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Asset Retirement Obligations - Roll Forward (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Asset Retirement Obligations        
Balance at beginning of period     $ 20,058  
Liabilities incurred     563  
Accretion of ARO liability $ 266 $ 297 467 $ 590
Liabilities settled due to sale of related properties     (60)  
Liabilities settled due to plugging and abandonment     (56)  
Liabilities related to assets held for sale     (1,143)  
Change in estimate     (168)  
Balance at end of period $ 19,661   $ 19,661