Summary of Charges for Employee Services and Omnibus Agreements (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Purchases - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	$ 280	$ 313	$ 556	$ 591
MPC | Rental cost of sales - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	41	36	87	79
MPC | Purchases - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	276	308	547	581
MPC | General and administrative expenses
Related Party Transaction [Line Items]
Labor and Related Expense	68	53	132	115
Other Affiliates [Member] | Purchases - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	4	5	9	10
Construction-in-progress | MPC
Related Party Transaction [Line Items]
Property, Plant and Equipment, Additions	$ 15	$ 38	$ 51	$ 79