28,364
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