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Asset Retirement Obligation (Asset Retirement Obligation Transactions) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Asset Retirement Obligation [Abstract]    
Balance $ 2,179  
Accretion expense 35 $ 36
Liabilities incurred  
Liabilities settled  
Balance $ 2,214