Equity - Changes in Partners Capital, Unit Rollforward (Details) - USD ($)							3 Months Ended							9 Months Ended
		Oct. 27, 2020	Aug. 17, 2020	Jul. 31, 2020	Jul. 30, 2019	Jul. 29, 2019	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019		Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 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							$ (654,000,000)	$ (634,000,000)	$ 2,736,000,000	$ (669,000,000)		$ (636,000,000)	$ (669,000,000)
Partners, Total Subsequent Distribution Amount							745,000,000			734,000,000				$ 2,250,000,000	$ 2,011,000,000
Contributions from:							13,000,000	6,000,000	225,000,000	295,000,000		13,000,000	15,000,000
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders							(736,000,000)	(728,000,000)	(738,000,000)	(715,000,000)		(764,000,000)	(776,000,000)
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							13,095,000,000	13,262,000,000	13,356,000,000	17,892,000,000		17,611,000,000	17,730,000,000	13,095,000,000	17,892,000,000		$ 16,613,000,000	$ 17,731,000,000
Distribution Made to Limited Partner, Cash Distributions Declared							745,000,000			734,000,000				2,250,000,000	2,011,000,000
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)	(10,000,000)	(11,000,000)	(7,000,000)		0	0	(31,000,000)
Contributions from:							0	0	0	0		0	0
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders							(20,000,000)	0	(21,000,000)	(21,000,000)		0	0	(41,000,000)
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							$ 601,000,000	611,000,000	$ 601,000,000	601,000,000		0	0	$ 601,000,000	601,000,000		$ 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														395,091
Balance at September 30, 2020							1,040,168,474							1,040,168,474
Distribution Made to Limited Partner, Cash Distributions Declared	[1]						$ 715,000,000			704,000,000	[2]			$ 2,158,000,000	1,919,000,000	[2]
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
Limited Partners Common Units
Stockholders Equity [Line Items]
Partners, Total Subsequent Distribution Amount							715,000,000			704,000,000				2,158,000,000	1,919,000,000
Series B Preferred Stock [Member] | Preferred Partner [Member]
Stockholders Equity [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[1]						10,000,000			10,000,000				$ 31,000,000	31,000,000
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]
Partners, Total Subsequent Distribution Amount							10,000,000			10,000,000				$ 31,000,000	31,000,000
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders			$ (21,000,000)
Preferred Class A [Member]
Stockholders Equity [Line Items]
Partners, Total Subsequent Distribution Amount							20,000,000			20,000,000				$ 61,000,000	61,000,000
MPC | Limited Partners Common Units
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity							$ 399,000,000	388,000,000	$ (1,733,000,000)	(380,000,000)		293,000,000	307,000,000
Units issued					161,000,000		647,000,000							647,000,000			666,000,000
Contributions from:							$ 13,000,000	6,000,000	225,000,000	292,000,000		0	0
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders							(445,000,000)	(458,000,000)	(446,000,000)	(432,000,000)		(332,000,000)	(327,000,000)
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							$ 2,827,000,000	2,951,000,000	3,014,000,000	5,767,000,000		(1,671,000,000)	(1,632,000,000)	$ 2,827,000,000	5,767,000,000		$ 4,968,000,000	(1,612,000,000)
Units Outstanding							647,000,000							647,000,000			666,000,000
MPC | ANDX LP [Member]
Stockholders Equity [Line Items]
Contributions from:										12,500,000					25,000,000
Public | Limited Partners Common Units
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity							$ 236,000,000	229,000,000	(1,022,000,000)	(222,000,000)		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		0	0
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders							(271,000,000)	(270,000,000)	(271,000,000)	(262,000,000)		(191,000,000)	(188,000,000)
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							$ 9,436,000,000	$ 9,469,000,000	$ 9,509,000,000	$ 11,289,000,000		$ 8,305,000,000	$ 8,326,000,000	$ 9,436,000,000	$ 11,289,000,000		$ 10,800,000,000	$ 8,336,000,000
Units Outstanding							393,000,000							393,000,000			392,000,000
Western Refining Wholesale & Western Refining Product Transport [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Contributed				(18,582,088)										(18,582,088)
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							$ 5,000,000

[1] See Note 7 for distribution information.

[2] The three and nine months ended September 30, 2019 amounts are net of $12.5 million and $25 million, respectively, of waived distributions with respect to units held by MPC and its affiliates.