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Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Changes to the Company's asset retirement obligation liability      
Asset retirement obligation, beginning of period $ 36,610 $ 42,556  
Liabilities incurred 574 0  
Liabilities settled (2,486) (2,943)  
Accretion expense 1,671 1,896  
Revisions in estimates (161) 291  
Divestiture of oil and gas properties (248) (75)  
Asset retirement obligation, end of period 35,960 41,725  
Less: current portion of asset retirement obligation (1,630) (1,132) $ (4,160)
Long-term asset retirement obligation $ 34,330 $ 40,593 $ 32,450