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Debt (Senior Notes) (Details) - USD ($) $ in Millions			12 Months Ended
	Feb. 08, 2018	Mar. 01, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 10, 2018	Nov. 15, 2018	Feb. 10, 2017
Debt Instrument [Line Items]
Extinguishment of Debt, Gain (Loss), Net of Tax			$ 46	$ 0	$ 0
Ozark Pipeline
Debt Instrument [Line Items]
Business Combination, Cash Consideration Transferred Net of Liabilities Assumed		$ 1,500
MPLX LP | MPLX 364-Day Term Loan [Member]
Debt Instrument [Line Items]
Repayments of Short-term Debt	$ 4,100
MPLX LP | Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross	$ 5,500						$ 2,250
MPLX LP | Senior Notes | Senior Notes Due February 2029 [Member]
Debt Instrument [Line Items]
Long-term debt, gross			$ 750	0
Interest rate, stated percentage			4.80%
Percent of par							99.432%
MPLX LP | Senior Notes | Senior Notes Due February 2049 [Member]
Debt Instrument [Line Items]
Long-term debt, gross			$ 1,500	0
Interest rate, stated percentage			5.50%
Percent of par							98.031%
MPLX LP | Senior Notes | 5.500% senior notes due February 2023
Debt Instrument [Line Items]
Long-term debt, gross			$ 0	710		$ 750
Interest rate, stated percentage			5.50%			5.50%
Percent of par						101.833%
MPLX LP | Senior Notes | Senior Notes Due March 2023 [Member]
Debt Instrument [Line Items]
Long-term debt, gross			$ 500	0
Interest rate, stated percentage			3.375%
Percent of par	99.931%
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
Interest rate, stated percentage			4.125%					4.125%
Percent of par								99.834%
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
Interest rate, stated percentage			5.20%					5.20%
Percent of par								99.304%
MPLX LP | Senior Notes | Senior Notes Due March 2028 [Member]
Debt Instrument [Line Items]
Long-term debt, gross			$ 1,250	0
Interest rate, stated percentage			4.00%
Percent of par	99.551%
MPLX LP | Senior Notes | Senior Notes Due April 2038 [Member]
Debt Instrument [Line Items]
Long-term debt, gross			$ 1,750	0
Interest rate, stated percentage			4.50%
Percent of par	98.811%
MPLX LP | Senior Notes | Senior Notes Due April 2048 [Member]
Debt Instrument [Line Items]
Long-term debt, gross			$ 1,500	0
Interest rate, stated percentage			4.70%
Percent of par	99.348%
MPLX LP | Senior Notes | Senior Notes Due April 2058 [Member]
Debt Instrument [Line Items]
Long-term debt, gross			$ 500	$ 0
Interest rate, stated percentage			4.90%
Percent of par	99.289%
MarkWest | Senior Notes | 5.500% senior notes due February 2023
Debt Instrument [Line Items]
Long-term debt, gross						$ 40
Debt Premium [Member]
Debt Instrument [Line Items]
Extinguishment of Debt, Gain (Loss), Net of Tax			$ 14

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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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