Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 315	$ 493
Gross derivative liabilities	(1,651)	(1,455)
Commodity Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	315	493
Gross derivative liabilities	(133)	(238)
Commodity Derivatives [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	246	421
Gross derivative liabilities	(74)	(118)
Commodity Derivatives [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	42	1
Gross derivative liabilities	(16)	0
Commodity Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	27	71
Gross derivative liabilities	(43)	(114)
Commodity Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	0	(6)
Interest-Rate Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	(1,518)	(1,217)
Interest-Rate Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	(56)	0
Interest-Rate Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	$ (1,462)	$ (1,217)