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Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Changes to the Company's asset retirement obligation liability      
Asset retirement obligation, beginning of period $ 42,556 $ 54,970  
Liabilities incurred 0 392  
Liabilities settled (2,943) (1,826)  
Accretion expense 1,896 2,507  
Revisions in estimates 291 (5,345)  
Divestiture of oil and gas properties (75) (2,103)  
Asset retirement obligation, end of period 41,725 48,595  
Less: current portion of asset retirement obligation (1,132) (5,378) $ (6,015)
Long-term asset retirement obligation $ 40,593 $ 43,217 $ 36,541