Net Income (Loss) Per Limited Partner Unit (Schedule of Distributions by Partner Class) (Detail) - USD ($) shares in Millions, $ in Millions		3 Months Ended											12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	[1]	Mar. 31, 2019	[1]	Dec. 31, 2018	[1]	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Net Income Per Share [Line Items]
Potentially dilutive securities													1		1		1
Net income attributable to MPLX LP		$ (581)	$ 629	$ 482		$ 503		$ 434		$ 510	$ 453	$ 421	$ 1,033	[2]	$ 1,818	[2]	$ 794	[2]
Total cash distributions declared													2,758		2,060		1,288
Undistributed net loss attributable to MPLX LP													(1,725)		(242)		(494)
Preferred Partner [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[2]												81		75		65
Total cash distributions declared	[3]																65
General Partner
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[2]																318
Total cash distributions declared	[3],[4]												0		0		328
Limited Partners Common Units
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[2]												910		1,743		411
Total cash distributions declared	[3]												2,635		1,985		895
Series A Preferred Stock [Member] | Preferred Partner [Member]
Net Income Per Share [Line Items]
Total cash distributions declared	[3]												81		75
Series B Preferred Stock [Member] | Preferred Partner [Member]
Net Income Per Share [Line Items]
Total cash distributions declared	[3]												$ 42	[2]	$ 0		$ 0

[1] As discussed in Note 1, MPLX’s acquisition of ANDX is considered a transfer between entities under common control due to MPC’s prior relationship with ANDX. Transfers of businesses between entities under common control require prior periods to be retrospectively adjusted for those dates that the entity was under common control. Accordingly, the tables above include the historical results of ANDX beginning October 1, 2018. Amounts shown for the fourth quarter of 2018 as well as the first and second quarters of 2019 are different than amounts previously reported for Total revenues and other income, Income from operations and Net income as a results of this retrospective adjustment for ANDX. Total revenues and other income originally reported for the fourth quarter of 2018 and the first and second quarters of 2019 was $1,715 million, $1,646 million and $1,629 million, respectively. Income from operations originally reported for the fourth quarter of 2018 and the first and second quarters of 2019 was $666 million, $678 million and $659 million, respectively. Net income originally reported for the fourth quarter of 2018 and the first and second quarters of 2019 was $439 million, $509 million and $488 million, respectively.

[2] Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.

[3] See Note 8 for distribution information.

[4] Distributions declared on January 25, 2018 on general partner common units issued on February 1, 2018 in exchange for the economic general partner interest, including IDRs, are shown as general partner distributions declared.