Derivatives (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid (received)	$ 15,452	$ 28,273	$ 38,252
Net derivative asset position, Fair value of derivatives	(12,835)	1,599	13,491
Net derivative asset position, net	2,617	29,872	51,743
Net derivative liability position, Collateral paid	0	0	(480)
Net derivative liability position, Fair value of derivatives	0	0	1,480
Net derivative liability position, net	$ 0	$ 0	$ 1,000