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Equity - Changes in Partners Capital, Unit Rollforward (Details) - USD ($) $ in Millions |  |  |  | 3 Months Ended
--- | --- | --- | --- | ---
Feb. 01, 2018 | [2] | Sep. 01, 2017 | Mar. 01, 2017 | Mar. 31, 2018 | Sep. 30, 2017 | Mar. 31, 2017
Stockholders Equity [Line Items] |  |  |  |  |  |  |
Balance at December 31, 2017 |  |  |  |  | 415,438,793 |  |
Unit-based compensation awards | [1] |  |  |  | 150,397 |  |
Contribution of refining logistics and fuels distribution assets | [2] |  |  |  | 113,888,889 |  |
Conversion of GP economic interests |  |  |  |  | 264,413,309 |  |
Balance at March 31, 2018 |  |  |  |  | 793,891,388 |  |
Sale of units for GP Interest |  |  |  |  |  |  | $ 151.0
General partner ownership interest |  |  | 2.00% | 2.00% | 2.00% |  |
Contribution from MPC |  |  |  |  | $ 1,046.0 |  | $ 12.0
Limited Partners Common Units |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |
Balance at December 31, 2017 |  |  |  |  | 407,130,020 |  |
Unit-based compensation awards | [1] |  |  |  | 150,257 |  |
Contribution of refining logistics and fuels distribution assets | [2] |  |  |  | 111,611,111 |  |
Conversion of GP economic interests |  | 275,000,000 |  |  | 275,000,000 |  |
Balance at March 31, 2018 |  |  |  |  | 793,891,388 |  |
General Partner Units |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |
Balance at December 31, 2017 |  |  |  |  | 8,308,773 |  |
Unit-based compensation awards | [1] |  |  |  | 140 |  |
Contribution of refining logistics and fuels distribution assets | [2] |  |  |  | 2,277,778 |  |
Conversion of GP economic interests |  |  |  |  | (10,586,691) |  |
Balance at March 31, 2018 |  |  |  |  | 0 |  |
Sale of units for GP Interest |  |  |  |  | $ 1.0 |  |
MPC |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |
Contribution from MPC |  |  |  | $ 6.0 |  | $ 10.0 |
MPC | Refining Logistics & Fuels Distribution |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |
Contribution from MPC |  |  |  |  | $ 23.7 |  |
[1] | As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 140 general partner units to maintain its two percent GP Interest.
[2] | MPC agreed to waive approximately one-third of the first quarter 2018 distributions on the common units issued in connection with this transaction. As a result of this waiver, MPC will not receive $23.7 million of the distributions that would have otherwise accrued on such common units with respect to the first quarter 2018. See Note 4 for information regarding this acquisition.

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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://www.xbrl.org/2003/role/presentationRef -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 Total change in each class of partners' capital accounts during the year due to the sale of units. All partners include general, limited and preferred partners. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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 + Details
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