Fair Value Measurements - Level 3 Table and Phantom (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2013
Crude Oil [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value Assets Unobservable Input Description	Oil price volatility curves
Weighted Average Of Price Volatility Curve Percentage	13.62%
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ (265)
Crude Oil [Member] | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price Volatility Curve	0.00%
Crude Oil [Member] | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price Volatility Curve	23.65%
Natural Gas [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value Assets Unobservable Input Description	Natural gas price volatility curves
Weighted Average Of Price Volatility Curve Percentage	22.49%
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	(217)
Natural Gas [Member] | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price Volatility Curve	17.75%
Natural Gas [Member] | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price Volatility Curve	60.88%
Basis Swap [Member] | Crude Oil [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value Assets Unobservable Input Description	Physical pricing point forward curves
Weighted Average Of Price Forward Curve	$ 3.74
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	1
Basis Swap [Member] | Crude Oil [Member] | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price Forward Curve	3.51
Basis Swap [Member] | Crude Oil [Member] | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price Forward Curve	4.41
Basis Swap [Member] | Natural Gas [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value Assets Unobservable Input Description	Physical pricing point forward curves
Weighted Average Of Price Forward Curve	$ (0.46)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 3
Basis Swap [Member] | Natural Gas [Member] | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price Forward Curve	(1.03)
Basis Swap [Member] | Natural Gas [Member] | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price Forward Curve	(0.11)