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Debt - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended
Sep. 26, 2019
Sep. 09, 2019
Jul. 21, 2017
Mar. 31, 2020
Mar. 31, 2019
[1]
Dec. 31, 2019
Jul. 30, 2019
Debt Instrument [Line Items]              
Repayments of Long-term Debt       $ 581 $ 821    
MPLX Revolving Credit Facility due July 2024 [Member]              
Debt Instrument [Line Items]              
Line of Credit Facility, Current Borrowing Capacity             $ 3,500
Long-term Line of Credit       750   $ 0  
MPLX Revolving Credit Facility due July 2024 [Member] | MPLX LP              
Debt Instrument [Line Items]              
Proceeds from Lines of Credit       $ 1,325      
Debt Instrument, Interest Rate, Effective Percentage       2.139%      
Repayments of Long-term Lines of Credit       $ 575      
Letters of Credit Outstanding       1      
Line of Credit Facility, Remaining Borrowing Capacity       $ 2,750      
Line of Credit Facility, Remaining Borrowing Capacity, Percentage       78.60%      
Bank revolving credit facility due 2022 | MPLX LP              
Debt Instrument [Line Items]              
Debt Instrument, Term     5 years        
MPLX Term Loan [Member]              
Debt Instrument [Line Items]              
Line of Credit Facility, Maximum Borrowing Capacity $ 1,000            
Debt Instrument, Description of Variable Rate Basis (i) the Adjusted LIBO Rate (as defined in the Term Loan Agreement) plus a margin ranging from 75.0 basis points to 100.0 basis points per annum, depending on MPLX’s credit ratings, or (ii) the Alternate Base Rate (as defined in the Term Loan Agreement).            
Long-term Debt, Maturity Date       Sep. 26, 2021      
Long-term Line of Credit       $ 1,000   1,000  
MPLX Term Loan [Member] | MPLX LP              
Debt Instrument [Line Items]              
Debt Instrument, Interest Rate, Effective Percentage       2.273%      
Senior Notes [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross   $ 2,000          
Percent of Par   100.00%          
Senior Notes [Member] | ANDX LP [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross       $ 190   190  
Senior Notes [Member] | MarkWest [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross       $ 23   $ 23  
Senior Notes [Member] | Floating Rate Senior Notes Due September 2021 [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross   $ 1,000          
Debt Instrument, Description of Variable Rate Basis   LIBOR plus 0.9 percent per annum          
Senior Notes [Member] | Floating Rate Senior Notes Due September 2022 [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross   $ 1,000          
Debt Instrument, Description of Variable Rate Basis   LIBOR plus 1.1 percent per annum          
Minimum [Member] | Senior Notes [Member]              
Debt Instrument [Line Items]              
Debt Instrument, Interest Rate, Stated Percentage       3.375%      
Maximum [Member] | Senior Notes [Member]              
Debt Instrument [Line Items]              
Debt Instrument, Interest Rate, Stated Percentage       6.375%      
[1]
Financial information for the first quarter of 2019 has been retrospectively adjusted for the acquisition of ANDX. See Notes 1 and 3.