Equity (Net Income Allocation) (Details) - USD ($) $ 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
Equity [Abstract]
Net income attributable to MPLX LP	$ (581)	$ 629	$ 482		$ 503		$ 434		$ 510	$ 453	$ 421	$ 1,033	[2]	$ 1,818	[2]	$ 794	[2]
Distributions to preferred unitholders																(65)
General partner's IDRs and other																310
Net income attributable to MPLX LP available to general and limited partners																419
General partner's two percent GP Interest in net income attributable to MPLX LP																8
General partner's GP Interest in net income attributable to MPLX LP												$ 0		$ 0		$ 318

[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.