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Acquisitions (Joint-Interest Acquisition) (Details) $ in Millions |  |  |  |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Feb. 01, 2018 | Sep. 01, 2017 USD ($) in mi shares | Mar. 01, 2017 USD ($) | Mar. 31, 2016 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2018 USD ($) shares | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) shares
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Units, acquisitions | shares |  |  |  |  |  |  |  |  | 22,993,880
Aggregate percentage of general partner interest |  | 2.00% |  | 2.00% | 2.00% |  | 2.00% |  |
Income/(loss) from equity method investments | [1],[2] |  |  |  |  |  | $ 240 | $ 78 | $ (74)
Contribution from MPC |  |  |  |  |  |  | 1,046 | 675 | $ 225
Limited Partners Common Units |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Units, acquisitions | shares |  |  |  |  |  |  |  |  | 22,534,002
General Partner Units |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Units, acquisitions | shares | [3] |  |  |  |  |  |  |  | 459,878
MPC |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Contribution from MPC |  |  |  | $ 6 | $ 15 |  |  |  |
MPC | Limited Partners Common Units |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Contribution from MPC |  |  |  |  |  |  | 0 | 0 | $ 84
MPC | General Partner |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Contribution from MPC |  |  |  |  |  |  | $ 0 | 0 | $ 141
Illinois Extension |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Equity method investment, ownership percentage |  |  | 35.00% |  |  |  | 35.00% |  |
Pipeline length | mi |  |  | 168 |  |  |  |  |  |
Pipeline diameter | in |  |  | 24 |  |  |  |  |  |
Number of pump stations |  |  | 2 |  |  |  |  |  |
LOOP |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Equity method investment, ownership percentage |  |  | 41.00% |  |  |  | 41.00% |  |
LOCAP |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Equity method investment, ownership percentage |  |  | 59.00% |  |  |  | 59.00% |  |
Explorer |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Equity method investment, ownership percentage |  |  | 25.00% |  |  |  | 25.00% |  |
Pipeline length | mi |  |  | 1,830 |  |  |  |  |  |
LOOP LOCAP SAX and Explorer |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Payments to acquire interest in joint venture |  |  | $ 420 |  |  |  |  |  |
Equity interest issued or issuable, value assigned |  |  | 630 |  |  |  |  |  |
Total consideration, value assigned |  |  | 1,050 |  |  |  |  |  |
Equity interest issued or issuable, fair value assigned |  |  | 653 |  |  |  |  |  |
Business Acquisition, Total Consideration, Fair Value Assigned |  |  | $ 1,070 |  |  |  |  |  |
Units, acquisitions | shares |  |  |  |  |  |  | 264,413,309 |  |
Aggregate percentage of general partner interest |  |  | 2.00% |  |  |  |  |  |
Income/(loss) from equity method investments |  |  |  |  |  |  | $ 118 | $ 21 |
LOOP LOCAP SAX and Explorer | General Partner Common Units |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Units, acquisitions | shares |  |  | 13,719,017 |  |  |  |  |  |
LOOP LOCAP SAX and Explorer | General Partner Units |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Units, acquisitions | shares |  |  | 377,778 |  |  |  |  |  |
LOOP LOCAP SAX and Explorer | MPLX Holdings Inc | Limited Partners Common Units |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Units, acquisitions | shares |  |  | 1,441,224 |  |  |  |  |  |
LOOP LOCAP SAX and Explorer | MPLX Logistics LLC | Limited Partners Common Units |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Units, acquisitions | shares |  |  | 3,350,893 |  |  |  |  |  |
LOOP LOCAP SAX and Explorer | MPC |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Contribution from MPC |  |  |  |  |  | $ 10 |  |  |
LOOP LOCAP SAX and Explorer | MPC | General Partner |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Payments of Distributions to Affiliates |  |  |  |  |  |  | $ 32 |  |
[1] | Includes an impairment expense of $89 million related to one of MPLX’s equity method investments for the year ended December 31, 2016.
[2] | “Income/(loss) from equity method investments” includes the impact of any basis differential amortization or accretion.
[3] | 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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