Financial Risk Management Activities - Gross and Net Fair Values (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivatives Fair Value [Line Items]
Assets - Gross fair value of derivative contracts	$ 96	$ 484
Assets – Master netting arrangements	0	0
Assets - Net Fair Value of Derivative Contracts	96	484
Liabilities - Gross fair value of derivative contracts	(1)	(2)
Liabilities - Master netting arrangements	0	0
Liabilities - Net Fair Value of Derivative Contracts	(1)	(2)
Derivative Contracts Designated as Hedging Instruments
Derivatives Fair Value [Line Items]
Assets - Gross fair value of derivative contracts	96	484
Liabilities - Gross fair value of derivative contracts	(1)	(2)
Derivative Contracts Designated as Hedging Instruments \| Commodity \| Other Current Assets
Derivatives Fair Value [Line Items]
Assets - Gross fair value of derivative contracts	96	484
Liabilities - Gross fair value of derivative contracts	0	0
Derivative Contracts Designated as Hedging Instruments \| Commodity \| Other Liabilities and Deferred Credits
Derivatives Fair Value [Line Items]
Liabilities - Gross fair value of derivative contracts	(1)	(2)
Derivative Contracts Designated as Hedging Instruments \| Interest Rate \| Other Liabilities and Deferred Credits
Derivatives Fair Value [Line Items]
Assets - Gross fair value of derivative contracts	$ 0	$ 0

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