Derivatives (Details 1) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid	$ 1,351	$ 28,273
Commodity derivative assets	17,252	1,599
Net derivative asset position, net	18,603	29,872
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