Debt (Available Capacity under our Facilities) (Details) - USD ($) $ in Millions		Dec. 31, 2021	Dec. 31, 2020	Jul. 30, 2019
MPLX
Line of Credit Facility [Line Items]
Outstanding Borrowings		$ 300	$ 175
MPC 5-year revolver due October 2023
Line of Credit Facility [Line Items]
Total Capacity		5,000
Outstanding Borrowings		0
Outstanding Letters of Credit		1
Available Capacity		$ 4,999
Weighted Average Interest Rate		0.00%
Trade Receivables Securitization due September 2022
Line of Credit Facility [Line Items]
Total Capacity	[1]	$ 250
Outstanding Borrowings		0
Outstanding Letters of Credit		250
Available Capacity		$ 0
Weighted Average Interest Rate		0.00%
MPLX revolving credit facility due July 2024 | MPLX
Line of Credit Facility [Line Items]
Total Capacity		$ 3,500		$ 3,500
Outstanding Borrowings		300
Outstanding Letters of Credit		0
Available Capacity		$ 3,200
Weighted Average Interest Rate		1.33%

[1] The committed capacity of the trade receivables securitization facility is $100 million. The facility allows the banks to make loans and issue letters of credit of up to $400 million in excess of the committed capacity at their discretion if there is available borrowing capacity.