Equity - Cash Distributions (Details) - USD ($) $ / shares in Units, shares in Millions							3 Months Ended						6 Months Ended			12 Months Ended
		Aug. 17, 2020	Aug. 14, 2020	Aug. 07, 2020	Jul. 28, 2020	Jul. 29, 2019	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019		Mar. 31, 2019		Jun. 30, 2020	Jun. 30, 2019		Dec. 31, 2019	Jul. 30, 2019	Dec. 31, 2018
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Contributions from:							$ 6,000,000	$ 225,000,000	$ 13,000,000		$ 15,000,000	[1]
Distribution Made to Limited Partner, Cash Distributions Declared							746,000,000		734,000,000				$ 1,505,000,000	$ 1,277,000,000
Partners, Total Subsequent Distribution Amount							746,000,000		734,000,000				1,505,000,000	1,277,000,000
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders							8,000,000	9,000,000	6,000,000		6,000,000	[1]
Partners' Capital Account, Distributions							728,000,000	738,000,000	764,000,000		776,000,000	[1]
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							13,262,000,000	$ 13,356,000,000	17,611,000,000		$ 17,730,000,000	[1]	13,262,000,000	17,611,000,000		$ 16,613,000,000		$ 17,731,000,000
Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners, Total Subsequent Distribution Amount							715,000,000		692,000,000				1,443,000,000	1,215,000,000
Preferred Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners, Total Subsequent Distribution Amount							21,000,000		21,000,000				41,000,000	41,000,000
Preferred Class B [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners, Total Subsequent Distribution Amount							10,000,000		21,000,000				21,000,000	21,000,000
Subsequent Event
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Declaration Date					Jul. 28, 2020
Cash distributions declared per limited partner common unit					$ 0.6875
Date of record				Aug. 07, 2020
Subsequent Event | Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution date			Aug. 14, 2020
Subsequent Event | Preferred Class B [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution date		Aug. 17, 2020
Partners, Total Subsequent Distribution Amount		$ 21,000,000
Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
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]
Cash distributions declared per limited partner common unit							$ 0.6875	$ 0.6875	$ 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	[2]						$ 10,000,000		$ 21,000,000				21,000,000	21,000,000
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	[1]	$ 0	[1]
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders							0	0	0	[1]	0	[1]
Partners' Capital Account, Distributions							0	21,000,000	0	[1]	0	[1]	21,000,000
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							$ 611,000,000	601,000,000	0		0	[1]	$ 611,000,000	0		$ 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							666						666			666	161
Contributions from:							$ 6,000,000	225,000,000	0	[1]	0	[1]
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders							0	0	0		0	[1]
Partners' Capital Account, Distributions							458,000,000	446,000,000	332,000,000		327,000,000	[1]
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							$ 2,951,000,000	3,014,000,000	(1,671,000,000)		(1,632,000,000)	[1]	$ 2,951,000,000	(1,671,000,000)		$ 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					12,500,000
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													$ 2,000,000			$ 1,000,000
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	[1]
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders							0	0	0		0	[1]
Partners' Capital Account, Distributions							270,000,000	271,000,000	191,000,000		188,000,000	[1]
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							$ 9,469,000,000	$ 9,509,000,000	$ 8,305,000,000		$ 8,326,000,000	[1]	$ 9,469,000,000	$ 8,305,000,000		$ 10,800,000,000		$ 8,336,000,000

[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.