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
6 Months Ended 12 Months Ended
Jun. 30, 2020
Dec. 31, 2019
Asset Retirement Obligations [Abstract]    
Beginning balance $ 15,844 $ 14,816
Accretion 440 812
Additions 359 595
Revisions   (379)
Ending balance $ 16,643 $ 15,844