Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net (loss)/income	$ 674	$ 689	$ (1,387)	$ 2,035
Other comprehensive income/(loss), net of tax:
Remeasurements of pension and other postretirement benefits related to equity method investments, net of tax	0	0	(1)	1
Comprehensive income/(loss)	674	689	(1,388)	2,036
Less comprehensive income attributable to:
Noncontrolling interests	9	8	24	20
Comprehensive Income (loss), Net of Tax, Attributable to Predecessor	0	52	0	401
Comprehensive income/(loss) attributable to MPLX LP	$ 665	$ 629	$ (1,412)	$ 1,615