Summary of Charges for Employee Services and Omnibus Agreements (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Purchases - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	$ 276	$ 278
MPC | Rental cost of sales - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	46	43
MPC | Purchases - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	271	273
MPC | General and administrative expenses
Related Party Transaction [Line Items]
Labor and Related Expense	64	62
Other Affiliates [Member] | Purchases - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	5	5
Construction-in-progress | MPC
Related Party Transaction [Line Items]
Property, Plant and Equipment, Additions	$ 36	$ 41