Derivatives (Net Asset or Liability Positions in Balance Sheet) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 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)	$ (52,888)	$ 1,351	$ 15,452
Net derivative asset position, Fair value of derivatives	68,244	17,252	(12,835)
Net derivative asset position, net	15,356	18,603	2,617
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