Financial Risk Management Activities - Additional Information (Detail) bbl in Millions, MMBbls in Millions, $ in Millions	6 Months Ended
Sep. 30, 2020 USD ($) MMBbls bbl / d vlcc $ / bbl bbl	Dec. 31, 2019 USD ($) MMBbls
Derivative Instruments, Gain (Loss) [Line Items]
Number of very large crude carriers \| vlcc	3
Crude oil loaded in very large crude carriers (in barrels of oil) \| bbl	6.3
Liability for deferred unrealized losses	$ 29	$ 1
Net deferred realized and unrealized gains	$ (363)	$ (699)
Very Large Crude Carriers (VLCCs)
Derivative Instruments, Gain (Loss) [Line Items]
Number of very large crude carriers \| vlcc	3
Crude oil loaded in very large crude carriers (in barrels of oil) \| bbl	6.3
Commodity Derivative Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount of crude oil \| MMBbls	13.8	54.9
Commodity Derivative Contracts \| West Texas Intermediate
Derivative Instruments, Gain (Loss) [Line Items]
Average monthly floor price (US dollar per barrel) \| $ / bbl	55
Notional amount of crude oil \| bbl / d	130,000
Commodity Derivative Contracts \| Brent
Derivative Instruments, Gain (Loss) [Line Items]
Average monthly floor price (US dollar per barrel) \| $ / bbl	60
Notional amount of crude oil \| bbl / d	20,000
Derivative Contracts Designated as Hedging Instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Liability for deferred unrealized losses	$ 29	$ 0
Derivative Contracts Designated as Hedging Instruments: \| Crude Oil Put Options
Derivative Instruments, Gain (Loss) [Line Items]
Liability for deferred unrealized losses	0	$ 0
Derivative Contracts Designated as Hedging Instruments: \| Crude Oil Put Options \| West Texas Intermediate \| Very Large Crude Carriers (VLCCs)
Derivative Instruments, Gain (Loss) [Line Items]
Net deferred realized and unrealized gains	40
Derivative Contracts Designated as Hedging Instruments: \| Crude Oil Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Liability for deferred unrealized losses	29
Derivative Contracts Designated as Hedging Instruments: \| Crude Oil Swaps \| Brent
Derivative Instruments, Gain (Loss) [Line Items]
Liability for deferred unrealized losses	29
Net deferred realized and unrealized gains	$ 17

Inventory, Crude Oil, In Transit, Loading

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Number of VLCCs

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Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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The average floor rate on a group of price risk derivatives such as floors or collars. A payment or receipt is triggered if the market rate falls below the floor rate on the contract.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and elected not to be offset.

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Nominal number of units used to calculate payment on derivative.

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