Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Detail) - USD ($) shares in Millions		3 Months Ended								9 Months Ended
		Sep. 30, 2020		Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019		Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020		Sep. 30, 2019
Net Income Per Share [Line Items]
Potentially dilutive units		1				1				1		1
Net income attributable to MPLX LP		$ 665,000,000	[1]			$ 629,000,000	[2]			$ (1,411,000,000)	[3]	$ 1,614,000,000	[4]
Less: Distribution declared		745,000,000				734,000,000				2,250,000,000		2,011,000,000
Undistributed net income (loss) attributable to MPLX LP		(80,000,000)				(105,000,000)				(3,661,000,000)		(397,000,000)
Undistributed net loss attributable to MPLX LP		(80,000,000)				(105,000,000)				(3,661,000,000)		(397,000,000)
Capital Contribution By Related Party		13,000,000		$ 6,000,000	$ 225,000,000	295,000,000		$ 13,000,000	$ 15,000,000
Limited Partners Common Units
Net Income Per Share [Line Items]
Net income attributable to MPLX LP		635,000,000	[1]			599,000,000	[2]			(1,503,000,000)	[3]	1,522,000,000	[4]
Less: Distribution declared	[5]	715,000,000				704,000,000	[6]			2,158,000,000		1,919,000,000	[6]
Undistributed net income (loss) attributable to MPLX LP		(80,000,000)				(105,000,000)				(3,661,000,000)		(397,000,000)
MPC | Limited Partners Common Units
Net Income Per Share [Line Items]
Capital Contribution By Related Party		13,000,000		$ 6,000,000	$ 225,000,000	292,000,000		$ 0	$ 0
MPC | ANDX LP [Member]
Net Income Per Share [Line Items]
Capital Contribution By Related Party						12,500,000						25,000,000
Series A Preferred Stock [Member] | Series A Preferred Stock [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP		20,000,000	[1]			20,000,000	[2]			61,000,000	[3]	61,000,000	[4]
Less: Distribution declared	[5]	20,000,000				20,000,000				61,000,000		61,000,000
Undistributed net income (loss) attributable to MPLX LP		0				0				0		0
Series B Preferred Stock [Member] | Series A Preferred Stock [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP		10,000,000	[1]			10,000,000	[2]			31,000,000	[3]	31,000,000	[4]
Less: Distribution declared	[5]	10,000,000				10,000,000				31,000,000		31,000,000
Undistributed net income (loss) attributable to MPLX LP		$ 0				$ 0				$ 0		$ 0

[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.