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Equity - Changes in Partners Capital, Unit Rollforward (Details) - USD ($)
$ in Millions
 3 Months Ended
Feb. 01, 2018
 [2]
Sep. 01, 2017
Mar. 01, 2017
Mar. 31, 2018
Sep. 30, 2017
Mar. 31, 2017
Stockholders Equity [Line Items]            
Balance at December 31, 2017       415,438,793    
Unit-based compensation awards [1]       150,397    
Contribution of refining logistics and fuels distribution assets [2]       113,888,889    
Conversion of GP economic interests       264,413,309    
Balance at March 31, 2018       793,891,388    
Sale of units for GP Interest           $ 151.0
General partner ownership interest   2.00% 2.00% 2.00%    
Contribution from MPC       $ 1,046.0   $ 12.0
Limited Partners Common Units            
Stockholders Equity [Line Items]            
Balance at December 31, 2017       407,130,020    
Unit-based compensation awards [1]       150,257    
Contribution of refining logistics and fuels distribution assets [2]       111,611,111    
Conversion of GP economic interests 275,000,000     275,000,000    
Balance at March 31, 2018       793,891,388    
General Partner Units            
Stockholders Equity [Line Items]            
Balance at December 31, 2017       8,308,773    
Unit-based compensation awards [1]       140    
Contribution of refining logistics and fuels distribution assets [2]       2,277,778    
Conversion of GP economic interests       (10,586,691)    
Balance at March 31, 2018       0    
Sale of units for GP Interest       $ 1.0    
MPC            
Stockholders Equity [Line Items]            
Contribution from MPC     $ 6.0   $ 10.0  
MPC | Refining Logistics & Fuels Distribution            
Stockholders Equity [Line Items]            
Contribution from MPC       $ 23.7    
[1] As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 140 general partner units to maintain its two percent GP Interest.
[2] MPC agreed to waive approximately one-third of the first quarter 2018 distributions on the common units issued in connection with this transaction. As a result of this waiver, MPC will not receive $23.7 million of the distributions that would have otherwise accrued on such common units with respect to the first quarter 2018. See Note 4 for information regarding this acquisition.