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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Changes in ARO liability          
Asset retirement obligations, beginning of period     $ 21,285 $ 17,422  
Additional liabilities incurred     1,535 990  
Liabilities acquired     39 2,180  
Liabilities settled     (1,420) (149)  
Accretion expense $ 365 $ 350 720 673  
Revisions in estimated liabilities     15 (2)  
Asset retirement obligations, end of period 22,174 21,114 22,174 21,114  
Less current portion 394 1,575 394 1,575  
Asset retirement obligations - long-term $ 21,780 $ 19,539 $ 21,780 $ 19,539 $ 20,122