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Commitments and Contingencies - MarkWest Environmental Proceeding (Details) - Pending Litigation - MarkWest Liberty Midstream Pipeline Launcher/Receiver Site [Member]
$ in Millions
3 Months Ended
Mar. 31, 2017
USD ($)
Cash  
Loss Contingencies [Line Items]  
Alleged amount overcharged from customers $ 2.4
Other Liabilities  
Loss Contingencies [Line Items]  
Alleged amount overcharged from customers $ 3.6