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Net Income (Loss) Per Limited Partner Unit (Schedule of Distributions by Partner Class) (Detail) - USD ($)
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 Per Share [Line Items]                      
Potentially dilutive securities                 1 1 1
Net income attributable to MPLX LP $ 434 $ 510 $ 453 $ 421 $ 238 $ 216 $ 190 $ 150 $ 1,818 [1] $ 794 [1] $ 233 [1]
Total cash distributions declared                 2,060 1,288 938
Undistributed net loss attributable to MPLX LP                 (242) (494) (705)
Preferred Units                      
Net Income Per Share [Line Items]                      
Net income attributable to MPLX LP                 75 [1] 65 [1] 41
Total cash distributions declared $ 20 $ 19 $ 20 $ 16 $ 16 $ 16 $ 17 $ 16 75 [2] 65 [2] 41 [2]
General Partner                      
Net Income Per Share [Line Items]                      
Net income attributable to MPLX LP [1]                   318 191
Total cash distributions declared [2]                 0 [3] 328 205
Limited Partners Common Units                      
Net Income Per Share [Line Items]                      
Net income attributable to MPLX LP [1]                 1,743 411 1
Total cash distributions declared [2]                 $ 1,985 $ 895 $ 692
[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] See Note 8 for distribution information.
[3] 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.