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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 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Net Income Per Share [Line Items]        
Potentially Dilutive Units     1 10
Net income (loss) attributable to MPLX LP [1] $ 190 $ 19 $ 340 $ (41)
Less: Distribution declared 311 231 590 431
Undistributed net income (loss) attributable to MPLX LP (121) (212) (250) (472)
Preferred Units        
Net Income Per Share [Line Items]        
Net income (loss) attributable to MPLX LP [1] 17 9 33 9
Less: Distribution declared [2] 17 9 33 9
Undistributed net income (loss) attributable to MPLX LP 0 0 0 0
General Partner | MPC        
Net Income Per Share [Line Items]        
Net income (loss) attributable to MPLX LP [1] 74 45 136 85
Less: Distribution declared [2] 76 50 141 94
Undistributed net income (loss) attributable to MPLX LP (2) (5) (5) (9)
Limited Partners Common Units [Member]        
Net Income Per Share [Line Items]        
Net income (loss) attributable to MPLX LP [1] 99 (35) 171 (135)
Less: Distribution declared [2] 218 172 416 328
Undistributed net income (loss) attributable to MPLX LP $ (119) $ (207) $ (245) $ (463)
[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.