Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) (Not Designated as Hedging Instrument [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 116	$ 380
Gross derivative liabilities	(1,085)	(986)
Commodity Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	116	380
Gross derivative liabilities	(193)	(332)
Commodity Derivatives [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	40	181
Gross derivative liabilities	(15)	(102)
Commodity Derivatives [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	23	89
Gross derivative liabilities	(9)	(66)
Commodity Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	52	106
Gross derivative liabilities	(166)	(149)
Commodity Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	1	4
Gross derivative liabilities	(3)	(15)
Interest-Rate and Other Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	(892)	(654)
Interest-Rate and Other Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	(628)	(480)
Interest-Rate and Other Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	$ (264)	$ (174)