Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) (Not Designated as Hedging Instrument [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 451	$ 493
Gross derivative liabilities	(1,656)	(1,455)
Commodity Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	451	493
Gross derivative liabilities	(234)	(238)
Commodity Derivatives [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	351	421
Gross derivative liabilities	(86)	(118)
Commodity Derivatives [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	35	1
Gross derivative liabilities	0	0
Commodity Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	65	71
Gross derivative liabilities	(117)	(114)
Commodity Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	(31)	(6)
Interest-Rate Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	$ (1,422)	$ (1,217)