Derivatives (Net Asset or Liability Positions in Balance Sheet) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid (received)	$ 21,751	$ 14,944	$ 54,762
Net derivative asset position, Fair value of derivatives	38,580	22,285	(18,874)
Net derivative asset position, net	60,331	37,229	35,888
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