Acquisitions Andeavor Logistics (Details) - USD ($) $ in Millions			3 Months Ended				9 Months Ended
		Jul. 30, 2019	Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019		Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 09, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Cash and cash equivalents			$ 28				$ 28						$ 15
Receivables, net			477				477						593
Inventories			117				117						110
Other current assets			59				59						110
Assets, Current			1,272				1,272						1,484
Equity method investments			4,081				4,081						5,275
Property, plant and equipment, net			21,615				21,615						22,145
Intangibles, net			991				991						1,270
Other noncurrent assets			48				48						52
Assets			36,662				36,662						40,430
Accounts payable			140				140						242
Accrued Liabilities, Current			169				169						187
Accrued interest payable			207				207						210
Other current liabilities			158				158						127
Liabilities, Current			1,531				1,531						2,132
Long-term deferred revenue			291				291						217
Liabilities, Related Parties, Noncurrent			281				281						290
Long-term debt			20,042				20,042						19,704
Deferred credits and other liabilities			184				184						192
Liabilities			22,599				22,599						22,849
Series A preferred units			968				968						968
Total MPLX LP partners’ capital			12,848				12,848						16,364
Accumulated other comprehensive loss	[1]		(16)		$ (15)		(16)		$ (15)				(15)				$ (16)
Revenues			2,247		2,280		5,320		6,725
Goodwill			7,657				7,657					$ 7,700	9,536				10,016
Net Income (Loss) Attributable to Parent			665	[2]	629	[3]	(1,411)	[4]	1,614	[5]
Acquisition Costs, Period Cost			0		9		0		14
Noncontrolling interests			247				247						249
Partners' Capital, Including Portion Attributable to Noncontrolling Interest			13,095		17,892		13,095		17,892		$ 13,262	$ 13,356	16,613		$ 17,611	$ 17,730	17,731
Liabilities and Equity			36,662				36,662						40,430
Net (loss)/income			674		689		(1,387)		2,035
Andeavor Logistics [Member]
Business Acquisition [Line Items]
Revenues					612				1,789
Acquisition Costs, Period Cost									14
Net (loss)/income					$ 191				$ 539
ANDX LP [Member] | Public
Business Acquisition [Line Items]
Common Units Conversion Ratio - ANDX to MPLX		1.135
ANDX LP [Member] | Nonpublic [Member]
Business Acquisition [Line Items]
Common Units Conversion Ratio - ANDX to MPLX		1.0328
Senior Notes [Member]
Business Acquisition [Line Items]
Long-term Debt, Gross														$ 2,000
L&S
Business Acquisition [Line Items]
Intangibles, net			211				211						238
Goodwill			7,657				$ 7,657						7,722				7,234
L&S | Minimum [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life							6 years
L&S | Maximum [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life							8 years
G&P
Business Acquisition [Line Items]
Intangibles, net			780				$ 780						1,032
Goodwill			$ 0				$ 0						$ 1,814				$ 2,782
G&P | Minimum [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life							6 years
G&P | Maximum [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life							25 years

[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] 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.

[3] 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.

[4] 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.

[5] 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.