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Debt (MPLX Term Loan) (Details) - MPLX - MPLX 364-day term loan - USD ($)
$ in Millions
Feb. 08, 2018
Feb. 01, 2018
Jan. 02, 2018
Debt Instrument [Line Items]      
Line of credit facility, maximum borrowing capacity   $ 4,100 $ 4,100
Debt instrument, term   364 days  
Borrowings   $ 4,100  
Repayments of short-term debt $ 4,100