Fair Value Measurements (Details) (Recurring [Member], USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Commodity Derivative Instruments, Assets	$ 432	$ 454
Commodity Derivative Instruments, Liabilities	108	466
Quoted Price in Active Markets (Level 1) [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Commodity Derivative Instruments, Assets	0	0
Commodity Derivative Instruments, Liabilities	0	0
Significant Other Inputs (Level 2) [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Commodity Derivative Instruments, Assets	432	454
Commodity Derivative Instruments, Liabilities	108	466
Significant Unobservable Inputs (Level 3) [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Commodity Derivative Instruments, Assets	0	0
Commodity Derivative Instruments, Liabilities	0	0
Netting [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Commodity Derivative Instruments, Assets	(33)	(148)
Commodity Derivative Instruments, Liabilities	(33)	(148)
Carrying Amount [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Commodity Derivative Instruments, Assets	399	306
Commodity Derivative Instruments, Liabilities	$ 75	$ 318