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Debt - MPC 364-Day Bank Revolving Credit Facility (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Jul. 26, 2019
MPC 364-day revolver due September 2020    
Debt Instrument [Line Items]    
Current borrowing capacity   $ 1,000
MPC 364-day revolver due September 2019    
Debt Instrument [Line Items]    
Current borrowing capacity $ 1,000