Acquisitions Andeavor Logistics (Details) - USD ($) $ in Millions			3 Months Ended				6 Months Ended
		Jul. 30, 2019	Jun. 30, 2020		Jun. 30, 2019		Jun. 30, 2020		Jun. 30, 2019		Mar. 31, 2020	Dec. 31, 2019	Sep. 09, 2019	Mar. 31, 2019	[9]	Dec. 31, 2018
Business Acquisition [Line Items]
Cash and cash equivalents			$ 67				$ 67					$ 15
Receivables, net			562				562					593
Inventories			115				115					110
Other current assets			48				48					110
Assets, Current			1,386				1,386					1,484
Equity method investments			4,065				4,065					5,275
Property, plant and equipment, net			21,758				21,758					22,145
Intangibles, net			1,023				1,023					1,270
Other noncurrent assets			51				51					52
Assets			37,022				37,022					40,430
Accounts payable			145				145					242
Accrued Liabilities, Current			138				138					187
Accrued interest payable			207				207					210
Other current liabilities			143				143					136
Liabilities, Current			1,228				1,228					2,132
Long-term deferred revenue			261				261					217
Liabilities, Related Parties, Noncurrent			287				287					290
Long-term debt			20,556				20,556					19,704
Deferred credits and other liabilities			175				175					192
Liabilities			22,792				22,792					22,849
Series A preferred units			968				968					968
Total MPLX LP partners’ capital			13,015				13,015					16,364
Accumulated other comprehensive loss	[1]		(16)		$ (15)		(16)		$ (15)			(15)				$ (16)
Revenues			2,081		2,210	[2],[3]	3,073		4,445	[2],[3]
Goodwill			7,722				7,722				$ 7,700	9,536				10,016
Net Income (Loss) Attributable to Parent			648	[4]	482	[2],[5]	(2,076)	[6]	985	[2],[7]
Acquisition Costs, Period Cost			0		4	[8]	0		5
Noncontrolling interests			247				247					249
Partners' Capital, Including Portion Attributable to Noncontrolling Interest			13,262		17,611		13,262		17,611		$ 13,356	16,613		$ 17,730		17,731
Liabilities and Equity			37,022				37,022					40,430
Net (loss)/income			655		657	[2],[8],[10]	(2,061)		1,346	[2],[10],[11]
Andeavor Logistics [Member]
Business Acquisition [Line Items]
Revenues					588				1,177
Acquisition Costs, Period Cost									5
Net (loss)/income					$ 168				$ 348
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			220				220					238
Goodwill			7,722				$ 7,722					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			803				$ 803					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] Financial information for the three and six months ended June 30, 2019 has been retrospectively adjusted for the acquisition of ANDX. See Notes 1 and 3.

[3] Financial information for the three and six months ended June 30, 2019 has been retrospectively adjusted for the acquisition of ANDX. See Notes 1 and 3.

[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.

[6] 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.

[7] 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.

[8] Financial information for the three and six months ended June 30, 2019 has been retrospectively adjusted for the acquisition of ANDX. See Notes 1 and 3.

[9] Financial information for the first and second quarters of 2019 has been retrospectively adjusted for the acquisition of ANDX.

[10] Financial information for the three and six months ended June 30, 2019 has been retrospectively adjusted for the acquisition of ANDX. See Notes 1 and 3.

[11] Financial information for the six months ended June 30, 2019 has been retrospectively adjusted for the acquisition of ANDX. See Notes 1 and 3.