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Fair Value Measurements Level 3 Table and Phantom (Details) (USD $)
In Millions, except Per Share data, unless otherwise specified
6 Months Ended
Jun. 30, 2013
Oil Trades
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value $ (365)
Oil Trades | Oil Price Volatility Curve
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Unobservable Input Oil price volatility curves
Weighted average of price volatility curve, percentage 17.22%
Oil Trades | Oil Price Volatility Curve | Minimum
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Price Volatility Curve 10.94%
Oil Trades | Oil Price Volatility Curve | Maximum
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Price Volatility Curve 24.33%
Oil Basis Swaps
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Unobservable Input Physical pricing point forward curves
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 3
Oil Basis Swaps | Oil Price Volatility Curve | Minimum
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Price Forward Curve 6.03
Oil Basis Swaps | Oil Price Volatility Curve | Maximum
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Price Forward Curve 7.07
Oil Basis Swaps | Natural Gas Price Volatility Curve
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Weighted Average Of Price Forward Curve $ 6.55
Natural Gas Trades
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value (225)
Natural Gas Trades | Natural Gas Price Volatility Curve
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Unobservable Input Natural gas price volatility curves
Weighted average of price volatility curve, percentage 22.94%
Natural Gas Trades | Natural Gas Price Volatility Curve | Minimum
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Price Volatility Curve 20.25%
Natural Gas Trades | Natural Gas Price Volatility Curve | Maximum
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Price Volatility Curve 37.87%
Natural Gas Basis Swaps
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value $ (7)
Natural Gas Basis Swaps | Physical Pricing Point Forward Curves
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Unobservable Input Physical pricing point forward curves
Weighted Average Of Price Forward Curve $ (0.28)
Natural Gas Basis Swaps | Physical Pricing Point Forward Curves | Minimum
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Price Forward Curve (1.68)
Natural Gas Basis Swaps | Physical Pricing Point Forward Curves | Maximum
 
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Price Forward Curve (0.04)