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Acquisitions (Fair Value of Consideration Transferred - Andeavor) (Details) - USD ($)
$ in Millions
12 Months Ended
Oct. 01, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Business Acquisition [Line Items]        
Fair value of MPC shares issued   $ 0 $ 19,766 $ 0
Fair value of converted equity awards   $ 0 $ 203 $ 0
Andeavor        
Business Acquisition [Line Items]        
Fair value of MPC shares issued $ 19,766      
Cash payment to Andeavor stockholders 3,486      
Cash settlement of non-employee director units 7      
Fair value of converted equity awards 203      
Total fair value of consideration transferred $ 23,462