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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.