Net Income (Loss) Per Limited Partner Unit (Basic and Diluted Earnings Per Unit) (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended											12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Net income attributable to MPLX LP:
Total cash distributions declared													$ 2,758		$ 2,060		$ 1,288
Undistributed net income loss attributable to MPLX LP													(1,725)		(242)		(494)
Net income attributable to MPLX LP		$ (581)	$ 629	$ 482	[1]	$ 503	[1]	$ 434	[1]	$ 510	$ 453	$ 421	$ 1,033	[2]	$ 1,818	[2]	$ 794	[2]
Weighted average units outstanding:
Basic (shares)													906	[3]	761		393
Diluted (shares)													907	[3]	761		396
General Partner
Net income attributable to MPLX LP:
Total cash distributions declared	[4],[5]												$ 0		$ 0		$ 328
Undistributed net income loss attributable to MPLX LP																	(10)
Net income attributable to MPLX LP	[2]																$ 318
Weighted average units outstanding:
Basic (shares)																	8
Diluted (shares)																	8
Limited Partners Common Units
Net income attributable to MPLX LP:
Total cash distributions declared	[5]												2,635		1,985		$ 895
Undistributed net income loss attributable to MPLX LP													(1,725)		(242)		(484)
Net income attributable to MPLX LP	[2]												$ 910		$ 1,743		$ 411
Weighted average units outstanding:
Basic (shares)													906	[3]	761		385
Diluted (shares)													907	[3]	761		388
Net income attributable to MPLX LP subsequent to initial public offering per limited partner unit:
Basic (in USD per unit)		$ (0.58)	$ 0.61	$ 0.56		$ 0.61		$ 0.52		$ 0.62	$ 0.55	$ 0.61	$ 1.00		$ 2.29		$ 1.07
Diluted (in USD per unit)		$ (0.58)	$ 0.61	$ 0.55		$ 0.61		$ 0.52		$ 0.62	$ 0.55	$ 0.61	$ 1.00		$ 2.29		$ 1.06
Preferred Partner [Member]
Net income attributable to MPLX LP:
Total cash distributions declared	[5]																$ 65
Undistributed net income loss attributable to MPLX LP																	0
Net income attributable to MPLX LP	[2]												$ 81		$ 75		65
Series A Preferred Stock [Member] | Preferred Partner [Member]
Net income attributable to MPLX LP:
Total cash distributions declared	[5]												81		75
Undistributed net income loss attributable to MPLX LP													0		0
Series B Preferred Stock [Member] | Preferred Partner [Member]
Net income attributable to MPLX LP:
Total cash distributions declared	[5]												42	[2]	$ 0		$ 0
Undistributed net income loss attributable to MPLX LP													$ 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] The Series B preferred units and the MPLX common units issued in connection with the Merger were not outstanding during the entire year. See Notes 4 and 8 for additional information about the treatment of these units.

[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.

[5] See Note 8 for distribution information.