v3.3.0.814
Acquisitions - Additional Information (Detail)
$ in Millions
 3 Months Ended 9 Months Ended
Jul. 11, 2015
USD ($)
Dec. 01, 2014
USD ($)
Mar. 01, 2014
USD ($)
Sep. 30, 2015
USD ($)
Sep. 30, 2014
USD ($)
Sep. 30, 2015
USD ($)
Sep. 30, 2014
USD ($)
Business Acquisition [Line Items]              
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned   $ 200.0          
Additional interest in Pipe Line Holdings   22.875% 13.00%        
Purchase of additional interest in Pipe Line Holdings   $ 600.0 $ 310.0     $ 0.0 $ 310.0
Percentage Of Partners Interest Contributed   7.625%          
Percentage of Partners Interest Purchased and Contributed   30.50%          
Purchase of additional interest in Pipe Line Holdings, historical value             (310.0)
MPLX Pipe Line Holdings LP [Member]              
Business Acquisition [Line Items]              
Percent of the net income related to assets contributed to MPLX LP   99.50%          
General Partner [Member]              
Business Acquisition [Line Items]              
Purchase of additional interest in Pipe Line Holdings, excess cash paid   $ 465.2 172.5 $ 0.0 $ 0.0 0.0 172.5
Noncontrolling Interest Retained by MPC [Member]              
Business Acquisition [Line Items]              
Purchase of additional interest in Pipe Line Holdings, historical value   $ 334.8 $ 137.5        
Noncontrolling Interest Retained by MPC [Member] | MPC [Member]              
Business Acquisition [Line Items]              
Purchase of additional interest in Pipe Line Holdings, historical value             $ (137.5)
Percent of the net income related to assets contributed to MPLX LP   0.50%          
Pipe Line Holding [Member]              
Business Acquisition [Line Items]              
Business Acquisition, Effective Date of Acquisition   Dec. 01, 2014 Mar. 01, 2014        
MarkWest [Member]              
Business Acquisition [Line Items]              
Business Acquisition, Date of Acquisition Agreement Jul. 11, 2015            
Common Units Conversion Ratio 1.09            
General Partners' Contributed Capital $ 675.0     675.0   675.0  
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned       13,700.0   13,700.0  
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt       $ 4,600.0   4,600.0  
Business Combination, Acquisition Related Costs           $ 4.3  
Cash [Member] | Pipe Line Holding [Member]              
Business Acquisition [Line Items]              
Purchase of additional interest in Pipe Line Holdings     $ 40.0        
Revolving Credit Agreement [Member] | Pipe Line Holding [Member]              
Business Acquisition [Line Items]              
Purchase of additional interest in Pipe Line Holdings     $ 270.0