Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 611		$ 506
Gross derivative liabilities	(1,129)		(1,419)
Commodity Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	611		506
Gross derivative liabilities	(289)		(225)
Commodity Derivatives [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	486		475
Gross derivative liabilities	(212)		(197)
Commodity Derivatives [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	86		24
Gross derivative liabilities	(24)		(7)
Commodity Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	39		6
Gross derivative liabilities	(49)		(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]
Derivatives Fair Value [Line Items]
Gross derivative liabilities	(840)		(1,194)
Interest-Rate and Other Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative liabilities	(589)	[1]
Interest-Rate and Other Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative liabilities	$ (251)	[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.