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				9 Months Ended
		Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Potentially dilutive units		1		1		1		1
Distribution Made to Limited Partner, Cash Distributions Declared		$ 745		$ 734		$ 2,250		$ 2,011
Net income (loss) attributable to MPLX LP:
Undistributed net income (loss) attributable to MPLX LP		(80)		(105)		(3,661)		(397)
Net income attributable to MPLX LP		665	[1]	629	[2]	(1,411)	[3]	1,614	[4]
Limited Partners Common Units
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[5]	715		704	[6]	2,158		1,919	[6]
Net income (loss) attributable to MPLX LP:
Undistributed net income (loss) attributable to MPLX LP		(80)		(105)		(3,661)		(397)
Net income attributable to MPLX LP		$ 635	[1]	$ 599	[2]	$ (1,503)	[3]	$ 1,522	[4]
Weighted average units outstanding:
Common - basic (in shares)		1,046		974	[7]	1,054		855	[8]
Common - diluted (in shares)		1,047		975	[7]	1,054		855	[8]
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)		$ 0.61		$ 0.61		$ (1.43)		$ 1.78
Diluted (in USD per unit)		$ 0.61		$ 0.61		$ (1.43)		$ 1.78
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	[5]	$ 20		$ 20		$ 61		$ 61
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		20	[1]	20	[2]	61	[3]	61	[4]
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	[5]	10		10		31		31
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]	$ 10	[2]	$ 31	[3]	$ 31	[4]

[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] 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.

[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] See Note 7 for distribution information.

[6] The three and nine months ended September 30, 2019 amounts are net of $12.5 million and $25 million, respectively, of waived distributions with respect to units held by MPC and its affiliates.

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

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