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Supplemental Information on Oil and Natural Gas Producing Activities - Additional Information (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
MBbls
Dec. 31, 2011
MBbls
Dec. 31, 2010
MBbls
Dec. 31, 2009
MBbls
Reserve Quantities [Line Items]        
Percentage of proved reserves estimated by the Company 2.40%      
Standardized measure of discounted future net cash flows 5,840,368 [1] 5,216,337 [1] 3,683,515 [1]  
Oil Reserves
        
Reserve Quantities [Line Items]        
Change in proved reserves (in MBbls for Oil/MMcf for Natural Gas) 108,800 36,000 154,200  
Revisions of previous estimates (in MBbls for Oil and NGLs/MMcf for Natural Gas)   (9,278) 12,999  
Proved developed and undeveloped reserves, extensions, discoveries, and additions (in MBbls for Oil/MMcf for Natural Gas)   55,577 69,512  
Proved developed and undeveloped reserves, production (in MBbls for Oil/MMcf for Natural Gas)   (11,830) (7,386)  
Proved developed and undeveloped reserves, sales of reserves in place (in MBbls for Oil/MMcf for Natural Gas) 23,600 (43,331) 0  
Proved reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas)   244,785 [2] 252,114 105,349
Natural Gas Reserves
        
Reserve Quantities [Line Items]        
Change in proved reserves (in MBbls for Oil/MMcf for Natural Gas) 60,500 68,600    
Revisions of previous estimates (in MBbls for Oil and NGLs/MMcf for Natural Gas) (538,214) [3] (164,845) [3] 867,931 [3]  
Proved developed and undeveloped reserves, extensions, discoveries, and additions (in MBbls for Oil/MMcf for Natural Gas) 489,302 [3] 299,848 [3] 211,150 [3]  
Proved developed and undeveloped reserves, production (in MBbls for Oil/MMcf for Natural Gas) (93,549) [3] (69,306) [3] (76,226) [3]  
Proved developed and undeveloped reserves, sales of reserves in place (in MBbls for Oil/MMcf for Natural Gas) (548) [3] (476,212) [3] (207) [3]  
Proved reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas) 1,415,042 [2],[3] 1,355,056 [2],[3] 1,762,665 [3] 680,075 [3]
Oil Excluding NGLs
        
Reserve Quantities [Line Items]        
Revisions of previous estimates (in MBbls for Oil and NGLs/MMcf for Natural Gas) (37,394)      
Proved developed and undeveloped reserves, extensions, discoveries, and additions (in MBbls for Oil/MMcf for Natural Gas) 89,656      
Proved developed and undeveloped reserves, production (in MBbls for Oil/MMcf for Natural Gas) (15,868)      
Proved developed and undeveloped reserves, sales of reserves in place (in MBbls for Oil/MMcf for Natural Gas) (20,269)      
Proved reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas) 262,045 214,450 [2]    
Permian Properties
        
Reserve Quantities [Line Items]        
Proved developed and undeveloped reserves, net (in MMBoe) 198.9      
Proved Developed Reserves as a Percentage of Total Proved Reserves 55.00%      
Standardized measure of discounted future net cash flows 2,500,000      
Mid-Continent and Permian Basin | Oil Reserves
        
Reserve Quantities [Line Items]        
Revisions of previous estimates (in MBbls for Oil and NGLs/MMcf for Natural Gas) (22,300)      
Proved developed and undeveloped reserves, extensions, discoveries, and additions (in MBbls for Oil/MMcf for Natural Gas) 116,900      
[1] Includes approximately $952.7 million and $932.8 million attributable to noncontrolling interests at December 31, 2012 and 2011, respectively.
[2] 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.
[3] Natural gas reserves are computed at 14.65 pounds per square inch absolute and 60 degrees Fahrenheit.