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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.