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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
Mar. 31, 2018
Mar. 31, 2017
Net income (loss) attributable to MPLX LP:    
Distributions declared (including IDRs) $ 483 $ 279
Undistributed net income (loss) attributable to MPLX LP (62) (129)
Net income attributable to MPLX LP [1] $ 421 $ 150
Weighted average units outstanding:    
Common - basic (in shares) 692 400
Common - diluted (in shares) 692 405
Limited Partners Common Units    
Net income (loss) attributable to MPLX LP:    
Distributions declared (including IDRs) [2] $ 467 $ 198
Undistributed net income (loss) attributable to MPLX LP (62) (126)
Net income attributable to MPLX LP [1] $ 405 $ 72
Weighted average units outstanding:    
Common - basic (in shares) 661 362
Common - diluted (in shares) 661 367
Net income attributable to MPLX LP per limited partner unit:    
Basic (in USD per unit) $ 0.61 $ 0.20
Diluted (in USD per unit) $ 0.61 $ 0.19
Preferred Units    
Net income (loss) attributable to MPLX LP:    
Distributions declared (including IDRs) [2] $ 16 $ 16
Undistributed net income (loss) attributable to MPLX LP 0 0
Net income attributable to MPLX LP [1] $ 16 $ 16
Weighted average units outstanding:    
Common - basic (in shares) 31 31
Common - diluted (in shares) 31 31
MPC | General Partner    
Net income (loss) attributable to MPLX LP:    
Distributions declared (including IDRs) [2] $ 0 $ 65
Undistributed net income (loss) attributable to MPLX LP   (3)
Net income attributable to MPLX LP [1]   $ 62
Weighted average units outstanding:    
Common - basic (in shares)   7
Common - diluted (in shares)   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 8 for distribution information.