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Equity-Based Compensation Plan - Additional Information (Detail) - Officer - Performance Shares [Member] - MPLX LP 2012 Incentive Compensation Plan [Member]	3 Months Ended
	Mar. 31, 2018
Equity Transactions And Share Based Compensation [Line Items]
Percentage paid out in cash	25.00%
Percentage paid out in stock	75.00%

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Equity Transactions And Share Based Compensation [Line Items]

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