Quarterly report pursuant to Section 13 or 15(d)

Asset Retirement Obligations (Summary of Changes In Asset Retirement Obligations) (Details)

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Asset Retirement Obligations (Summary of Changes In Asset Retirement Obligations) (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Asset Retirement Obligations [Abstract]    
Beginning balance $ 14,816 $ 20,163
Accretion 198 1,180
Revisions   (6,527)
Ending balance $ 15,014 $ 14,816