Fair Value Measurements and Disclosures of Assets and Liabilities Measured on a Nonrecurring Basis (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Asset Impairment Charges [Abstract]
Impairment of Oil and Gas Properties	$ 63		$ 0		$ 63		$ 73
Discount Rate for Impairment Model	10.00%		10.00%		10.00%		10.00%
Quoted Prices in Active Markets (Level 1)
Asset Impairment Charges [Abstract]
Impairment of Oil and Gas Properties	0		0		0		0
Significant Other Observable Inputs (Level 2)
Asset Impairment Charges [Abstract]
Impairment of Oil and Gas Properties	0		0		0		0
Significant Unobservable Inputs (Level 3)
Asset Impairment Charges [Abstract]
Impairment of Oil and Gas Properties	75		0		75		172
Net Book Value
Asset Impairment Charges [Abstract]
Impairment of Oil and Gas Properties	$ 138	[1]	$ 0	[1]	$ 138	[1]	$ 245	[1]

[1] Amount represents net book value at the date of assessment.