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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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