Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 506	$ 1,080
Gross derivative liabilities	(1,419)	(1,617)
Commodity Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	506	1,080
Gross derivative liabilities	(225)	(418)
Commodity Derivatives [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	475	924
Gross derivative liabilities	(197)	(353)
Commodity Derivatives [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	24	150
Gross derivative liabilities	(7)	(15)
Commodity Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	6	5
Gross derivative liabilities	(14)	(33)
Commodity Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	1	1
Gross derivative liabilities	(7)	(17)
Interest-Rate and Other Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative liabilities	(1,194)	(1,199)
Interest-Rate and Other Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative liabilities		(391)
Interest-Rate and Other Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative liabilities	$ (1,194)	$ (808)