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
		Mar. 31, 2020		Mar. 31, 2019
Net income (loss) attributable to MPLX LP:
Undistributed net income (loss) attributable to MPLX LP		$ (3,483)		$ (40)
Net income attributable to MPLX LP		(2,724)	[1]	503	[2],[3]
Limited Partners Common Units
Net income (loss) attributable to MPLX LP:
Undistributed net income (loss) attributable to MPLX LP		(3,483)		(40)
Net income attributable to MPLX LP		$ (2,755)	[1]	$ 483	[3]
Weighted average units outstanding:
Common - basic (in shares)		1,058		794	[2]
Common - diluted (in shares)		1,058		795	[2]
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)		$ (2.60)		$ 0.61	[2]
Diluted (in USD per unit)		$ (2.60)		$ 0.61	[2]
Series A Preferred Stock [Member] | Series A Preferred Stock [Member]
Net income (loss) attributable to MPLX LP:
Undistributed net income (loss) attributable to MPLX LP		$ 0		$ 0
Net income attributable to MPLX LP		20	[1]	$ 20	[3]
Series B Preferred Stock [Member] | Series A Preferred Stock [Member]
Net income (loss) attributable to MPLX LP:
Undistributed net income (loss) attributable to MPLX LP		0
Net income attributable to MPLX LP	[1]	$ 11

[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.