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Commitments and Contingencies (Detail) - USD ($)
$ in Millions
 12 Months Ended
Sep. 06, 2016
Dec. 31, 2018
Dec. 31, 2017
Commitments And Contingencies [Line Items]      
Accrual for environmental loss contingencies   $ 14.0 $ 13.0
Payable to MPC for indemnification of environmental costs   203.0 516.0
Contractual commitments to acquire property, plant and equipment   746.0  
MPC      
Commitments And Contingencies [Line Items]      
Payable to MPC for indemnification of environmental costs [1]   131.0 470.0
Environmental Loss Contingency [Member] | MPC      
Commitments And Contingencies [Line Items]      
Receivable from MPC for indemnification of environmental costs   0.0  
Payable to MPC for indemnification of environmental costs     $ 1.0
MarkWest [Member]      
Commitments And Contingencies [Line Items]      
Damages sought   10.0  
Apex      
Commitments And Contingencies [Line Items]      
Litigation settlement, amount $ 10.0    
Bilfinger Westcon, Inc. [Member]      
Commitments And Contingencies [Line Items]      
Damages sought   40.0  
MarkWest, MarkWest Liberty Midstream, Ohio Fractionation, MarkWest Utica EMG [Member]      
Commitments And Contingencies [Line Items]      
EPA proposed penalty   0.9  
Estimated cost of proposed supplemental environmental projects   3.3  
Markwest Liberty Midstream [Member]      
Commitments And Contingencies [Line Items]      
EPA proposed penalty   0.6  
Estimated cost of proposed supplemental environmental projects   $ 2.4  
[1] Balance includes $386 million related to the MPC Loan Agreement as of December 31, 2017. There was no outstanding balance on the MPC Loan Agreement as of December 31, 2018.