Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Net Income Per Share [Line Items]
Net Income (Loss) Attributable to Parent	[1]	$ 875	$ 706	$ 1,700	$ 1,445
Distribution Made to Limited Partner, Cash Distributions Declared		745	736	1,490	1,474
Undistributed net income (loss) attributable to MPLX LP		130	(30)	210	(29)
Limited Partners Common Units [Member]
Net Income Per Share [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared		714	705	1,427	1,412
Undistributed net income (loss) attributable to MPLX LP		126	(30)	204	(29)
Series A Preferred Stock [Member] | Series A Preferred Stock [Member]
Net Income Per Share [Line Items]
Net Income (Loss) Attributable to Parent	[1]	25	21	48	41
Distribution Made to Limited Partner, Cash Distributions Declared		21	21	42	41
Undistributed net income (loss) attributable to MPLX LP		4	0	6	0
Series B Preferred Stock [Member] | Series A Preferred Stock [Member]
Net Income Per Share [Line Items]
Net Income (Loss) Attributable to Parent	[1]	10	10	21	21
Distribution Made to Limited Partner, Cash Distributions Declared		10	10	21	21
Undistributed net income (loss) attributable to MPLX LP		0	0	0	0
Common Stock | Limited Partners Common Units [Member]
Net Income Per Share [Line Items]
Net Income (Loss) Attributable to Parent	[1]	$ 840	$ 675	$ 1,631	$ 1,383

[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.