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Equity-Based Compensation Plan - Additional Information (Detail) - Performance Shares [Member] - MPLX LP 2012 Incentive Compensation Plan [Member]	6 Months Ended
	Jun. 30, 2017 $ / shares
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit)	$ 0.90
Officer [Member]
Equity Transactions And Share Based Compensation [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Percentage Paid Out In Cash	75.00%
Share Based Compensation Arrangement By Share Based Payment Award Percentage Paid Out In Stock	25.00%

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Equity Transactions And Share Based Compensation [Line Items]

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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