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Redeemable Preferred Units (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions |  | 12 Months Ended
--- | --- | ---
May 13, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Redeemable Noncontrolling Interest [Line Items] |  |  |  |
Issuance of preferred units |  |  | 14,129,589 | 26,885,597
Issuance of redeemable preferred units |  | $ 0 | $ 0 | $ 984
Series A Convertible Preferred Units |  |  |  |
Redeemable Noncontrolling Interest [Line Items] |  |  |  |
Issuance of preferred units | 30,800,000 |  |  |
Preferred units, dividend rate, percentage | 6.50% |  |  |
Units issued, price per unit | $ 32.50 |  |  |
Issuance of redeemable preferred units | $ 984 |  |  |
Preferred units, dividend rate, per-dollar-amount | $ 0.528125 |  |  |
Preferred units, description | The holders may convert their preferred units into common units at any time after the third anniversary of the issuance date or prior to liquidation, dissolution or winding up of the Partnership, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, MPLX may convert the preferred units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX LP common units is greater than $48.75 for the 20-day trading period immediately preceding the conversion notice date. The conversion rate for the preferred units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable preferred unit, divided by (b) $32.50, subject to adjustment for unit distributions, unit splits and similar transactions. The holders of the preferred units are entitled to vote on an as-converted basis with the common unitholders and will have certain other class voting rights with respect to any amendment to the Partnership Agreement that would adversely affect any rights, preferences or privileges of the preferred units. In addition, upon certain events involving a change of control, the holders of preferred units may elect, among other potential elections, to convert their preferred units to common units at the then applicable change of control conversion rate. |  |  |

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- Definition Description of the various rights, preferences, and privileges of the preferred partners' ownership units including: redemption, conversion, liquidation, tax status of distribution per unit and the sharing in distributions. + 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 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055 Reference 3: 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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