Derivatives (Net Asset or Liability Positions in Balance Sheet) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid (received)	$ 799	$ 56,005	$ 109,346
Net derivative asset position, Fair value of derivatives	21,363	(10,323)	(5,996)
Net derivative asset position, net	22,162	45,682	103,350
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