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Equity - Changes in Partners Capital, Unit Rollforward (Details) - shares
 6 Months Ended
Mar. 01, 2017
Jun. 30, 2017
Stockholders Equity [Line Items]    
Balance at December 31, 2016   368,555,271
Unit-based compensation awards [1]   (172,063)
Balance at June 30, 2017   394,873,292
Limited Partners Common Units [Member]    
Stockholders Equity [Line Items]    
Balance at December 31, 2016   357,193,288
Unit-based compensation awards [1]   (168,622)
Balance at June 30, 2017   382,984,949
Class B Units [Member]    
Stockholders Equity [Line Items]    
Balance at December 31, 2016 [2]   3,990,878
Balance at June 30, 2017   3,990,878
General Partner Units [Member]    
Stockholders Equity [Line Items]    
Balance at December 31, 2016   7,371,105
Unit-based compensation awards [1]   (3,441)
Balance at June 30, 2017   7,897,465
ATM Program [Member]    
Stockholders Equity [Line Items]    
Issuance of units under the ATM Program [3]   12,921,085
ATM Program [Member] | Limited Partners Common Units [Member]    
Stockholders Equity [Line Items]    
Issuance of units under the ATM Program [3]   12,662,663
ATM Program [Member] | General Partner Units [Member]    
Stockholders Equity [Line Items]    
Issuance of units under the ATM Program [3]   258,422
HST & WHC & MPLXT [Member]    
Stockholders Equity [Line Items]    
Contribution of HST/WHC/MPLXT [4]   13,224,873
HST & WHC & MPLXT [Member] | Limited Partners Common Units [Member]    
Stockholders Equity [Line Items]    
Contribution of HST/WHC/MPLXT [4]   12,960,376
HST & WHC & MPLXT [Member] | General Partner Units [Member]    
Stockholders Equity [Line Items]    
Contribution of HST/WHC/MPLXT [4] 264,497 264,497
[1] As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 3,441 general partner units to maintain its two percent GP Interest.
[2] On July 1, 2017, 3,990,878 Class B units converted to 4,350,057 common units and will be eligible to receive the second quarter 2017 distribution.
[3] As a result of common units issued under the ATM Program during the period, MPLX GP contributed $9 million in exchange for 258,422 general partner units to maintain its two percent GP Interest.
[4] See Note 3 for information regarding the HST, WHC and MPLXT acquisition.