Summary of Charges for Employee Services and Omnibus Agreements (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Related Party Transaction [Line Items]
Restructuring Charges	$ 36	$ 0	$ 36	$ 0
Purchases - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	297	303	853	894
MPC \| Rental cost of sales - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	32	45	119	124
MPC \| Purchases - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	293	297	840	878
MPC \| General and administrative expenses
Related Party Transaction [Line Items]
Labor and Related Expense	63	59	195	174
Other Affiliates [Member] \| Purchases - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	4	6	13	16
Construction-in-progress \| MPC
Related Party Transaction [Line Items]
Property, Plant and Equipment, Additions	$ 29	$ 48	$ 80	$ 127

Amount of expense for salary, wage, profit sharing; incentive and equity-based compensation; and other employee benefit.

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