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Related Party Agreements and Transactions (Intercompany Loans with Related Parties) (Detail) - USD ($)
$ in Millions
12 Months Ended
Jul. 31, 2019
Apr. 27, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 21, 2018
Apr. 26, 2018
Related Party Transaction [Line Items]            
Revenue from related parties     $ 10 $ 6    
Purchases from related parties     5 8    
MPC            
Related Party Transaction [Line Items]            
Current liabilities - related parties     911 360    
Other            
Related Party Transaction [Line Items]            
Current liabilities - related parties     37 76    
MPC Investment | Related Party Revolving Credit Agreement            
Related Party Transaction [Line Items]            
Line of Credit Facility, Current Borrowing Capacity $ 1,500 $ 1,000       $ 500
Debt Instrument, Description of Variable Rate Basis LIBOR plus 1.25 percent LIBOR plus 1.50 percent        
Proceeds from lines of credit     $ 8,540 $ 3,962    
Interest rate during period     3.441% 3.473%    
Repayments of lines of credit     $ 7,946 $ 4,347    
Outstanding balance     594 $ 0    
ANDX LP [Member] | MPC Investment | Related Party Revolving Credit Agreement            
Related Party Transaction [Line Items]            
Line of Credit Facility, Current Borrowing Capacity         $ 500  
Proceeds from lines of credit     $ 773      
Interest rate during period     4.249%      
Repayments of lines of credit     $ 773      
Outstanding balance     $ 0