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Asset Retirement Obligation (Asset Retirement Obligation Transactions) (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Asset Retirement Obligation [Abstract]    
Balance $ 1,998  
Accretion expense 63 $ 67
Liabilities incurred  
Liabilities settled  
Liabilities relieved - sold properties (50)  
Balance $ 2,011