Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 218	$ 506
Gross derivative liabilities	(1,377)	(1,419)
Commodity Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	218	506
Gross derivative liabilities	(276)	(225)
Commodity Derivatives [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	43	475
Gross derivative liabilities	(20)	(197)
Commodity Derivatives [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	22	24
Gross derivative liabilities	(7)	(7)
Commodity Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	153	6
Gross derivative liabilities	(245)	(14)
Commodity Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets		1
Gross derivative liabilities	(4)	(7)
Interest-Rate and Other Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative liabilities	$ (1,101)	$ (1,194)