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Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended
	Sep. 06, 2016	Mar. 31, 2018	Dec. 31, 2017
Commitments And Contingencies [Line Items]
Accrued liabilities for environmental remediation		$ 15.0	$ 13.0
Payables - related parties		146.0	516.0
Contractual commitments to acquire property, plant and equipment		718.0
MPC
Commitments And Contingencies [Line Items]
Payables - related parties		115.0	470.0
Environmental Loss Contingency [Member] | MPC
Commitments And Contingencies [Line Items]
Payables - related parties			$ 1.0
Receivables from MPC for indemnification of environmental costs		3.0
MarkWest, MarkWest Liberty Midstream, MarkWest Bluestone, Ohio Fractionation, MarkWest Utica EMG [Member] [Member]
Commitments And Contingencies [Line Items]
Loss Contingency, Damages Sought, Value		10.0
Bilfinger Westcon, Inc. [Member]
Commitments And Contingencies [Line Items]
Loss Contingency, Damages Sought, Value		40.0
Apex [Member]
Commitments And Contingencies [Line Items]
Apex litigation settlement amount	$ 10.0
Markwest Liberty Midstream [Member]
Commitments And Contingencies [Line Items]
EPA proposed penalty		0.6
Estimated cost of proposed supplemental environmental projects		$ 2.4

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Total costs accrued as of the balance sheet date for environmental loss contingencies.

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