Acquisitions Andeavor Logistics (Details) - USD ($) $ in Millions			3 Months Ended
		Jul. 30, 2019	Mar. 31, 2020		Mar. 31, 2019		Dec. 31, 2019	Sep. 09, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Cash and cash equivalents			$ 57				$ 15
Receivables, net			522				593
Inventories			105				110
Other current assets			45				110
Assets, Current			1,329				1,484
Equity method investments			3,992				5,275
Property, plant and equipment, net			21,829				22,145
Intangibles, net			1,055				1,270
Other noncurrent assets			50				52
Assets			37,006				40,430
Accounts payable			138				242
Accrued Liabilities, Current			135				187
Accrued interest payable			214				210
Other current liabilities			129				136
Liabilities, Current			1,214				2,132
Long-term deferred revenue			241				217
Liabilities, Related Parties, Noncurrent			290				290
Long-term debt			20,467				19,704
Deferred credits and other liabilities			175				192
Liabilities			22,682				22,849
Series A preferred units			968				968
Total MPLX LP partners’ capital			13,108				16,364
Accumulated other comprehensive loss	[1]		(16)		$ (15)		(15)		$ (16)
Revenues			992		2,235	[2],[3]
Goodwill			7,722				9,536		10,016
Net Income (Loss) Attributable to Parent			(2,724)	[4]	503	[2],[5]
Acquisition Costs, Period Cost			0		1	[6]
Noncontrolling interests			248				249
Partners' Capital, Including Portion Attributable to Noncontrolling Interest			13,356		17,730	[7]	16,613		17,731
Liabilities and Equity			37,006				40,430
Net income/(loss)			(2,716)		689	[2],[6],[8],[9]
Andeavor Logistics [Member]
Business Acquisition [Line Items]
Revenues					589
Acquisition Costs, Period Cost					1
Net income/(loss)					$ 180
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			229				238
Goodwill			$ 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			$ 826				1,032
Goodwill			$ 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 first quarter of 2019 has been retrospectively adjusted for the acquisition of ANDX. See Notes 1 and 3.

[3] Financial information for the first quarter of 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] Financial information for the first quarter of 2019 has been retrospectively adjusted for the acquisition of ANDX. See Notes 1 and 3.

[7] Financial information for the first quarter of 2019 has been retrospectively adjusted for the acquisition of ANDX. See Notes 1 and 3.

[8] Financial information for the first quarter of 2019 has been retrospectively adjusted for the acquisition of ANDX. See Notes 1 and 3.

[9] Financial information for the first quarter of 2019 has been retrospectively adjusted for the acquisition of ANDX. See Notes 1 and 3.