Derivative Instruments and Hedging Activities (Details 1) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Commodity Derivative Instruments, Assets	$ 307	$ 332
Recurring [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Commodity Derivative Instruments, Assets	410	428
Commodity Derivative Instruments, Liabilities	308	250
Recurring [Member] | Quoted Price in Active Markets (Level 1) [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Commodity Derivative Instruments, Assets
Commodity Derivative Instruments, Liabilities
Recurring [Member] | Significant Other Inputs (Level 2) [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Commodity Derivative Instruments, Assets	410	428
Commodity Derivative Instruments, Liabilities	308	250
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Commodity Derivative Instruments, Assets
Commodity Derivative Instruments, Liabilities
Recurring [Member] | Netting [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Commodity Derivative Instruments, Assets	(103)	(96)
Commodity Derivative Instruments, Liabilities	(103)	(96)
Recurring [Member] | Carrying Amount [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Commodity Derivative Instruments, Assets	307	332
Commodity Derivative Instruments, Liabilities	$ 205	$ 154