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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 | 6 Months Ended
--- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Net income (loss) attributable to MPLX LP: |  |  |  |  |
Distributions declared (including IDRs) |  | $ 311 | $ 231 | $ 590 | $ 431
Undistributed net income (loss) attributable to MPLX LP |  | (121) | (212) | (250) | (472)
Net income (loss) attributable to MPLX LP | [1] | $ 190 | $ 19 | $ 340 | $ (41)
Weighted average units outstanding: |  |  |  |  |
Common - basic (in shares) |  | 416 | 355 | 409 | 331
Common - diluted (in shares) |  | 421 | 355 | 413 | 331
Limited Partners Common Units [Member] |  |  |  |  |
Net income (loss) attributable to MPLX LP: |  |  |  |  |
Distributions declared (including IDRs) | [2] | $ 218 | $ 172 | $ 416 | $ 328
Undistributed net income (loss) attributable to MPLX LP |  | (119) | (207) | (245) | (463)
Net income (loss) attributable to MPLX LP | [1] | $ 99 | $ (35) | $ 171 | $ (135)
Weighted average units outstanding: |  |  |  |  |
Common - basic (in shares) |  | 377 | 331 | 370 | 316
Common - diluted (in shares) |  | 382 | 331 | 374 | 316
Net income attributable to MPLX LP per limited partner unit: |  |  |  |  |
Basic (in USD per unit) |  | $ 0.26 | $ (0.11) | $ 0.46 | $ (0.43)
Diluted (in USD per unit) |  | $ 0.26 | $ (0.11) | $ 0.46 | $ (0.43)
Preferred Units |  |  |  |  |
Net income (loss) attributable to MPLX LP: |  |  |  |  |
Distributions declared (including IDRs) | [2] | $ 17 | $ 9 | $ 33 | $ 9
Undistributed net income (loss) attributable to MPLX LP |  | 0 | 0 | 0 | 0
Net income (loss) attributable to MPLX LP | [1] | $ 17 | $ 9 | $ 33 | $ 9
Weighted average units outstanding: |  |  |  |  |
Common - basic (in shares) |  | 31 | 17 | 31 | 8
Common - diluted (in shares) |  | 31 | 17 | 31 | 8
MPC | General Partner |  |  |  |  |
Net income (loss) attributable to MPLX LP: |  |  |  |  |
Distributions declared (including IDRs) | [2] | $ 76 | $ 50 | $ 141 | $ 94
Undistributed net income (loss) attributable to MPLX LP |  | (2) | (5) | (5) | (9)
Net income (loss) attributable to MPLX LP | [1] | $ 74 | $ 45 | $ 136 | $ 85
Weighted average units outstanding: |  |  |  |  |
Common - basic (in shares) |  | 8 | 7 | 8 | 7
Common - diluted (in shares) |  | 8 | 7 | 8 | 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 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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- 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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- Definition The earnings that is allocated to common stock and participating securities to the extent that each security may share in earnings as if all of the earnings for the period had been distributed. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 65 -URI http://asc.fasb.org/extlink&oid=96947427&loc=d3e2793-109256 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60A -URI http://asc.fasb.org/extlink&oid=96947427&loc=SL5780132-109256 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 66 -URI http://asc.fasb.org/extlink&oid=96947427&loc=d3e2814-109256 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60 -URI http://asc.fasb.org/extlink&oid=96947427&loc=d3e2740-109256 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60B -URI http://asc.fasb.org/extlink&oid=96947427&loc=SL5780133-109256 + Details
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