Basis of Presentation and Significant Accounting Policies (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary of Significant Accounting Policies (Details)
Impairments of oil and gas properties	$ 0	$ 15	$ 64
Impairment of long lived assets	8	7	15
Future reimbursement of decommissioning liabilities	0	7
Reduction in decommissioning liabilities for work performed	$ 87	$ 95	$ 88