Derivatives (Net Asset or Liability Positions in Balance Sheet) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 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)	$ 14,942	$ 1,351	$ 27,737
Net derivative asset position, Fair value of derivatives	36,653	17,252	(999)
Net derivative asset position, net	51,595	18,603	26,738
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