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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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- Definition Total costs accrued as of the balance sheet date for environmental loss contingencies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346 + Details
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