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Debt - MPLX Floating Rate Senior Notes (Details) - MPLX LP - Senior notes - USD ($)
$ in Millions
Sep. 23, 2019
Sep. 09, 2019
Debt Instrument [Line Items]    
Long-term debt, gross $ 3,060 $ 2,000
Floating Rate Senior Notes Due September 2021    
Debt Instrument [Line Items]    
Long-term debt, gross   1,000
Floating Rate Senior Notes Due September 2022    
Debt Instrument [Line Items]    
Long-term debt, gross   $ 1,000