Net Income Per Limited Partner Unit - Basic and Diluted Earnings Per Unit (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2020		Jun. 30, 2019		Jun. 30, 2020		Jun. 30, 2019
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Potentially dilutive units				1		1		1
Distribution Made to Limited Partner, Cash Distributions Declared		$ 746		$ 734		$ 1,505		$ 1,277
Net income (loss) attributable to MPLX LP:
Undistributed net income (loss) attributable to MPLX LP		(98)		(252)		(3,581)		(292)
Net income attributable to MPLX LP		648	[1]	482	[2],[3]	(2,076)	[4]	985	[2],[5]
Limited Partners Common Units
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[6]	715		692	[7]	1,443		1,215	[7]
Net income (loss) attributable to MPLX LP:
Undistributed net income (loss) attributable to MPLX LP		(98)		(252)		(3,581)		(292)
Net income attributable to MPLX LP		$ 617	[1]	$ 440	[3]	$ (2,138)	[4]	$ 923	[5]
Weighted average units outstanding:
Common - basic (in shares)		1,059		794	[2],[8]	1,059		794	[2],[9]
Common - diluted (in shares)		1,059		795	[2],[8]	1,059		795	[2],[9]
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)		$ 0.58		$ 0.56	[2]	$ (2.02)		$ 1.16	[2]
Diluted (in USD per unit)		$ 0.58		$ 0.55	[2]	$ (2.02)		$ 1.16	[2]
Series A Preferred Stock [Member] | Series A Preferred Stock [Member]
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[6]	$ 21		$ 21		$ 41		$ 41
Net income (loss) attributable to MPLX LP:
Undistributed net income (loss) attributable to MPLX LP		0		0		0		0
Net income attributable to MPLX LP		21	[1]	21	[3]	41	[4]	41	[5]
Series B Preferred Stock [Member] | Series A Preferred Stock [Member]
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[6]	10		21		21		21
Net income (loss) attributable to MPLX LP:
Undistributed net income (loss) attributable to MPLX LP		0		0		0		0
Net income attributable to MPLX LP		$ 10	[1]	$ 21	[3]	$ 21	[4]	$ 21	[5]

[1] Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.

[2] Financial information for the three and six months ended June 30, 2019 has been retrospectively adjusted for the acquisition of ANDX. See Notes 1 and 3.

[3] Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.

[4] Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.

[5] Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.

[6] See Note 7 for distribution information.

[7] The three and six months ended June 30, 2019 amounts are net of $12.5 million of waived distributions with respect to units held by MPC and its affiliates.

[8] The Series B preferred units and the MPLX common units issued in connection with the Merger were not outstanding during the three months ended June 30, 2019. See Notes 3 and 7 for additional information about the treatment of these units.

[9] The Series B preferred units and the MPLX common units issued in connection with the Merger were not outstanding during the six months ended June 30, 2019. See Notes 3 and 7 for additional information about the treatment of these units.