Financial Risk Management Activities - Fair Values of Financial Risk Management Derivative Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivatives, Fair Value [Line Items]
Assets - Gross fair value of derivative contracts	$ 111	$ 69
Liabilities - Gross fair value of derivative contracts	0	(55)
Assets - Gross amounts offset in the Consolidated Balance Sheet	0	(13)
Liabilities - Gross amounts offset in the Consolidated Balance Sheet	0	13
Derivative assets	111	56
Derivative liabilities	0	(42)
Derivative Contracts Designated as Hedging Instruments:
Derivatives, Fair Value [Line Items]
Assets - Gross fair value of derivative contracts	110	69
Liabilities - Gross fair value of derivative contracts	0	(54)
Derivative Contracts Designated as Hedging Instruments: \| Commodity Options / Collars
Derivatives, Fair Value [Line Items]
Assets - Gross fair value of derivative contracts	106	64
Liabilities - Gross fair value of derivative contracts	0	0
Derivative Contracts Designated as Hedging Instruments: \| Crude Oil Swaps
Derivatives, Fair Value [Line Items]
Assets - Gross fair value of derivative contracts		0
Liabilities - Gross fair value of derivative contracts		(54)
Derivative Contracts Designated as Hedging Instruments: \| Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Assets - Gross fair value of derivative contracts	4	5
Liabilities - Gross fair value of derivative contracts	0	0
Derivative Contracts Not Designated as Hedging Instruments:
Derivatives, Fair Value [Line Items]
Assets - Gross fair value of derivative contracts	1	0
Liabilities - Gross fair value of derivative contracts	0	(1)
Derivative Contracts Not Designated as Hedging Instruments: \| Foreign Exchange Forwards / Swaps
Derivatives, Fair Value [Line Items]
Assets - Gross fair value of derivative contracts	1	0
Liabilities - Gross fair value of derivative contracts	$ 0	$ (1)

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