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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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