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Debt (Credit Agreements) (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2025
USD ($)
Dec. 31, 2024
USD ($)
Debt Instrument [Line Items]    
Debt Instrument, Covenant Description The MPLX Credit Agreement contains certain representations and warranties, affirmative and restrictive covenants and events of default that MPLX considers to be usual and customary for an agreement of this type, including a financial covenant that requires MPLX to maintain a ratio of Consolidated Total Debt as of the end of each fiscal quarter to Consolidated EBITDA (both as defined in the MPLX Credit Agreement) for the prior four fiscal quarters of no greater than 5.0 to 1.0 (or 5.5 to 1.0 for up to two fiscal quarters following certain acquisitions). Consolidated EBITDA is subject to adjustments, including for certain acquisitions and dispositions completed and capital projects undertaken during the relevant period. Other covenants restrict MPLX and/or certain of its subsidiaries from incurring debt, creating liens on our assets and entering into transactions with affiliates.  
Debt Instrument, Covenant Compliance As of December 31, 2025, MPLX was in compliance with the covenants contained in the MPLX Credit Agreement.  
MPLX Revolving Credit Facility due July 2027    
Debt Instrument [Line Items]    
Maximum borrowing capacity $ 2,000  
Debt Instrument, Description of Variable Rate Basis Adjusted Term SOFR or the Alternate Base Rate, both as defined in the MPLX Credit Agreement, plus an applicable margin  
Number of renewals 2  
Debt Instrument, Renewal Term 1 year  
Repayments of long-term lines of credit $ 106  
Proceeds from long-term lines of credit 106  
MPLX Revolving Credit Facility due July 2027 | Maximum    
Debt Instrument [Line Items]    
Additional borrowing capacity 1,000  
MPLX Revolving Credit Facility due July 2027 | Letter of Credit    
Debt Instrument [Line Items]    
Maximum borrowing capacity $ 150  
Revolving Credit Facility    
Debt Instrument [Line Items]    
Repayments of long-term lines of credit   $ 0
Proceeds from long-term lines of credit   $ 0