Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Detail) - USD ($) shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2020		Mar. 31, 2019
Net Income Per Share [Line Items]
Potentially dilutive units		1		1
Net income attributable to MPLX LP		$ (2,724)	[1]	$ 503	[2],[3]
Less: Distribution declared		759		543
Undistributed net income (loss) attributable to MPLX LP		(3,483)		(40)
Undistributed net loss attributable to MPLX LP		(3,483)		(40)
Contributions from:		225		15	[4]
Limited Partners Common Units
Net Income Per Share [Line Items]
Net income attributable to MPLX LP		(2,755)	[1]	483	[3]
Less: Distribution declared	[5]	728		523
Undistributed net income (loss) attributable to MPLX LP		(3,483)		(40)
MPC | Limited Partners Common Units
Net Income Per Share [Line Items]
Contributions from:		225		0	[4]
Series A Preferred Stock [Member] | Series A Preferred Stock [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP		20	[1]	20	[3]
Less: Distribution declared	[5]	20		20
Undistributed net income (loss) attributable to MPLX LP		0		0
Series B Preferred Stock [Member] | Series A Preferred Stock [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	11
Less: Distribution declared	[5]	11		$ 0
Undistributed net income (loss) attributable to MPLX LP		$ 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 first quarter of 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] Financial information for the first quarter of 2019 has been retrospectively adjusted for the acquisition of ANDX. See Notes 1 and 3.

[5] See Note 7 for distribution information.