Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 337	$ 493
Gross derivative liabilities	(1,312)	(1,455)
Commodity Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	337	493
Gross derivative liabilities	(202)	(238)
Commodity Derivatives [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	238	421
Gross derivative liabilities	(73)	(118)
Commodity Derivatives [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	5	1
Gross derivative liabilities	0	0
Commodity Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	60	71
Gross derivative liabilities	(89)	(114)
Commodity Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	34	0
Gross derivative liabilities	(40)	(6)
Interest-Rate Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	$ (1,110)	$ (1,217)