Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 1,080	$ 801
Gross derivative liabilities	(1,617)	(724)
Commodity Contract [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	1,080	801
Gross derivative liabilities	(418)	(489)
Commodity Contract [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	924	444
Gross derivative liabilities	(353)	(274)
Commodity Contract [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	150	242
Gross derivative liabilities	(15)	(56)
Commodity Contract [Member] | Accrued expense [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	5	89
Gross derivative liabilities	(33)	(131)
Commodity Contract [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	1	26
Gross derivative liabilities	(17)	(28)
Interest Rate Contract and Other [Member]
Derivatives Fair Value [Line Items]
Gross derivative liabilities	(1,199)	(235)
Interest Rate Contract and Other [Member] | Accrued expense [Member]
Derivatives Fair Value [Line Items]
Gross derivative liabilities	(391)	(190)
Interest Rate Contract and Other [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative liabilities	$ (808)	$ (45)