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Net Income (Loss) Per Limited Partner Unit (Basic and Diluted Earnings Per Unit) (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Net income attributable to MPLX LP: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total cash distributions declared |  |  |  |  |  |  |  |  |  | $ 2,060 |  | $ 1,288 |  | $ 938 |
Undistributed net income loss attributable to MPLX LP |  |  |  |  |  |  |  |  |  | (242) |  | (494) |  | (705) |
Net income attributable to MPLX LP |  | $ 434 | $ 510 | $ 453 | $ 421 | $ 238 | $ 216 | $ 190 | $ 150 | $ 1,818 | [1] | $ 794 | [1] | $ 233 | [1]
Weighted average units outstanding: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basic (shares) |  |  |  |  |  |  |  |  |  | 761 |  | 393 |  | 338 |
Diluted (shares) |  |  |  |  |  |  |  |  |  | 761 |  | 396 |  | 345 |
General Partner |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income attributable to MPLX LP: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total cash distributions declared | [3] |  |  |  |  |  |  |  |  | $ 0 | [2] | $ 328 |  | $ 205 |
Undistributed net income loss attributable to MPLX LP |  |  |  |  |  |  |  |  |  |  |  |  |  | (14) |
Net income attributable to MPLX LP | [1] |  |  |  |  |  |  |  |  |  |  | 318 |  | $ 191 |
Weighted average units outstanding: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basic (shares) |  |  |  |  |  |  |  |  |  |  |  |  |  | 7 |
Diluted (shares) |  |  |  |  |  |  |  |  |  |  |  |  |  | 7 |
Limited Partners Common Units |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income attributable to MPLX LP: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total cash distributions declared | [3] |  |  |  |  |  |  |  |  | 1,985 |  | 895 |  | $ 692 |
Undistributed net income loss attributable to MPLX LP |  |  |  |  |  |  |  |  |  | (242) |  | (484) |  | (691) |
Net income attributable to MPLX LP | [1] |  |  |  |  |  |  |  |  | $ 1,743 |  | $ 411 |  | $ 1 |
Weighted average units outstanding: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basic (shares) |  |  |  |  |  |  |  |  |  | 761 |  | 385 |  | 331 |
Diluted (shares) |  |  |  |  |  |  |  |  |  | 761 |  | 388 |  | 338 |
Net income attributable to MPLX LP subsequent to initial public offering per limited partner unit: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basic (in USD per unit) |  | $ 0.52 | $ 0.62 | $ 0.55 | $ 0.61 | $ 0.31 | $ 0.29 | $ 0.26 | $ 0.20 | $ 2.29 |  | $ 1.07 |  | $ 0.00 |
Diluted (in USD per unit) |  | $ 0.52 | $ 0.62 | $ 0.55 | $ 0.61 | $ 0.31 | $ 0.29 | $ 0.26 | $ 0.19 | $ 2.29 |  | $ 1.06 |  | $ 0.00 |
Preferred Units |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income attributable to MPLX LP: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total cash distributions declared |  | $ 20 | $ 19 | $ 20 | $ 16 | $ 16 | $ 16 | $ 17 | $ 16 | $ 75 | [3] | $ 65 | [3] | $ 41 | [3]
Undistributed net income loss attributable to MPLX LP |  |  |  |  |  |  |  |  |  | 0 |  | 0 |  | 0 |
Net income attributable to MPLX LP |  |  |  |  |  |  |  |  |  | $ 75 | [1] | 65 | [1] | $ 41 |
Marathon Petroleum Corporation [Member] | General Partner |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income attributable to MPLX LP: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total cash distributions declared |  |  |  |  |  |  |  |  |  |  |  | 328 |  |  |
Undistributed net income loss attributable to MPLX LP |  |  |  |  |  |  |  |  |  |  |  | $ (10) |  |  |
Weighted average units outstanding: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basic (shares) |  |  |  |  |  |  |  |  |  |  |  | 8 |  |  |
Diluted (shares) |  |  |  |  |  |  |  |  |  |  |  | 8 |  |  |
Marathon Petroleum Corporation [Member] | Limited Partners Common Units |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income attributable to MPLX LP: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total cash distributions declared |  | $ 327 | $ 322 | $ 316 | $ 288 | $ 171 | $ 62 | $ 56 | $ 49 |  |  |  |  |  |
[1] | Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period were distributed based on the current period distribution priorities.
[2] | Distributions declared on January 25, 2018 on general partner common units issued on February 1, 2018 in exchange for the economic general partner interest, including IDRs, are shown as general partner distributions declared.
[3] | See Note 8 for distribution information.

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