Basis of Presentation and Summary of Significant Account Policies (Accounts Receivable) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Accounts Receivable [Abstract]
Natural gas and oil sales	$ 1,457	$ 1,089
Joint interest	592	1,171
Oilfield services	24	43
Related parties(a)	23	45
Other	168	176
Allowance for doubtful accounts	(19)	(19)
Total accounts receivable	$ 2,245	$ 2,505