Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 96	$ 357
Gross derivative liabilities	(1,393)	(1,378)
Commodity derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	45	301
Gross derivative liabilities	(29)	(132)
Commodity derivatives [Member] | Other current assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	45	300
Gross derivative liabilities	(27)	(126)
Commodity derivatives [Member] | Other current liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	1
Gross derivative liabilities	(2)	(6)
Interest-rate derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	51	56
Gross derivative liabilities	(1,320)	(1,238)
Interest-rate derivatives [Member] | Other current assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	21	22
Gross derivative liabilities	0	0
Interest-rate derivatives [Member] | Other assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	30	34
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	(83)	(82)
Interest-rate derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	(1,237)	(1,156)
WES [Member] | Interest-rate derivatives [Member] | Other current liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	$ (44)	$ (8)