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Asset Retirement Obligation (Asset Retirement Obligation Transactions) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Asset Retirement Obligation [Abstract]    
Balance $ 1,998 $ 2,179
Accretion expense 125 132
Liabilities incurred 12
Liabilities settled (25) (83)
Liabilities sold properties (50) (55)
Revisions in estimated liabilities 69 (187)
Balance $ 2,117 $ 1,998