v3.3.0.814
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 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Net income attributable to MPLX LP:        
Distributions declared (including IDRs) $ 46.5 $ 27.9 $ 125.0 $ 79.4
Undistributed net income attributable to MPLX LP (5.0) 1.2 13.3 12.7
Net income attributable to MPLX LP [1] $ 41.5 $ 29.1 $ 138.3 $ 92.1
Weighted average units outstanding:        
Weighted Average Limited Partnership Units Outstanding, Basic 82.0 75.5 82.0 75.4
Weighted Average Limited Partnership Units Outstanding, Diluted 82.0 75.5 82.0 75.5
Limited Partners Common Units [Member]        
Net income attributable to MPLX LP:        
Distributions declared (including IDRs) [2] $ 37.7 $ 13.2 $ 74.6 $ 38.0
Undistributed net income attributable to MPLX LP (4.9) 0.5 4.6 4.8
Net income attributable to MPLX LP [1] $ 32.8 $ 13.7 $ 79.2 $ 42.8
Weighted average units outstanding:        
Weighted Average Limited Partnership Units Outstanding, Basic 80.4 37.1 55.9 37.0
Weighted Average Limited Partnership Units Outstanding, Diluted 80.4 37.1 55.9 37.1
Net income attributable to MPLX LP per limited partner unit:        
Basic (in USD per unit) $ 0.41 $ 0.37 $ 1.42 $ 1.16
Diluted (in USD per unit) $ 0.41 $ 0.37 $ 1.42 $ 1.16
MPC [Member] | General Partner [Member]        
Net income attributable to MPLX LP:        
Distributions declared (including IDRs) [2] $ 8.8 $ 1.5 $ 19.0 $ 3.4
Undistributed net income attributable to MPLX LP (0.1) 0.3 6.6 3.2
Net income attributable to MPLX LP [1] $ 8.7 $ 1.8 $ 25.6 $ 6.6
Weighted average units outstanding:        
Weighted Average Limited Partnership Units Outstanding, Basic 1.6 1.4 1.6 1.4
Weighted Average Limited Partnership Units Outstanding, Diluted 1.6 1.4 1.6 1.4
MPC [Member] | Limited Partners Subordinated Units [Member]        
Net income attributable to MPLX LP:        
Distributions declared (including IDRs) [2] $ 0.0 $ 13.2 $ 31.4 $ 38.0
Undistributed net income attributable to MPLX LP 0.0 0.4 2.1 4.7
Net income attributable to MPLX LP [1] $ 0.0 $ 13.6 $ 33.5 $ 42.7
Weighted average units outstanding:        
Weighted Average Limited Partnership Units Outstanding, Basic 0.0 37.0 24.5 37.0
Weighted Average Limited Partnership Units Outstanding, Diluted 0.0 37.0 24.5 37.0
Net income attributable to MPLX LP per limited partner unit:        
Basic (in USD per unit) $ 0.00 $ 0.37 $ 1.36 $ 1.16
Diluted (in USD per unit) $ 0.00 $ 0.37 $ 1.36 $ 1.16
[1] Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[2] See Note 6 for distribution information.