Derivatives (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 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)	$ 27,737	$ 28,273	$ 13,358
Net derivative asset position, Fair value of derivatives	(999)	1,599	16,258
Net derivative asset position, net	26,738	29,872	29,616
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