Derivatives (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 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 (received)	$ (2,769)	$ 28,273	$ 27,498
Net derivative asset position, Fair value of derivatives	32,310	1,599	11,389
Net derivative asset position, net	29,541	29,872	38,887
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