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Debt (Summary of Outstanding Borrowings) (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 10, 2018
Nov. 15, 2018
Feb. 08, 2018
Dec. 31, 2017
Feb. 10, 2017
Debt Instrument [Line Items]            
Debt and capital lease obligations $ 13,856       $ 7,362  
Unamortized debt issuance costs 97       27  
Unamortized discount 366       389  
Amounts due within one year 1       1  
Total long-term debt due after one year 13,392       6,945  
MPLX LP | Line of Credit | MPLX Revolver due July 2022            
Debt Instrument [Line Items]            
Long-term debt, gross 0       505  
MPLX LP | Senior Notes            
Debt Instrument [Line Items]            
Long-term debt, gross     $ 2,250 $ 5,500    
MPLX LP | Senior Notes | 5.500% senior notes due February 2023            
Debt Instrument [Line Items]            
Long-term debt, gross 0 $ 750     710  
MPLX LP | Senior Notes | Senior Notes Due March 2023 [Member]            
Debt Instrument [Line Items]            
Long-term debt, gross 500       0  
MPLX LP | Senior Notes | 4.500% senior notes due July 2023            
Debt Instrument [Line Items]            
Long-term debt, gross 989       989  
MPLX LP | Senior Notes | 4.875% senior notes due December 2024            
Debt Instrument [Line Items]            
Long-term debt, gross 1,149       1,149  
MPLX LP | Senior Notes | 4.000% senior notes due February 2025            
Debt Instrument [Line Items]            
Long-term debt, gross 500       500  
MPLX LP | Senior Notes | 4.875% senior notes due June 2025            
Debt Instrument [Line Items]            
Long-term debt, gross 1,189       1,189  
MPLX LP | Senior Notes | 4.125% senior notes due March 2027            
Debt Instrument [Line Items]            
Long-term debt, gross 1,250       1,250 $ 1,250
MPLX LP | Senior Notes | Senior Notes Due March 2028 [Member]            
Debt Instrument [Line Items]            
Long-term debt, gross 1,250       0  
MPLX LP | Senior Notes | Senior Notes Due February 2029 [Member]            
Debt Instrument [Line Items]            
Long-term debt, gross 750       0  
MPLX LP | Senior Notes | Senior Notes Due April 2038 [Member]            
Debt Instrument [Line Items]            
Long-term debt, gross 1,750       0  
MPLX LP | Senior Notes | 5.200% senior notes due March 2047            
Debt Instrument [Line Items]            
Long-term debt, gross 1,000       1,000 $ 1,000
MPLX LP | Senior Notes | Senior Notes Due April 2048 [Member]            
Debt Instrument [Line Items]            
Long-term debt, gross 1,500       0  
MPLX LP | Senior Notes | Senior Notes Due February 2049 [Member]            
Debt Instrument [Line Items]            
Long-term debt, gross 1,500       0  
MPLX LP | Senior Notes | Senior Notes Due April 2058 [Member]            
Debt Instrument [Line Items]            
Long-term debt, gross 500       0  
MarkWest | Senior Notes | 5.500% senior notes due February 2023            
Debt Instrument [Line Items]            
Long-term debt, gross   $ 40        
MarkWest | Senior Notes | MarkWest senior notes            
Debt Instrument [Line Items]            
Long-term debt, gross 23       63  
Marathon Pipe Line LLC | Capital Lease Obligations            
Debt Instrument [Line Items]            
Capital lease obligations due 2020 $ 6       $ 7