Equity - Cash Distributions (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended
		Apr. 28, 2020	Mar. 31, 2020	Mar. 31, 2019
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Contributions from:			$ 225	$ 15	[1]
Distribution Made to Limited Partner, Cash Distributions Declared			759	543
Partners, Total Subsequent Distribution Amount			759	543
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders			9	6	[1]
Partners' Capital Account, Distributions			738	776	[1]
Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners, Total Subsequent Distribution Amount			728	523	[2]
Preferred Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners, Total Subsequent Distribution Amount			20	20
Preferred Class B [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners, Total Subsequent Distribution Amount			11	0
Subsequent Event
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Declaration Date		Apr. 28, 2020
Cash distributions declared per limited partner common unit		$ 0.6875
Distribution date		May 15, 2020
Date of record		May 08, 2020
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	[3]		$ 11	0
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
Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[3]		$ 728	$ 523
Cash distributions declared per limited partner common unit			$ 0.6875	$ 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		$ 728
Preferred Class B [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Contributions from:			$ 0	$ 0	[1]
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders			0	0	[1]
Partners' Capital Account, Distributions			21	0	[1]
MPC | Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Contributions from:			225	0	[1]
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders			0	0	[1]
Partners' Capital Account, Distributions			$ 446	$ 327	[1]
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%

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

[2] The distribution on common units for the three months ended March 31, 2019 does not include the impact of the issuance of units in connection with the merger.

[3] See Note 7 for distribution information.