Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 346	$ 116
Gross derivative liabilities	(1,692)	(1,592)
Commodity derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	292	85
Gross derivative liabilities	(279)	(216)
Commodity derivatives [Member] | Other current assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	58	10
Gross derivative liabilities	(40)	(3)
Commodity derivatives [Member] | Other assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	33	9
Gross derivative liabilities	(29)	0
Commodity derivatives [Member] | Other current liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	38	66
Gross derivative liabilities	(44)	(201)
Commodity derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	163	0
Gross derivative liabilities	(166)	(12)
Interest-rate derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	54	31
Gross derivative liabilities	(1,413)	(1,376)
Interest-rate derivatives [Member] | Other current assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	12	8
Gross derivative liabilities	0	0
Interest-rate derivatives [Member] | Other assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	42	23
Gross derivative liabilities	0	0
Interest-rate derivatives [Member] | Other current liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	(237)	(48)
Interest-rate derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	$ (1,176)	$ (1,328)