Derivatives (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid (received)	$ 28,585	$ 79,646	$ (79,711)
Net derivative asset position, Fair value of derivatives	5,733	(10,981)	147,983
Net derivative asset position, net	34,318	68,665	68,272
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