Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Detail) - USD ($) shares in Millions		3 Months Ended							6 Months Ended
		Jun. 30, 2020		Mar. 31, 2020	Jun. 30, 2019		Mar. 31, 2019	[6]	Jun. 30, 2020		Jun. 30, 2019
Net Income Per Share [Line Items]
Potentially dilutive units					1				1		1
Net income attributable to MPLX LP		$ 648,000,000	[1]		$ 482,000,000	[2],[3]			$ (2,076,000,000)	[4]	$ 985,000,000	[2],[5]
Less: Distribution declared		746,000,000			734,000,000				1,505,000,000		1,277,000,000
Undistributed net income (loss) attributable to MPLX LP		(98,000,000)			(252,000,000)				(3,581,000,000)		(292,000,000)
Undistributed net loss attributable to MPLX LP		(98,000,000)			(252,000,000)				(3,581,000,000)		(292,000,000)
Capital Contribution By Related Party		6,000,000		$ 225,000,000	13,000,000		$ 15,000,000
Limited Partners Common Units
Net Income Per Share [Line Items]
Net income attributable to MPLX LP		617,000,000	[1]		440,000,000	[3]			(2,138,000,000)	[4]	923,000,000	[5]
Less: Distribution declared	[7]	715,000,000			692,000,000	[8]			1,443,000,000		1,215,000,000	[8]
Undistributed net income (loss) attributable to MPLX LP		(98,000,000)			(252,000,000)				(3,581,000,000)		(292,000,000)
MPC | Limited Partners Common Units
Net Income Per Share [Line Items]
Capital Contribution By Related Party		6,000,000		$ 225,000,000	0	[6]	$ 0
MPC | ANDX LP [Member]
Net Income Per Share [Line Items]
Capital Contribution By Related Party					12,500,000						12,500,000
Series A Preferred Stock [Member] | Series A Preferred Stock [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP		21,000,000	[1]		21,000,000	[3]			41,000,000	[4]	41,000,000	[5]
Less: Distribution declared	[7]	21,000,000			21,000,000				41,000,000		41,000,000
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]		21,000,000	[3]			21,000,000	[4]	21,000,000	[5]
Less: Distribution declared	[7]	10,000,000			21,000,000				21,000,000		21,000,000
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] 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] Financial information for the first and second quarters of 2019 has been retrospectively adjusted for the acquisition of ANDX.

[7] See Note 7 for distribution information.

[8] 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.