Annual report pursuant to Section 13 and 15(d)

Supplemental Oil and Natural Gas Reserve Data (unaudited) (Narrative) (Details)

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Supplemental Oil and Natural Gas Reserve Data (unaudited) (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
MBoe
Dec. 31, 2011
MBoe
Dec. 31, 2010
MBoe
Dec. 31, 2009
MBoe
Reserve Quantities [Line Items]        
Proved Developed and Proved Undeveloped Reserves (BOE), Net 14,072,000 15,928,000    
Proved Developed and Proved Undeveloped Reserves (BOE), Percentage Change Between Periods (12.00%)      
Proved Undeveloped Reserves (BOE), Percentage Change Between Periods (18.00%)      
Proved Undeveloped Reserve (BOE) 1 7,337,000 8,925,000 7,019,000 3,267,000
Proved Developed And Proved Undeveloped Reserves Extensions Discoveries And Additions 2,344,000      
Proved Developed And Proved Undeveloped Reserves Transfers of Proved Developed Production Reserves from Development Program (557,000)      
Proved Developed And Proved Undeveloped Reserves Sales Of Minerals In Place (1,148,000)      
Proved Developed And Proved Undeveloped Reserves Revisions Of Previous Estimates Increase (Decrease) (2,227,000)      
Proved Undeveloped Reserves Conversion to Proved Developed Reserves, Percentage Transfered   (6.00%)    
Proved Undeveloped Reserves Conversion to Proved Developed Reserves Total Cost $ 19      
Permian Basin [Member]
       
Reserve Quantities [Line Items]        
Exploratory Wells Drilled, Net Productive 27      
Medusa fields in the Gulf of Mexico [Member]
       
Reserve Quantities [Line Items]        
Proved Undeveloped Reserve (BOE) 1 1,297,000