Related Party Agreements and Transactions (Summary of Employee Services Expenses by Income Statement Line Item) (Detail) - MPC - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 640	$ 487	$ 449
Rental cost of sales - related parties [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	3	1	0
Purchases - related parties
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	528	385	349
General and Administrative Expense - Related Party [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 109	$ 101
General and administrative expenses
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements			$ 100