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Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2016
Jun. 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 0 397  
Liabilities settled (2,574) (1,186)  
Accretion expense 1,226 1,682  
Revisions in estimates 291 554  
Divestiture of oil and gas properties (17) (1,794)  
Asset retirement obligation, end of period 41,482 54,623  
Less: current portion of asset retirement obligation (1,496) (1,405) $ (6,015)
Long-term asset retirement obligation $ 39,986 $ 53,218 $ 36,541