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Supplemental Information on Oil and Natural Gas Producing Activities - Summary of Changes in Estimated Oil and Natural Gas Reserves (Unaudited) (Parenthetical) (Details)
Dec. 31, 2012
MMcf
Dec. 31, 2011
MMcf
Dec. 31, 2010
MMcf
Dec. 31, 2009
MMcf
Oil Reserves
        
Reserve Quantities [Line Items]        
Proved reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas)   244,785 [1] 252,114 105,349
Natural Gas Liquids
        
Reserve Quantities [Line Items]        
Proved reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas) 67,994 [1],[2] 30,335 [1],[2]    
Natural Gas Reserves
        
Reserve Quantities [Line Items]        
Proved reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas) 1,415,042 [1],[3] 1,355,056 [1],[3] 1,762,665 [3] 680,075 [3]
Oil Excluding NGLs
        
Reserve Quantities [Line Items]        
Proved reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas) 262,045 214,450 [1]    
Noncontrolling Interest | Natural Gas Liquids
        
Reserve Quantities [Line Items]        
Proved reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas) 5,132 1,782    
Noncontrolling Interest | Natural Gas Reserves
        
Reserve Quantities [Line Items]        
Proved reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas) 94,543 45,500    
Noncontrolling Interest | Oil Excluding NGLs
        
Reserve Quantities [Line Items]        
Proved reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas) 17,340 17,018    
[1] Proved reserves at December 31, 2012 and 2011, respectively, include approximately 17,340 MBbls and 17,018 MBbls of oil, 5,132 MBbls and 1,782 MBbls of NGLs and 94,543 MMcf and 45,500 MMcf of natural gas attributable to noncontrolling interests.
[2] Prior to 2012, NGLs did not comprise a significant portion of total proved reserves and were included with oil reserves, which affects the comparability of estimated reserves for 2012, 2011 and 2010.
[3] Natural gas reserves are computed at 14.65 pounds per square inch absolute and 60 degrees Fahrenheit.