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Supplemental Cash Flow Information - Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures (Details) - USD ($) $ in Millions | 6 Months Ended
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Jun. 30, 2026 | Jun. 30, 2025
Supplemental Cash Flow Elements [Abstract] |  |
Additions to property, plant and equipment | $ 1,400 | $ 568
Increase in capital accruals | 84 | 41
Total capital expenditures | $ 1,484 | $ 609

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- Definition Capital Expenditures + References No definition available. + Details
Name: | mplx_CapitalExpenditures
Namespace Prefix: | mplx_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Increase (Decrease) In Accruals for Capital Expenditures + References No definition available. + Details
Name: | mplx_IncreaseDecreaseInAccrualsForCapitalExpenditures
Namespace Prefix: | mplx_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 13 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482740/230-10-45-13 + Details
Name: | us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_SupplementalCashFlowElementsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration