Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	[1]	$ (16)	$ (15)	$ (15)	$ (16)
Other Comprehensive Income (Loss), Net of Tax	[2]	(1)	1
Pension Plan [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	[1]	(14)	(14)	(14)	(14)
Other Comprehensive Income (Loss), Net of Tax	[2]	0	0
Other Postretirement Benefits Plan [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	[1]	(2)	(1)	$ (1)	$ (2)
Other Comprehensive Income (Loss), Net of Tax	[2]	$ (1)	$ 1

[1] These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”

[2] Components of other comprehensive income/loss - remeasurements relate to actuarial gains and losses as well as amortization of prior service costs. MPLX records an adjustment to “Comprehensive income” in accordance with its ownership interest in LOOP and Explorer.