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Acquisitions (Acquisition of Hardin Street Marine LLC) (Details) $ in Millions |  |  |  | 12 Months Ended
--- | --- | --- | --- | ---
Feb. 01, 2018 | Mar. 01, 2017 USD ($) | Mar. 31, 2016 USD ($) shares | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |
Aggregate percentage of general partner interest | 2.00% | 2.00% | 2.00% | 2.00% |  |
Contribution from MPC |  |  |  | $ 1,046 | $ 675 | $ 225
Number of tow boats |  |  | 18 |  |  |
Number of barges |  |  | 219 |  |  |
MPC |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Contribution from MPC |  | $ 6 | $ 15 |  |  |
Percent of volumes shipped |  |  | 60.00% |  |  |
MPC | General Partner |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Contribution from MPC |  |  |  | $ 0 | $ 0 | $ 141
Hardin Street Marine |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Equity interest issued or issuable, value assigned |  |  | $ 600 |  |  |
Effective date of acquisition |  |  | Mar. 31, 2016 |  |  |
Hardin Street Marine | Limited Partner |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Equity interest issued or issuable, number of shares | shares |  |  | 22,534,002 |  |  |
Equity interest issued or issuable, fair value assigned |  |  | $ 669 |  |  |
Hardin Street Marine | General Partner |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Equity interest issued or issuable, number of shares | shares |  |  | 459,878 |  |  |
Equity interest issued or issuable, fair value assigned |  |  | $ 14 |  |  |

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