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Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Detail) - USD ($)
shares in Millions, $ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Net Income Per Share [Line Items]    
Potentially dilutive units 1 1
Net income attributable to MPLX LP [1] $ 421 $ 150
Less: Distribution declared 483 279
Undistributed net loss attributable to MPLX LP (62) (129)
Preferred Units    
Net Income Per Share [Line Items]    
Net income attributable to MPLX LP [1] 16 16
Less: Distribution declared [2] 16 16
General Partner | MPC    
Net Income Per Share [Line Items]    
Net income attributable to MPLX LP [1]   62
Less: Distribution declared [2] 0 65
Limited Partners Common Units    
Net Income Per Share [Line Items]    
Net income attributable to MPLX LP [1] 405 72
Less: Distribution declared [2] $ 467 $ 198
[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.