Equity - Changes in Partners Capital, Unit Rollforward (Details) - USD ($)						3 Months Ended						6 Months Ended			12 Months Ended
		Aug. 17, 2020	Jul. 28, 2020	Jul. 30, 2019	Jul. 29, 2019	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019		Mar. 31, 2019	[1]	Jun. 30, 2020	Jun. 30, 2019		Dec. 31, 2019	Dec. 31, 2018
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity						$ (634,000,000)	$ 2,736,000,000	$ (636,000,000)		$ (669,000,000)
Partners, Total Subsequent Distribution Amount						746,000,000		734,000,000				$ 1,505,000,000	$ 1,277,000,000
Contributions from:						6,000,000	225,000,000	13,000,000		15,000,000
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders						(728,000,000)	(738,000,000)	(764,000,000)		(776,000,000)
Partners' Capital, Including Portion Attributable to Noncontrolling Interest						13,262,000,000	13,356,000,000	17,611,000,000		17,730,000,000		13,262,000,000	17,611,000,000		$ 16,613,000,000	$ 17,731,000,000
Distribution Made to Limited Partner, Cash Distributions Declared						746,000,000		734,000,000				1,505,000,000	1,277,000,000
Preferred Class B [Member]
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity						(10,000,000)	(11,000,000)	0	[1]	0
Contributions from:						0	0	0	[1]	0
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders						0	(21,000,000)	0	[1]	0		(21,000,000)
Partners' Capital, Including Portion Attributable to Noncontrolling Interest						$ 611,000,000	$ 601,000,000	0		0		$ 611,000,000	0		$ 611,000,000	0
Limited Partners Common Units
Stockholders Equity [Line Items]
Balance at December 31, 2019							1,058,355,471					1,058,355,471
Unit-based compensation awards												253,291
Balance at June 30, 2020						1,058,608,762						1,058,608,762			1,058,355,471
Distribution Made to Limited Partner, Cash Distributions Declared	[2]					$ 715,000,000		692,000,000	[3]			$ 1,443,000,000	1,215,000,000	[3]
Andeavor Logistics [Member]
Stockholders Equity [Line Items]
Preferred Units, Outstanding					600,000
ANDX LP [Member]
Stockholders Equity [Line Items]
Dividend rate, percentage					6.875%
Price per share					$ 1,000
Subsequent Event | Limited Partners Common Units
Stockholders Equity [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared			$ 715,000,000
Series B Preferred Stock [Member] | Preferred Partner [Member]
Stockholders Equity [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[2]					10,000,000		21,000,000				$ 21,000,000	21,000,000
Series B Preferred Stock [Member] | MPLX LP
Stockholders Equity [Line Items]
Dividend rate, percentage												4.652%
Preferred Stock, Dividend Payment Rate, Variable												LIBOR plus
Preferred Class B [Member]
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity												$ (21,000,000)
Partners, Total Subsequent Distribution Amount						10,000,000		21,000,000				$ 21,000,000	21,000,000
Preferred Class B [Member] | Subsequent Event
Stockholders Equity [Line Items]
Partners, Total Subsequent Distribution Amount		$ 21,000,000
MPC | Limited Partners Common Units
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity						$ 388,000,000	$ (1,733,000,000)	293,000,000		307,000,000
Units issued				161,000,000		666,000,000						666,000,000			666,000,000
Contributions from:						$ 6,000,000	225,000,000	0	[1]	0
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders						(458,000,000)	(446,000,000)	(332,000,000)		(327,000,000)
Partners' Capital, Including Portion Attributable to Noncontrolling Interest						$ 2,951,000,000	3,014,000,000	(1,671,000,000)		(1,632,000,000)		$ 2,951,000,000	(1,671,000,000)		$ 4,968,000,000	(1,612,000,000)
Units Outstanding						666,000,000						666,000,000			666,000,000
MPC | ANDX LP [Member]
Stockholders Equity [Line Items]
Contributions from:								12,500,000					12,500,000
Public | Limited Partners Common Units
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity						$ 229,000,000	(1,022,000,000)	168,000,000		176,000,000
Units issued				102,000,000		393,000,000						393,000,000			392,000,000
Contributions from:						$ 0	0	0		0
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders						(270,000,000)	(271,000,000)	(191,000,000)		(188,000,000)
Partners' Capital, Including Portion Attributable to Noncontrolling Interest						$ 9,469,000,000	$ 9,509,000,000	$ 8,305,000,000		$ 8,326,000,000		$ 9,469,000,000	$ 8,305,000,000		$ 10,800,000,000	$ 8,336,000,000
Units Outstanding						393,000,000						393,000,000			392,000,000
Nonpublic [Member] | ANDX LP [Member]
Stockholders Equity [Line Items]
Common Units Conversion Ratio - ANDX to MPLX				1.0328
Public | ANDX LP [Member]
Stockholders Equity [Line Items]
Common Units Conversion Ratio - ANDX to MPLX				1.135
TexNew Mex units [Member] | MPLX LP
Stockholders Equity [Line Items]
Units Outstanding				80,000
Distribution Made to Limited Partner, Cash Distributions Declared												$ 2,000,000			$ 1,000,000

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

[2] See Note 7 for distribution information.

[3] The three and six months ended June 30, 2019 amounts are net of $12.5 million of waived distributions with respect to units held by MPC and its affiliates.