Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 351	$ 526
Gross derivative liabilities	(1,758)	(1,722)
Commodity Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	329	470
Gross derivative liabilities	(118)	(180)
Commodity Derivatives [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	322	462
Gross derivative liabilities	(116)	(177)
Commodity Derivatives [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	7	8
Gross derivative liabilities	0	0
Commodity Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	(2)	(3)
Interest-Rate Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	22	56
Gross derivative liabilities	(1,640)	(1,542)
Interest-Rate Derivatives [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	2	2
Gross derivative liabilities	0	0
Interest-Rate Derivatives [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	20	54
Gross derivative liabilities	0	0
Interest-Rate Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	(90)	(54)
Interest-Rate Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	$ (1,550)	$ (1,488)