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Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Detail) - USD ($) shares in Millions, $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Net Income Per Share [Line Items] |  |  |
Potentially dilutive units |  | 1 | 1
Net income attributable to MPLX LP | [1] | $ 421 | $ 150
Less: Distribution declared |  | 483 | 279
Undistributed net loss attributable to MPLX LP |  | (62) | (129)
Preferred Units |  |  |
Net Income Per Share [Line Items] |  |  |
Net income attributable to MPLX LP | [1] | 16 | 16
Less: Distribution declared | [2] | 16 | 16
General Partner | MPC |  |  |
Net Income Per Share [Line Items] |  |  |
Net income attributable to MPLX LP | [1] |  | 62
Less: Distribution declared | [2] | 0 | 65
Limited Partners Common Units |  |  |
Net Income Per Share [Line Items] |  |  |
Net income attributable to MPLX LP | [1] | 405 | 72
Less: Distribution declared | [2] | $ 467 | $ 198
[1] | Allocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[2] | See Note 8 for distribution information.

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- Definition Amount of equity impact of cash distribution declared to unit-holder of 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 1 -URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70434-108055 Reference 2: 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 Reference 3: 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 + Details
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