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Equity (Units Outstanding) (Details) - shares				12 Months Ended
	Feb. 01, 2018	Mar. 01, 2017	Mar. 31, 2016	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Stockholders Equity [Line Items]
Number of units outstanding				794,089,518		415,438,793		368,555,271	311,469,407
Aggregate percentage of general partner interest	2.00%	2.00%	2.00%	2.00%
Limited Partners Common Units
Stockholders Equity [Line Items]
Number of units outstanding				794,089,518		407,130,020		357,193,288	296,687,176
Limited Partners Common Units | MPC
Stockholders Equity [Line Items]
Common units outstanding				505,000,000		118,000,000
General Partner And Limited Partner Units | MPC
Stockholders Equity [Line Items]
Common units outstanding				504,701,934
General Partner Units
Stockholders Equity [Line Items]
Number of units outstanding				0	[1]	8,308,773	[1]	7,371,105	6,800,475	[1]
[1] Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisitions of HSM, HST, WHC, MPLXT, the Joint-Interest Acquisition and Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two percent GP Interest.

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Stockholders Equity [Line Items]

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Percentage investment held by the managing member or general partner of the limited liability company (LLC) or limited partnership (LP).

+ References

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-Publisher FASB
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-Topic 272
-SubTopic 10
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- Definition

The number of limited partner units outstanding.

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The number of each class of partnership units outstanding at the balance sheet date. Units represent shares of ownership of the general, limited, and preferred partners.

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