Derivatives (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid (received)	$ 38,252	$ 3,008	$ 0
Net derivative asset position, Fair value of derivatives	13,491	25,356	0
Net derivative asset position, net	51,743	28,364	0
Net derivative liability position, Collateral paid	(480)	0	49,007
Net derivative liability position, Fair value of derivatives	1,480	0	(58,195)
Net derivative liability position, net	$ 1,000	$ 0	$ (9,188)