Equity-Based Compensation Plan - Additional Information (Detail)	3 Months Ended
Mar. 31, 2020 $ / shares
Officer \| Performance Shares [Member] \| MPLX LP 2012 Incentive Compensation Plan [Member]
Equity Transactions And Share Based Compensation [Line Items]
Percentage paid out in cash	75.00%
Share Based Compensation Arrangement By Share Based Payment Award Percentage Paid Out In Stock	25.00%
2020 [Member] \| Performance Units Market Condition [Member]
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit)	$ 0.80
2018 [Member] \| Performance Units Market Condition [Member]
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit)	0.45
Maximum [Member] \| 2020 [Member] \| Performance Unit Performance Condition [Member]
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit)	2.00
Maximum [Member] \| 2018 [Member] \| Performance Unit Performance Condition [Member]
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit)	$ 2.00

Equity Transactions And Share Based Compensation [Line Items]

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- Definition

Share Based Compensation Arrangement By Share Based Payment Award Percentage Paid Out In Cash

+ References

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- Definition

Share Based Compensation Arrangement By Share Based Payment Award Percentage Paid Out In Stock

+ References

No definition available.

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- Definition

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://fasb.org/us-gaap/role/ref/legacyRef
-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=116856206&loc=d3e5070-113901

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