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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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- Definition Percentage investment held by the managing member or general partner of the limited liability company (LLC) or limited partnership (LP). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 272 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055 + Details
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- Definition The number of limited partner units outstanding. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 5 -Subparagraph (SAB Topic 4.F) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187171-122770 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 272 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70434-108055 + Details
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