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Asset Retirement Obligation (Asset Retirement Obligation Transactions) (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Asset Retirement Obligation [Abstract]    
Balance $ 2,222  
Accretion expense 73 $ 63
Liabilities incurred  
Liabilities settled (49)  
Balance $ 2,246