Derivatives (Fair Value of Commodity Derivatives with Cash Collateral) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid	$ 14,944	$ 1,351
Commodity derivative assets	22,285	17,252
Net derivative asset position, net	37,229	18,603
Net derivative liability position, Collateral paid	0	0
Net derivative liability position, Fair value of derivatives	0	0
Net derivative liability position, net	$ 0	$ 0