Derivative Instruments - Effect of Derivative Instruments - Balance Sheet Table (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivatives Fair Value [Line Items]
Gross derivative assets	$ 108	$ 116
Gross derivative liabilities	(1,628)	(1,592)
Commodity Derivatives [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	54	85
Gross derivative liabilities	(209)	(216)
Commodity Derivatives [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	7	10
Gross derivative liabilities	(1)	(3)
Commodity Derivatives [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	2	9
Gross derivative liabilities	0	0
Commodity Derivatives [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	45	66
Gross derivative liabilities	(206)	(201)
Commodity Derivatives [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	(2)	(12)
Interest Rate Swap [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	54	31
Gross derivative liabilities	(1,419)	(1,376)
Interest Rate Swap [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	14	8
Gross derivative liabilities	0	0
Interest Rate Swap [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	40	23
Gross derivative liabilities	0	0
Interest Rate Swap [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	(236)	(48)
Interest Rate Swap [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Gross derivative assets	0	0
Gross derivative liabilities	$ (1,183)	$ (1,328)