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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 | 6 Months Ended
--- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Net Income Per Share [Line Items] |  |  |  |  |
Potentially Dilutive Units |  |  |  | 1 | 10
Net income (loss) attributable to MPLX LP | [1] | $ 190 | $ 19 | $ 340 | $ (41)
Less: Distribution declared |  | 311 | 231 | 590 | 431
Undistributed net income (loss) attributable to MPLX LP |  | (121) | (212) | (250) | (472)
Preferred Units |  |  |  |  |
Net Income Per Share [Line Items] |  |  |  |  |
Net income (loss) attributable to MPLX LP | [1] | 17 | 9 | 33 | 9
Less: Distribution declared | [2] | 17 | 9 | 33 | 9
Undistributed net income (loss) attributable to MPLX LP |  | 0 | 0 | 0 | 0
General Partner | MPC |  |  |  |  |
Net Income Per Share [Line Items] |  |  |  |  |
Net income (loss) attributable to MPLX LP | [1] | 74 | 45 | 136 | 85
Less: Distribution declared | [2] | 76 | 50 | 141 | 94
Undistributed net income (loss) attributable to MPLX LP |  | (2) | (5) | (5) | (9)
Limited Partners Common Units [Member] |  |  |  |  |
Net Income Per Share [Line Items] |  |  |  |  |
Net income (loss) attributable to MPLX LP | [1] | 99 | (35) | 171 | (135)
Less: Distribution declared | [2] | 218 | 172 | 416 | 328
Undistributed net income (loss) attributable to MPLX LP |  | $ (119) | $ (207) | $ (245) | $ (463)
[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 7 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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