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Equity (Reorganization Transactions) (Details) $ / shares in Units, $ in Millions						12 Months Ended
		Feb. 01, 2018 USD ($) shares	Jul. 01, 2017 Agreement $ / shares shares	Sep. 01, 2016 USD ($) shares	Jul. 01, 2016 Agreement $ / shares shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares
Class of Stock [Line Items]
Partners' Capital Account, Units, Converted							366,509	366,509
Partners' Capital Account, Exchanges and Conversions | $						$ 0	$ 0	$ 0
Contributions from MPC | $						$ 0	$ 0	$ 225
Units contributed								6,716,419
Limited Partners Common Units
Class of Stock [Line Items]
Partners' Capital Account, Units, Converted		275,000,000				275,000,000	4,350,057	4,350,057
Partners' Capital Account, Exchanges and Conversions | $		$ 10,400
Units contributed				21,401,137				7,153,177
Limited Partners Common Units | MPC
Class of Stock [Line Items]
Partners' Capital Account, Exchanges and Conversions | $						$ 7,926	$ 0	$ 0
Contributions from MPC | $				$ 84
Class B Units
Class of Stock [Line Items]
Partners' Capital Account, Units, Converted							(3,990,878)	(3,990,878)
General Partner Units
Class of Stock [Line Items]
Partners' Capital Account, Units, Converted	[1]					(10,586,691)	7,330	7,330
Units contributed				436,758				(436,758)	[1]
General Partner | MPC
Class of Stock [Line Items]
Partners' Capital Account, Exchanges and Conversions | $						$ (7,926)	$ 0	$ 0
Contributions from MPC | $				$ 141
MarkWest
Class of Stock [Line Items]
Common Units Conversion Ratio			1.09		1.09
Cash consideration to unitholders | $ / shares			$ 6.20		$ 6.20
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%
[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.

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