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
9 Months Ended 12 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Asset Retirement Obligations [Abstract]    
Beginning balance $ 17,334 $ 15,844
Accretion 1,179 893
Additions 14,564 359
Revisions   238
Ending balance $ 33,077 $ 17,334