Equity - Cash Distributions (Details) - USD ($) $ / shares in Units, shares in Millions							3 Months Ended							9 Months Ended
		Nov. 13, 2020	Nov. 06, 2020	Oct. 27, 2020	Aug. 17, 2020	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	Jul. 30, 2019	Dec. 31, 2018
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Contributions from:							$ 13,000,000	$ 6,000,000	$ 225,000,000	$ 295,000,000		$ 13,000,000	$ 15,000,000
Distribution Made to Limited Partner, Cash Distributions Declared							745,000,000			734,000,000				$ 2,250,000,000	$ 2,011,000,000
Partners' Capital Account, Distributions							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
Preferred Class B [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution date					Aug. 17, 2020
Partners' Capital Account, Distributions					$ 21,000,000
Subsequent Event
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Declaration Date				Oct. 27, 2020
Cash distributions declared per limited partner common unit				$ 0.6875
Date of record			Nov. 06, 2020
Subsequent Event | Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution date		Nov. 13, 2020
Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
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]
Cash distributions declared per limited partner common unit							$ 0.6875	$ 0.6875	$ 0.6875	$ 0.6775		$ 0.6675	$ 0.6575
Limited Partners Common Units | Subsequent Event
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared				$ 715,000,000
Series A Preferred Stock [Member] | Series B Preferred Stock [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[1]						$ 10,000,000			$ 10,000,000				31,000,000	31,000,000
Series A Preferred Stock [Member] | Preferred Class B [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Cash distributions declared per limited partner common unit							$ 68.75
Preferred Class B [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Contributions from:							$ 0	$ 0	$ 0	0		$ 0	$ 0
Partners' Capital Account, Distributions							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
MPC | Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Limited Partners' Capital Account, Units Issued							647							647			666	161
Contributions from:							$ 13,000,000	6,000,000	225,000,000	292,000,000		0	0
Partners' Capital Account, Distributions							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)
MPC | ANDX LP [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Contributions from:										12,500,000					25,000,000
ANDX LP [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Preferred Stock, Dividend Rate, Percentage						6.875%
MPLX LP | Series B Preferred Stock [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Preferred Stock, Dividend Payment Rate, Variable														LIBOR plus
Preferred Stock, Dividend Rate, Percentage														4.652%
TexNew Mex units [Member] | MPLX LP
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared							$ 5,000,000
Public | Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Limited Partners' Capital Account, Units Issued							393							393			392	102
Contributions from:							$ 0	0	0	0		0	0
Partners' Capital Account, Distributions							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

[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.