Derivative Instrument and Hedging Activities - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity Derivative Instruments, Assets	$ 33	$ 332
Recurring [Member] | Quoted Price in Active Markets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity Derivative Instruments, Assets
Commodity Derivative Instruments, Liabilities
Recurring [Member] | Significant Other Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity Derivative Instruments, Assets	48	428
Commodity Derivative Instruments, Liabilities	131	250
Recurring [Member] | Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity Derivative Instruments, Assets
Commodity Derivative Instruments, Liabilities
Recurring [Member] | Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity Derivative Instruments, Assets	48	428
Commodity Derivative Instruments, Liabilities	131	250
Recurring [Member] | Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity Derivative Instruments, Assets	(15)	(96)
Commodity Derivative Instruments, Liabilities	(15)	(96)
Recurring [Member] | Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity Derivative Instruments, Assets	33	332
Commodity Derivative Instruments, Liabilities	$ 116	$ 154