Annual report pursuant to Section 13 and 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
1 Months Ended 12 Months Ended
Dec. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Asset Retirement Obligations [Abstract]      
Beginning balance   $ 15,844 $ 14,816
Accretion   893 812
Additions   359 595
Revisions $ (400) 238 (379)
Ending balance $ 15,844 $ 17,334 $ 15,844