Equity-Based Compensation Plan - Summary of Phantom Unit Award Activity (Details) - $ / shares		6 Months Ended
Jul. 30, 2019	Jun. 30, 2020
Phantom Units
Number of Units
Outstanding at December 31, 2019		1,109,568
Granted		195,461
Settled		323,884
Forfeited		4,627
Outstanding at June 30, 2020		976,518
Weighted Average Fair Value
Outstanding at December 31, 2019		$ 35.97
Granted		19.43
Settled		35.67
Forfeited		36.40
Outstanding at June 30, 2020		$ 32.76
ANDX LP [Member] \| Public
Equity Transactions And Share Based Compensation [Line Items]
Common Units Conversion Ratio - ANDX to MPLX	1.135

Ratio applied to the conversion of ANDX common units to MPLX common units

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Equity Transactions And Share Based Compensation [Line Items]

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Share Based Compensation Arrangement By Share Based Payment Award Non Option Equity Instruments Weighted Average Grant Date Fair Value Issued

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Share Based Compensation Arrangement By Share Based Payment Award Non Option Equity Instruments Weighted Average Grant Date Fair Value Outstanding

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Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments. Weighted Average Grant Fair Value [Roll Forward]

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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).

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Number of non-option equity instruments exercised by participants.

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Number of shares under non-option equity instrument agreements that were cancelled as a result of occurrence of a terminating event.

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Net number of non-option equity instruments granted to participants.

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Number of equity instruments other than options outstanding, including both vested and non-vested instruments.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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