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Net Income Per Limited Partner Unit - Basic and Diluted Earnings Per Unit (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Net income (loss) attributable to MPLX LP: |  |  |
Distributions declared (including IDRs) |  | $ 483 | $ 279
Undistributed net income (loss) attributable to MPLX LP |  | (62) | (129)
Net income attributable to MPLX LP | [1] | $ 421 | $ 150
Weighted average units outstanding: |  |  |
Common - basic (in shares) |  | 692 | 400
Common - diluted (in shares) |  | 692 | 405
Limited Partners Common Units |  |  |
Net income (loss) attributable to MPLX LP: |  |  |
Distributions declared (including IDRs) | [2] | $ 467 | $ 198
Undistributed net income (loss) attributable to MPLX LP |  | (62) | (126)
Net income attributable to MPLX LP | [1] | $ 405 | $ 72
Weighted average units outstanding: |  |  |
Common - basic (in shares) |  | 661 | 362
Common - diluted (in shares) |  | 661 | 367
Net income attributable to MPLX LP per limited partner unit: |  |  |
Basic (in USD per unit) |  | $ 0.61 | $ 0.20
Diluted (in USD per unit) |  | $ 0.61 | $ 0.19
Preferred Units |  |  |
Net income (loss) attributable to MPLX LP: |  |  |
Distributions declared (including IDRs) | [2] | $ 16 | $ 16
Undistributed net income (loss) attributable to MPLX LP |  | 0 | 0
Net income attributable to MPLX LP | [1] | $ 16 | $ 16
Weighted average units outstanding: |  |  |
Common - basic (in shares) |  | 31 | 31
Common - diluted (in shares) |  | 31 | 31
MPC | General Partner |  |  |
Net income (loss) attributable to MPLX LP: |  |  |
Distributions declared (including IDRs) | [2] | $ 0 | $ 65
Undistributed net income (loss) attributable to MPLX LP |  |  | (3)
Net income attributable to MPLX LP | [1] |  | $ 62
Weighted average units outstanding: |  |  |
Common - basic (in shares) |  |  | 7
Common - diluted (in shares) |  |  | 7
[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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- Definition The portion of profit or loss for the period, net of income taxes, which is attributable to the parent. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(22)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(18)) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(20)) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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