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Debt Debt - Summary of Outstanding Borrowings - Interest Rates and Table Due Dates (Details)	3 Months Ended
	Mar. 31, 2018	Feb. 10, 2017
MarkWest [Member] | Senior Notes [Member] | 5.500% senior notes due February 2023
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.50%
Debt Instrument, Maturity Date	Feb. 15, 2023
MarkWest [Member] | Senior Notes [Member] | 4.500% senior notes due July 2023
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.50%
Debt Instrument, Maturity Date	Jul. 15, 2023
MarkWest [Member] | Senior Notes [Member] | 4.875% senior notes due December 2024
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Dec. 01, 2024
MarkWest [Member] | Senior Notes [Member] | 4.875% senior notes due June 2025
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Jun. 01, 2025
MPLX LP | Line of Credit [Member] | Bank revolving credit facility due 2022
Debt Instrument [Line Items]
Expiration date	Jul. 21, 2022
MPLX LP | Senior Notes [Member] | 5.500% senior notes due February 2023
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.50%
Debt Instrument, Maturity Date	Feb. 15, 2023
MPLX LP | Senior Notes [Member] | Senior Notes Due March 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.375%
Debt Instrument, Maturity Date	Mar. 15, 2023
MPLX LP | Senior Notes [Member] | 4.500% senior notes due July 2023
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.50%
Debt Instrument, Maturity Date	Jul. 15, 2023
MPLX LP | Senior Notes [Member] | 4.875% senior notes due December 2024
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Dec. 01, 2024
MPLX LP | Senior Notes [Member] | 4.000% senior notes due February 2025
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.00%
Debt Instrument, Maturity Date	Feb. 15, 2025
MPLX LP | Senior Notes [Member] | 4.875% senior notes due June 2025
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Jun. 01, 2025
MPLX LP | Senior Notes [Member] | 4.125% senior notes due March 2027
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.125%	4.125%
Debt Instrument, Maturity Date	Mar. 01, 2027
MPLX LP | Senior Notes [Member] | Senior Notes Due March 2028 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.00%
Debt Instrument, Maturity Date	Mar. 15, 2028
MPLX LP | Senior Notes [Member] | Senior Notes Due April 2038 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.50%
Debt Instrument, Maturity Date	Apr. 15, 2038
MPLX LP | Senior Notes [Member] | 5.200% senior notes due March 2047
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.20%	5.20%
Debt Instrument, Maturity Date	Mar. 01, 2047
MPLX LP | Senior Notes [Member] | Senior Notes Due April 2048 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.70%
Debt Instrument, Maturity Date	Apr. 15, 2048
MPLX LP | Senior Notes [Member] | Senior Notes Due April 2058 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.90%
Debt Instrument, Maturity Date	Apr. 15, 2058
Marathon Pipe Line LLC [Member] | Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Capital Lease Due Date Year	2020

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- Definition

The year in which the entity's capital leases are due, in CCYY format.

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Name Accounting Standards Codification
-Topic 210
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-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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