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Debt (Summary of Outstanding Borrowings) (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 10, 2018 | Nov. 15, 2018 | Feb. 08, 2018 | Dec. 31, 2017 | Feb. 10, 2017
--- | --- | --- | --- | --- | --- | ---
Debt Instrument [Line Items] |  |  |  |  |  |
Debt and capital lease obligations | $ 13,856 |  |  |  | $ 7,362 |
Unamortized debt issuance costs | 97 |  |  |  | 27 |
Unamortized discount | 366 |  |  |  | 389 |
Amounts due within one year | 1 |  |  |  | 1 |
Total long-term debt due after one year | 13,392 |  |  |  | 6,945 |
MPLX LP | Line of Credit | MPLX Revolver due July 2022 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 0 |  |  |  | 505 |
MPLX LP | Senior Notes |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross |  |  | $ 2,250 | $ 5,500 |  |
MPLX LP | Senior Notes | 5.500% senior notes due February 2023 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 0 | $ 750 |  |  | 710 |
MPLX LP | Senior Notes | Senior Notes Due March 2023 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 500 |  |  |  | 0 |
MPLX LP | Senior Notes | 4.500% senior notes due July 2023 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 989 |  |  |  | 989 |
MPLX LP | Senior Notes | 4.875% senior notes due December 2024 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 1,149 |  |  |  | 1,149 |
MPLX LP | Senior Notes | 4.000% senior notes due February 2025 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 500 |  |  |  | 500 |
MPLX LP | Senior Notes | 4.875% senior notes due June 2025 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 1,189 |  |  |  | 1,189 |
MPLX LP | Senior Notes | 4.125% senior notes due March 2027 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 1,250 |  |  |  | 1,250 | $ 1,250
MPLX LP | Senior Notes | Senior Notes Due March 2028 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 1,250 |  |  |  | 0 |
MPLX LP | Senior Notes | Senior Notes Due February 2029 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 750 |  |  |  | 0 |
MPLX LP | Senior Notes | Senior Notes Due April 2038 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 1,750 |  |  |  | 0 |
MPLX LP | Senior Notes | 5.200% senior notes due March 2047 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 1,000 |  |  |  | 1,000 | $ 1,000
MPLX LP | Senior Notes | Senior Notes Due April 2048 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 1,500 |  |  |  | 0 |
MPLX LP | Senior Notes | Senior Notes Due February 2049 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 1,500 |  |  |  | 0 |
MPLX LP | Senior Notes | Senior Notes Due April 2058 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 500 |  |  |  | 0 |
MarkWest | Senior Notes | 5.500% senior notes due February 2023 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross |  | $ 40 |  |  |  |
MarkWest | Senior Notes | MarkWest senior notes |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt, gross | 23 |  |  |  | 63 |
Marathon Pipe Line LLC | Capital Lease Obligations |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Capital lease obligations due 2020 | $ 6 |  |  |  | $ 7 |

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