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Related Party Agreements and Transactions - Payables to Related Parties (Detail) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Related Party Transaction [Line Items]    
Payables - related parties $ 146 $ 516
MPC    
Related Party Transaction [Line Items]    
Payables - related parties 115 470
MarkWest Utica EMG    
Related Party Transaction [Line Items]    
Payables - related parties 20 29
Ohio Gathering    
Related Party Transaction [Line Items]    
Payables - related parties 0 8
Sherwood Midstream    
Related Party Transaction [Line Items]    
Payables - related parties 11 8
Other    
Related Party Transaction [Line Items]    
Payables - related parties 0 1
Related Party Revolving Credit Agreement | MPC Investment    
Related Party Transaction [Line Items]    
Intercompany loan outstanding $ 0 $ 386