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Asset Retirement Obligations - Roll Forward (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Asset Retirement Obligations    
Balance at beginning of period $ 20,058  
Liabilities incurred 413  
Accretion of ARO liability 201 $ 293
Liabilities settled due to plugging and abandonment (37)  
Balance at end of period $ 20,635