Quarterly report pursuant to Section 13 or 15(d)

Basis of Presentation and Significant Accounting Policies (Details)

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Basis of Presentation and Significant Accounting Policies (Details) (USD $)
3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
item
Sep. 30, 2012
Basis of Presentation and Significant Accounting Policies        
Other comprehensive income (loss)     $ 0  
Derivative Financial Instruments        
Fair value of commodity derivative contracts 708,000,000   708,000,000  
Impairment of Unproved Properties        
Impairment of unproved properties 3,000,000 2,000,000 10,000,000 5,000,000
Industry Segment and Geographic Information        
Number of operating segments     1  
BNP Paribas
       
Derivative Financial Instruments        
Fair value of commodity derivative contracts 156,000,000   156,000,000  
Credit Suisse
       
Derivative Financial Instruments        
Fair value of commodity derivative contracts 174,000,000   174,000,000  
Wells Fargo
       
Derivative Financial Instruments        
Fair value of commodity derivative contracts 116,000,000   116,000,000  
JP Morgan
       
Derivative Financial Instruments        
Fair value of commodity derivative contracts 113,000,000   113,000,000  
Barclays
       
Derivative Financial Instruments        
Fair value of commodity derivative contracts 112,000,000   112,000,000  
Citi Bank
       
Derivative Financial Instruments        
Fair value of commodity derivative contracts 23,000,000   23,000,000  
Deutsche Bank
       
Derivative Financial Instruments        
Fair value of commodity derivative contracts 11,000,000   11,000,000  
Union Bank
       
Derivative Financial Instruments        
Fair value of commodity derivative contracts 1,000,000   1,000,000  
Toronto Dominion Bank
       
Derivative Financial Instruments        
Fair value of commodity derivative contracts $ 2,000,000   $ 2,000,000