Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 121	$ 526
Gross derivative liabilities	(1,904)	(1,722)
Commodity Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	99	470
Gross derivative liabilities	(50)	(180)
Commodity Derivatives [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	65	462
Gross derivative liabilities	(8)	(177)
Commodity Derivatives [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	29	8
Gross derivative liabilities	(2)	0
Commodity Derivatives [Member] | Accrued Expenses [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	2	0
Gross derivative liabilities	(18)	(3)
Commodity Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	3	0
Gross derivative liabilities	(22)	0
Interest-Rate Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	22	56
Gross derivative liabilities	(1,854)	(1,542)
Interest-Rate Derivatives [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	4	2
Gross derivative liabilities	0	0
Interest-Rate Derivatives [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	18	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	(48)	(54)
Interest-Rate Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	$ (1,806)	$ (1,488)