Fair Value Measurements and Disclosures of Assets and Liabilities Measured on a Nonrecurring Basis (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties	$ 104		$ 757		$ 144
Discount Rate for Impairment Model	10.00%
Quoted Prices in Active Markets (Level 1)
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties	0	[1]	0	[1]	0	[1]
Significant Other Observable Inputs (Level 2)
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties	0	[1]	0	[1]	0	[1]
Significant Unobservable Inputs (Level 3)
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties	228	[1]	213	[1]	30	[1]
Net Book Value
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties	$ 332	[2]	$ 970	[2]	$ 174	[2]

[1] See Note 1. Summary of Significant Accounting Policies - Fair Value Measurements for a description of the fair value hierarchy.

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