Fair Value Measurement (Unaudited) Unobservable Inputs (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Liability	$ (208)		$ (20)
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Liability	(229)	[1]	0	[1],[2]
Plains Offshore Warrants [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Liability	(11)		0
Derivative financial instruments assumed in the PXP acquisition	(12)
Net unrealized (losses) gains included in earnings related to assets and liabilities still held at the end of the period	1	[3]
Plains Offshore Warrants [Member] | Minimum [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Assumptions, Expected Volatility Rate	20.00%
Plains Offshore Warrants [Member] | Maximum [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Assumptions, Expected Volatility Rate	54.00%
Plains Offshore Warrants [Member] | Weighted Average [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Assumptions, Expected Volatility Rate	28.00%
Crude Oil Options [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Liability	(218)		0
Derivative financial instruments assumed in the PXP acquisition	(83)
Net unrealized (losses) gains included in earnings related to assets and liabilities still held at the end of the period	$ (135)	[4]
Crude Oil Options [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Deferred Premium Price	5.15
Crude Oil Options [Member] | Minimum [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Assumptions, Expected Volatility Rate	18.00%
Crude Oil Options [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Deferred Premium Price	7.22
Crude Oil Options [Member] | Maximum [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Assumptions, Expected Volatility Rate	46.00%
Crude Oil Options [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Deferred Premium Price	6.35
Crude Oil Options [Member] | Weighted Average [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Assumptions, Expected Volatility Rate	25.00%

[1] Recorded at fair value.

[2] Crude oil options are net of $461 million for deferred premiums and accrued interest at September 30, 2013. Refer to Note 9 for further discussion and balance sheet classifications.

[3] Realized and unrealized (losses) gains are recorded in other income. There were no realized gains for the first nine months of 2013.

[4] Realized and unrealized (losses) gains are recorded in revenue. There were no realized gains for the first nine months of 2013.