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Equity (Changes in Partners Capital, Unit Rollforward) (Details) - shares |  |  |  |  |  | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Feb. 01, 2018 | Sep. 01, 2017 | Mar. 01, 2017 | Sep. 01, 2016 | Mar. 31, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance, number of units |  |  |  |  |  |  | 415,438,793 |  | 368,555,271 |  | 311,469,407 |
Unit-based compensation awards, in units |  |  |  |  |  |  | 348,527 |  | 273,639 |  | 123,459 |
Units issued |  |  |  |  |  |  |  |  | 14,129,589 |  | 26,885,597 |
Unit conversion |  |  |  |  |  |  |  |  | 366,509 |  | 366,509 |
Units, acquisitions |  |  |  |  |  |  |  |  |  |  | 22,993,880 |
Units contributed |  |  |  |  |  |  |  |  |  |  | 6,716,419 |
Ending balance, number of units |  |  |  |  |  |  | 794,089,518 |  | 415,438,793 |  | 368,555,271 |
Aggregate percentage of general partner interest |  | 2.00% |  | 2.00% |  | 2.00% | 2.00% |  |  |  |  |
LOOP LOCAP SAX and Explorer |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Units, acquisitions |  |  |  |  |  |  | 264,413,309 |  |  |  |  |
Aggregate percentage of general partner interest |  |  | 2.00% |  |  |  |  |  |  |  |  |
Refining Logistics & Fuels Distribution [Member] |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Units, acquisitions |  |  |  |  |  |  | 113,888,889 |  |  |  |  |
HST & WHC & MPLXT |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Units, acquisitions |  |  |  |  |  |  |  |  | 13,224,873 |  |  |
LOOP LOCAP SAX and Explorer |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Units, acquisitions |  |  |  |  |  |  |  |  | 18,888,912 |  |  |
Limited Partners Common Units |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance, number of units |  |  |  |  |  |  | 407,130,020 |  | 357,193,288 |  | 296,687,176 |
Unit-based compensation awards, in units |  |  |  |  |  |  | 348,387 |  | 268,167 |  | 120,989 |
Units issued |  |  |  |  |  |  | 0 |  | 13,846,998 |  | 26,347,887 |
Unit conversion |  | 275,000,000 |  |  |  |  | 275,000,000 |  | 4,350,057 |  | 4,350,057 |
Units, acquisitions |  |  |  |  |  |  |  |  |  |  | 22,534,002 |
Units contributed |  |  |  |  | 21,401,137 |  |  |  |  |  | 7,153,177 |
Ending balance, number of units |  |  |  |  |  |  | 794,089,518 |  | 407,130,020 |  | 357,193,288 |
Limited Partners Common Units | Refining Logistics & Fuels Distribution [Member] |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Units, acquisitions |  |  |  |  |  |  | 111,611,111 |  |  |  |  |
Limited Partners Common Units | HST & WHC & MPLXT |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Units, acquisitions |  |  |  |  |  |  |  |  | 12,960,376 |  |  |
Limited Partners Common Units | LOOP LOCAP SAX and Explorer |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Units, acquisitions |  |  |  |  |  |  |  |  | 18,511,134 |  |  |
Class B Units |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance, number of units |  |  |  |  |  |  | 0 |  | 3,990,878 |  | 7,981,756 |
Unit conversion |  |  |  |  |  |  |  |  | (3,990,878) |  | (3,990,878) |
Ending balance, number of units |  |  |  |  |  |  | 0 |  | 0 |  | 3,990,878 |
General Partner Units |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance, number of units |  |  |  |  |  |  | 8,308,773 | [1] | 7,371,105 |  | 6,800,475 | [1]
Unit-based compensation awards, in units | [1] |  |  |  |  |  | 140 |  | 5,472 |  | 2,470 |
Units issued | [1] |  |  |  |  |  |  |  | 282,591 |  | 537,710 |
Unit conversion | [1] |  |  |  |  |  | (10,586,691) |  | 7,330 |  | 7,330 |
Units, acquisitions | [1] |  |  |  |  |  |  |  |  |  | 459,878 |
Units contributed |  |  |  |  | 436,758 |  |  |  |  |  | (436,758) | [1]
Ending balance, number of units |  |  |  |  |  |  | 0 | [1] | 8,308,773 | [1] | 7,371,105 |
General Partner Units | LOOP LOCAP SAX and Explorer |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Units, acquisitions |  |  | 377,778 |  |  |  |  |  |  |  |  |
General Partner Units | Refining Logistics & Fuels Distribution [Member] |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Units, acquisitions | [1] |  |  |  |  |  | 2,277,778 |  |  |  |  |
General Partner Units | HST & WHC & MPLXT |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Units, acquisitions |  |  |  | 264,497 |  |  |  |  | 264,497 | [1] |  |
General Partner Units | LOOP LOCAP SAX and Explorer |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Units, acquisitions | [1] |  |  |  |  |  |  |  | 377,778 |  |  |
[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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