Debt - Available Capacity under our Facilities (Details) $ in Millions	Jun. 30, 2019 USD ($)
Debt Instrument [Line Items]
Total Capacity	$ 11,100
Outstanding Borrowings	2,115
Outstanding Letters of Credit	35
Available Capacity	8,950
MPC 364-day bank revolving credit facility
Debt Instrument [Line Items]
Total Capacity	1,000
Outstanding Borrowings	0
Outstanding Letters of Credit	0
Available Capacity	$ 1,000
Weighted Average Interest Rate	0.00%
MPC bank revolving credit facility
Debt Instrument [Line Items]
Total Capacity	$ 5,000
Outstanding Borrowings	0
Outstanding Letters of Credit	32
Available Capacity	$ 4,968
Weighted Average Interest Rate	0.00%
MPC trade receivables securitization facility
Debt Instrument [Line Items]
Total Capacity	$ 750
Outstanding Borrowings	0
Outstanding Letters of Credit	0
Available Capacity	$ 750
Weighted Average Interest Rate	0.00%
MPLX bank revolving credit facility
Debt Instrument [Line Items]
Total Capacity	$ 2,250
Outstanding Borrowings	615
Outstanding Letters of Credit	3
Available Capacity	$ 1,632
Weighted Average Interest Rate	3.80%
ANDX revolving & dropdown credit facilities
Debt Instrument [Line Items]
Total Capacity	$ 2,100
Outstanding Borrowings	1,500	[1]
Outstanding Letters of Credit	0
Available Capacity	$ 600
Weighted Average Interest Rate	3.90%

[1] Western Refining Southwest, Inc., a wholly-owned subsidiary of MPC and unitholder of ANDX, has guaranteed certain outstanding borrowings under the ANDX dropdown credit facility that were made in connection with the August 2018 dropdown transaction.