Quarterly report pursuant to Section 13 or 15(d)

Asset Retirement Obligations - Summary of Changes to Asset Retirement Obligation (Details)

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Asset Retirement Obligations - Summary of Changes to Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]      
Asset retirement obligations, beginning of period $ 390,568    
Asset retirement obligation settlements (21,939) $ (30,338)  
Accretion of discount 10,930    
Disposition of properties (965)    
Liabilities assumed through acquisition 2,944    
Liabilities incurred 4,671    
Revisions of estimated liabilities [1] 3,858    
Asset retirement obligations, end of period 390,067    
Less current portion 41,494   $ 36,003
Long-term $ 348,573   $ 354,565
[1] Revisions were primarily attributable to increases from non-operated properties.