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Credit Agreement (Detail) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2025
Dec. 31, 2024
Debt Instrument [Line Items]    
Long-term Line of Credit $ 0 $ 0
Line of Credit Facility, Remaining Borrowing Capacity, Percentage 100.00%  
MPLX Revolving Credit Facility due June 2027    
Debt Instrument [Line Items]    
Long-term Line of Credit $ 0  
Line of Credit Facility, 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.  
Letters of Credit Outstanding, Amount $ 1  
Line of Credit Facility, Remaining Borrowing Capacity 2,000  
Proceeds from Long-Term Lines of Credit 106  
Repayments of Long-Term Lines of Credit $ 106  
Debt, Weighted Average Interest Rate 7.74%  
MPLX Revolving Credit Facility due June 2027 | Letter of Credit    
Debt Instrument [Line Items]    
Line of Credit Facility, Maximum Borrowing Capacity $ 150