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Debt - MPLX Senior Notes (Details) - Senior notes - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2019
Sep. 23, 2019
Sep. 09, 2019
Jul. 30, 2019
ANDX        
Debt Instrument [Line Items]        
Long-term debt, gross   $ 690    
ANDX | Senior Notes Due October 2019        
Debt Instrument [Line Items]        
Long-term debt, gross   500    
Repayments of debt $ 500      
ANDX | Senior Notes Due October 2019        
Debt Instrument [Line Items]        
Interest paid $ 14      
MPLX LP        
Debt Instrument [Line Items]        
Long-term debt, gross   $ 3,060 $ 2,000  
MPLX LP | ANDX        
Debt Instrument [Line Items]        
Long-term debt       $ 3,750