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Asset Retirement Obligation (Asset Retirement Obligation Transactions) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Asset Retirement Obligation [Abstract]      
Balance $ 2,046 $ 2,222  
Accretion expense 141 143 $ 126
Liabilities incurred 1 2  
Liabilities settled (45) (86)  
Liabilities sold properties (11) (25)  
Revisions in estimated liabilities 138 (210) 112
Balance $ 2,270 $ 2,046 $ 2,222