Equity (Reorganization Transactions) (Details) $ / shares in Units, $ in Millions							2 Months Ended	3 Months Ended								12 Months Ended
		Jul. 29, 2019 $ / shares shares	Feb. 01, 2018 USD ($) shares	Jul. 01, 2017 Agreement $ / shares shares	Mar. 01, 2017 USD ($)	Jul. 01, 2016 Agreement $ / shares shares	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Mar. 31, 2019 $ / shares	Dec. 31, 2018 USD ($) $ / shares	Sep. 30, 2018 $ / shares	Jun. 30, 2018 $ / shares	Mar. 31, 2018 $ / shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Jul. 30, 2019 USD ($) shares	Dec. 31, 2016 USD ($)
Class of Stock [Line Items]
Contribution from MPC																$ 346.0	$ 11,980.0	$ 675.0
Partners' Capital, Including Portion Attributable to Noncontrolling Interest								$ 16,613.0				$ 17,731.0				$ 16,613.0	$ 17,731.0	$ 9,973.0		$ 11,110.0
Partners' Capital Account, Units, Converted | shares																1,148,330	264,413,309	366,509
Partners' Capital Account, Exchanges and Conversions																$ (36.0)	$ 0.0	$ 0.0
Net income (loss)																(1,381.0)	(1,931.0)	(771.0)
(Placeholder)																$ (30.0)	$ (17.0)	$ (7.0)
MPC
Class of Stock [Line Items]
Contribution from MPC					$ 6.0
Limited Partners Common Units
Class of Stock [Line Items]
Cash distributions declared per limited partner common unit | $ / shares								$ 0.6875	$ 0.6775	$ 0.6675	$ 0.6575	$ 0.6475	$ 0.6375	$ 0.6275	$ 0.6175	$ 2.6900	$ 2.5300
Partners' Capital Account, Units, Converted | shares			275,000,000													1,148,330	275,000,000	4,350,057
Partners' Capital Account, Exchanges and Conversions			$ 10,400.0
Limited Partners Common Units | MPC
Class of Stock [Line Items]
Contribution from MPC																$ 315.0	$ 0.0	$ 0.0
Partners' Capital, Including Portion Attributable to Noncontrolling Interest								$ 4,968.0				$ (1,612.0)				4,968.0	(1,612.0)	2,099.0		$ 1,069.0
Partners' Capital Account, Exchanges and Conversions																0.0	7,926.0	0.0
Net income (loss)																(595.0)	(1,076.0)	(110.0)
(Placeholder)																$ 0.0	$ 0.0	$ 0.0
Class B Units
Class of Stock [Line Items]
Partners' Capital Account, Units, Converted | shares																		3,990,878
General Partner Units
Class of Stock [Line Items]
Partners' Capital Account, Units, Converted | shares	[1]															0	10,586,691	7,330
MarkWest
Class of Stock [Line Items]
Common Units Conversion Ratio | shares				1.09		1.09
Cash consideration to unitholders | $ / shares				$ 6.20		$ 6.20
ANDX LP [Member] | MPC
Class of Stock [Line Items]
Contribution from MPC								$ 12.5	$ 12.5							$ 37.5
Andeavor Logistics [Member]
Class of Stock [Line Items]
Preferred Units, Outstanding | shares		600,000																	600,000
ANDX LP [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage		6.875%
Units issued, price per unit | $ / shares		$ 1,000
Maximum | M&R MWE Liberty LLC [Member]
Class of Stock [Line Items]
Number of Underwritten Offerings Allowable to be Demanded | Agreement				3		3
Number of Underwritten Offerings Allowable to be Demanded in 12 Month Period | Agreement				1		1
Maximum | M&R MWE Liberty LLC [Member] | Limited Partners Common Units
Class of Stock [Line Items]
Percentage of Limited Partners' Capital Account, Units Outstanding				5.00%		5.00%
Maximum | M&R MWE Liberty LLC [Member] | Class B Units
Class of Stock [Line Items]
Percentage of Units Sold in Public Offering				20.00%		20.00%
Series B Preferred Stock [Member] | MPLX LP [Member]
Class of Stock [Line Items]
Preferred Units, Outstanding | shares								600,000								600,000			600,000
Preferred Stock, Dividend Rate, Percentage																4.652%
Preferred Class B [Member] | Preferred Partner [Member]
Class of Stock [Line Items]
Partners' Capital, Including Portion Attributable to Noncontrolling Interest								$ 611.0								$ 611.0			$ 615.0
Cash distributions declared per limited partner common unit | $ / shares								$ 68.75
Net income (loss)							$ 17.0
(Placeholder)							$ (21.0)	$ 21.0	$ 21.0	$ 21.0

[1] Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisitions of HSM, HST, WHC, MPLXT, the Joint-Interest Acquisition and Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two percent GP Interest.