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Debt (Summary of Outstanding Borrowings Interest Rates and Table Due Dates) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 10, 2018	Feb. 10, 2017
MPLX LP | MPLX Revolver due July 2022
Debt Instrument [Line Items]
Proceeds from long-term lines of credit	$ 1,410	$ 670
MPLX LP | Line of Credit | MPLX Revolver due July 2022
Debt Instrument [Line Items]
Expiration date	Jul. 21, 2022
MPLX LP | Senior Notes | 5.500% senior notes due February 2023
Debt Instrument [Line Items]
Interest rate, stated percentage	5.50%		5.50%
Maturity date	Feb. 15, 2023
MPLX LP | Senior Notes | Senior Notes Due March 2023 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	3.375%
Maturity date	Mar. 15, 2023
MPLX LP | Senior Notes | 4.500% senior notes due July 2023
Debt Instrument [Line Items]
Interest rate, stated percentage	4.50%
Maturity date	Jul. 15, 2023
MPLX LP | Senior Notes | 4.875% senior notes due December 2024
Debt Instrument [Line Items]
Interest rate, stated percentage	4.875%
Maturity date	Dec. 01, 2024
MPLX LP | Senior Notes | 4.000% senior notes due February 2025
Debt Instrument [Line Items]
Interest rate, stated percentage	4.00%
Maturity date	Feb. 15, 2025
MPLX LP | Senior Notes | 4.875% senior notes due June 2025
Debt Instrument [Line Items]
Interest rate, stated percentage	4.875%
Maturity date	Jun. 01, 2025
MPLX LP | Senior Notes | 4.125% senior notes due March 2027
Debt Instrument [Line Items]
Interest rate, stated percentage	4.125%			4.125%
Maturity date	Mar. 01, 2027
MPLX LP | Senior Notes | Senior Notes Due March 2028 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.00%
Maturity date	Mar. 15, 2028
MPLX LP | Senior Notes | Senior Notes Due February 2029 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.80%
Maturity date	Feb. 15, 2029
MPLX LP | Senior Notes | Senior Notes Due April 2038 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.50%
Maturity date	Apr. 15, 2038
MPLX LP | Senior Notes | 5.200% senior notes due March 2047
Debt Instrument [Line Items]
Interest rate, stated percentage	5.20%			5.20%
Maturity date	Mar. 01, 2047
MPLX LP | Senior Notes | Senior Notes Due April 2048 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.70%
Maturity date	Apr. 15, 2048
MPLX LP | Senior Notes | Senior Notes Due November 2049 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	5.50%
Maturity date	Feb. 15, 2049
MPLX LP | Senior Notes | Senior Notes Due April 2058 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.90%
Maturity date	Apr. 15, 2058
MarkWest | Senior Notes | 4.500% senior notes due July 2023
Debt Instrument [Line Items]
Interest rate, stated percentage	4.50%
Maturity date	Jul. 15, 2023
MarkWest | Senior Notes | 4.875% senior notes due December 2024
Debt Instrument [Line Items]
Interest rate, stated percentage	4.875%
Maturity date	Dec. 01, 2024
MarkWest | Senior Notes | 4.875% senior notes due June 2025
Debt Instrument [Line Items]
Interest rate, stated percentage	4.875%
Maturity date	Jun. 01, 2025
Marathon Pipe Line LLC | Capital Lease Obligations
Debt Instrument [Line Items]
Capital lease due date year	2020

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Contractual interest rate for funds borrowed, under the debt agreement.

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