Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 112	$ 108
Gross derivative liabilities	(1,604)	(1,628)
Commodity derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	37	54
Gross derivative liabilities	(496)	(209)
Commodity derivatives [Member] | Other current assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	2	7
Gross derivative liabilities	0	(1)
Commodity derivatives [Member] | Other assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	2
Gross derivative liabilities	0	0
Commodity derivatives [Member] | Other current liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	20	45
Gross derivative liabilities	(440)	(206)
Commodity derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	15	0
Gross derivative liabilities	(56)	(2)
Interest-rate derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	75	54
Gross derivative liabilities	(1,108)	(1,419)
Interest-rate derivatives [Member] | Other current assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	21	14
Gross derivative liabilities	0	0
Interest-rate derivatives [Member] | Other assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	54	40
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	(80)	(236)
Interest-rate derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	$ (1,028)	$ (1,183)