Derivatives (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 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)	$ 0	$ 79,646	$ 61,992
Net derivative asset position, Fair value of derivatives	0	(10,981)	52,731
Net derivative asset position, net	0	68,665	114,723
Net derivative liability position, Collateral paid	49,007	0	0
Net derivative liability position, Fair value of derivatives	(58,195)	0	0
Net derivative liability position, net	$ (9,188)	$ 0	$ 0