Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 457		$ 506
Gross derivative liabilities	(1,011)		(1,419)
Commodity Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	457		506
Gross derivative liabilities	(246)		(225)
Commodity Derivatives [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	359		475
Gross derivative liabilities	(170)		(197)
Commodity Derivatives [Member] | Other Noncurrent Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	50		24
Gross derivative liabilities	(13)		(7)
Commodity Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	48		6
Gross derivative liabilities	(61)		(14)
Commodity Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0		1
Gross derivative liabilities	(2)		(7)
Interest-Rate and Other Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	(765)		(1,194)
Interest-Rate and Other Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	(550)	[1]	0
Interest-Rate and Other Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	$ (215)	[1]	$ (1,194)	[1]

[1] Interest-rate swaps with June 2014 maturity dates were reclassified from other liabilities to accrued expenses during the second quarter of 2013.