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Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2017
Jun. 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 (1,357) (2,574)  
Accretion expense 1,100 1,226  
Revisions in estimates (161) 291  
Divestiture of oil and gas properties (248) (17)  
Asset retirement obligation, end of period 36,518 41,482  
Less: current portion of asset retirement obligation (2,759) (1,496) $ (4,160)
Long-term asset retirement obligation $ 33,759 $ 39,986 $ 32,450