Related Party Agreements and Transactions (Summary of Expenses by Income Statement RP Line Item) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
MPC
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 1,639	$ 1,155	$ 595
MPC | Rental cost of sales - related parties [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	165	31	2
MPC | Purchases - related parties
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	1,210	919	455
MPC | General and Administrative Expense - Related Party [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	243	199	138
Other Affiliates [Member] | Purchases - related parties
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	21	6	0
Construction-in-progress | MPC
Related Party Transaction [Line Items]
Property, Plant and Equipment, Additions	$ 169	$ 152	$ 42