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Debt (Credit Agreements) (Detail) $ in Millions			12 Months Ended
	Jul. 21, 2017 USD ($)	Oct. 27, 2015	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Feb. 19, 2017
Debt Instrument [Line Items]
Long-term debt - repayments			$ 6,780	$ 416	$ 1,312
MPLX Revolver | MPLX LP
Debt Instrument [Line Items]
Maximum borrowing capacity					2,000
Term		5 years
Description of variable rate basis	Adjusted LIBOR or the Alternate Base Rate (as defined in the MPLX Credit Agreement), at our election, plus a specified margin
Number of prior quarterly reporting periods used in determining compliance with covenant of ratio of consolidated net debt to consolidated EBITDA	4
MPLX Revolver | MPLX LP | Maximum
Debt Instrument [Line Items]
Additional borrowing capacity	$ 500
Covenant of ratio of consolidated net debt to consolidated EBITDA (in ratio)	5.0
Covenant of ratio of consolidated net debt to consolidated EBITDA following certain acquisitions (in ratio)	5.5
MPLX Revolver | MPLX LP | Minimum
Debt Instrument [Line Items]
Covenant of ratio of consolidated net debt to consolidated EBITDA (in ratio)	1.0
Covenant of ratio of consolidated net debt to consolidated EBITDA following certain acquisitions (in ratio)	1.0
MPLX Revolver | MPLX LP | Swingline Loan
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 100
Term loan facility due 2019
Debt Instrument [Line Items]
Maximum borrowing capacity					$ 250
Term loan facility due 2019 | MPLX LP
Debt Instrument [Line Items]
Interest rate, effective percentage						2.407%
MPLX Revolver due July 2022 | MPLX LP
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 2,250
Proceeds from long-term lines of credit			$ 1,410	$ 670
Term	5 years
Interest rate, effective percentage			3.464%	2.748%
Repayments of long-term lines of credit			$ 1,915	$ 165
Letters of credit outstanding, amount			3	3
Remaining borrowing capacity			$ 2,200	$ 1,700
Remaining borrowing capacity, percentage			99.90%	77.40%
MPLX Revolver due July 2022 | MPLX LP | Letter of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 222
Number of renewals	2
Debt Instrument, Renewal Term	1 year
Line of Credit | MPLX Revolver | MPLX LP
Debt Instrument [Line Items]
Proceeds from long-term lines of credit				$ 0
Line of Credit | MPLX Revolver due July 2022 | MPLX LP
Debt Instrument [Line Items]
Expiration date			Jul. 21, 2022
Long-term debt, outstanding			$ 0	$ 505

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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