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Equity-Based Compensation Plan (Tables)
 3 Months Ended
Mar. 31, 2018
Phantom Units  
Equity Transactions And Share Based Compensation [Line Items]  
Summary of Share-based Compensation, Restricted Stock Units Award Activity
The following is a summary of phantom unit award activity of MPLX LP common units for the three months ended March 31, 2018:
 
Number
of Units
 
Weighted
Average
Fair Value
Outstanding at December 31, 2017
1,351,523

 
$
34.53

Granted
169,310

 
35.00

Settled
(217,093
)
 
33.83

Forfeited
(46,330
)
 
34.95

Outstanding at March 31, 2018
1,257,410

 
34.70
Performance Shares [Member]  
Equity Transactions And Share Based Compensation [Line Items]  
Summary of Share-based Compensation, Restricted Stock Units Award Activity
The following is a summary of the activity for performance unit awards to be settled in MPLX LP common units for the three months ended March 31, 2018:
 
Number of
Units
Outstanding at December 31, 2017
2,536,594

Granted
—

Settled
(538,594
)
Forfeited
(50,000
)
Outstanding at March 31, 2018
1,948,000