Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2020	Dec. 31, 2019
Commitments And Contingencies [Line Items]
Accrued liabilities for environmental remediation	$ 18	$ 19
Loss Contingency, Damages Sought, Value	187
Contractual commitments to acquire property, plant and equipment	293
MPC
Commitments And Contingencies [Line Items]
Due to Related Parties, Current	260	911
Environmental Loss Contingency [Member] \| MPC
Commitments And Contingencies [Line Items]
Due to Related Parties, Current	$ 0	$ 0
Whistler Pipeline LLC [Member]
Commitments And Contingencies [Line Items]
Equity method investment, ownership percentage	38.00%
Indirect Ownership Interest [Member] \| Bakken Pipeline System [Member]
Commitments And Contingencies [Line Items]
Equity method investment, ownership percentage	9.19%
Guarantee Type, Other [Member]
Commitments And Contingencies [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 230

Commitments And Contingencies [Line Items]

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Total costs accrued as of the balance sheet date for environmental loss contingencies.

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Carrying amount as of the balance sheet date of obligations due all related parties. For classified balance sheets, represents the current portion of such liabilities (due within one year or within the normal operating cycle if longer).

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

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The value (monetary amount) of the award the plaintiff seeks in the legal matter.

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Minimum amount to be expended to satisfy the terms of arrangements in which the entity has agreed to expend funds to procure goods or services, excluding long-term purchase commitments or unconditional purchase obligations.

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