Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) (Not Designated as Hedging Instrument [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 289	$ 380
Gross derivative liabilities	(997)	(986)
Commodity Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	289	380
Gross derivative liabilities	(72)	(332)
Commodity Derivatives [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	255	181
Gross derivative liabilities	(49)	(102)
Commodity Derivatives [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	27	89
Gross derivative liabilities	(11)	(66)
Commodity Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	6	106
Gross derivative liabilities	(10)	(149)
Commodity Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	1	4
Gross derivative liabilities	(2)	(15)
Interest-Rate and Other Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	(925)	(654)
Interest-Rate and Other Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	0	(480)
Interest-Rate and Other Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	$ (925)	$ (174)