Derivatives (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid (received)	$ 54,762	$ 1,351	$ (2,769)
Net derivative asset position, Fair value of derivatives	(18,874)	17,252	32,310
Net derivative asset position, net	35,888	18,603	29,541
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