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Equity - Net Income Allocation (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Equity [Abstract]        
Net income attributable to MPLX LP [1] $ 41.5 $ 29.1 $ 138.3 $ 92.1
General partner's incentive distribution rights and other 7.9 1.0 16.9 1.9
Net income attributable to MPLX LP available to general and limited partners 33.6 28.1 121.4 90.2
General partner's two percent interest in net income attributable to MPLX LP 0.7 0.5 2.5 1.8
Less: General partner’s interest in net income attributable to MPLX LP $ 8.6 $ 1.5 $ 19.4 $ 3.7
[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.