Equity - Changes in Partners Capital, Unit Rollforward (Details) - USD ($) $ / shares in Units, $ in Millions					3 Months Ended			12 Months Ended
		Apr. 28, 2020	Jul. 30, 2019	Jul. 29, 2019	Mar. 31, 2020	Mar. 31, 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					$ 2,736	$ (669)	[1]
Partners, Total Subsequent Distribution Amount					759	543
Contributions from:					225	15	[1]
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders					(738)	(776)	[1]
Partners' Capital, Including Portion Attributable to Noncontrolling Interest					13,356	17,730	[1]	$ 16,613	$ 17,731
Distribution Made to Limited Partner, Cash Distributions Declared					759	543
Preferred Class B [Member]
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity					(11)	0	[1]
Contributions from:					0	0	[1]
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders					(21)	0	[1]
Partners' Capital, Including Portion Attributable to Noncontrolling Interest					$ 601	0	[1]	$ 611	0
Limited Partners Common Units
Stockholders Equity [Line Items]
Balance at December 31, 2019					1,058,355,471
Unit-based compensation awards					151,878
Balance at March 31, 2020					1,058,507,349			1,058,355,471
Distribution Made to Limited Partner, Cash Distributions Declared	[2]				$ 728	523
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		$ 728
Series B Preferred Stock [Member] | Preferred Partner [Member]
Stockholders Equity [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[2]				$ 11	0
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					$ 11	0
Preferred Class B [Member] | Preferred Partner [Member]
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity					(11)
Partners' Capital, Including Portion Attributable to Noncontrolling Interest					601			$ 611
MPC | Limited Partners Common Units
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity					$ (1,733)	307	[1]
Units issued			161,000,000		666,000,000			666,000,000
Contributions from:					$ 225	0	[1]
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders					(446)	(327)	[1]
Partners' Capital, Including Portion Attributable to Noncontrolling Interest					$ 3,014	(1,632)	[1]	$ 4,968	(1,612)
Units Outstanding					666,000,000			666,000,000
Public | Limited Partners Common Units
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity					$ (1,022)	176	[1]
Units issued			102,000,000		393,000,000			392,000,000
Contributions from:					$ 0	0	[1]
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders					(271)	(188)	[1]
Partners' Capital, Including Portion Attributable to Noncontrolling Interest					$ 9,509	$ 8,326	[1]	$ 10,800	$ 8,336
Units Outstanding					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					$ 0			$ 1

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

[2] See Note 7 for distribution information.