Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 579	$ 1,080
Gross derivative liabilities	(1,655)	(1,617)
Commodity Contract [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	579	1,080
Gross derivative liabilities	(304)	(418)
Commodity Contract [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	485	924
Gross derivative liabilities	(228)	(353)
Commodity Contract [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	76	150
Gross derivative liabilities	(34)	(15)
Commodity Contract [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	7	5
Gross derivative liabilities	(22)	(33)
Commodity Contract [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	11	1
Gross derivative liabilities	(20)	(17)
Interest Rate Contract and Other [Member]
Derivatives Fair Value [Line Items]
Gross derivative liabilities	(1,351)	(1,199)
Interest Rate Contract and Other [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative liabilities	(67)	(391)
Interest Rate Contract and Other [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative liabilities	$ (1,284)	$ (808)