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Asset Retirement Obligations - Roll Forward (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Asset Retirement Obligations        
Balance at beginning of period     $ 20,058  
Liabilities incurred     698  
Accretion of ARO liability $ 253 $ 323 720 $ 913
Liabilities settled due to sale of related properties     (1,202)  
Liabilities settled due to plugging and abandonment     (174)  
Change in estimate     (261)  
Balance at end of period $ 19,839   $ 19,839