Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

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Debt (Commercial Paper) (Details) - USD ($) $ in Millions		12 Months Ended
Debt (Commercial Paper) (Details) - USD ($) $ in Millions	Feb. 26, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Commercial paper – issued		$ 0	$ 300	$ 1,263
Commercial paper - repayments		0	300	$ 1,263
Commercial Paper
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 2,000
Debt instrument, term	397 days
Marathon Petroleum Corporation
Debt Instrument [Line Items]
Commercial paper outstanding		$ 0	$ 0