Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) (Not Designated as Hedging Instrument [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 380		$ 506
Gross derivative liabilities	(986)		(1,419)
Commodity Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	380		506
Gross derivative liabilities	(332)		(225)
Commodity Derivatives [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	181		475
Gross derivative liabilities	(102)		(197)
Commodity Derivatives [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	89		24
Gross derivative liabilities	(66)		(7)
Commodity Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	106		6
Gross derivative liabilities	(149)		(14)
Commodity Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	4		1
Gross derivative liabilities	(15)		(7)
Interest-Rate and Other Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	(654)		(1,194)
Interest-Rate and Other Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	(480)	[1]	0
Interest-Rate and Other Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	$ (174)	[1]	$ (1,194)	[1]

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