Subsequent Events	12 Months Ended
Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Event(s)	Subsequent Event(s) We have evaluated events that occurred after December 31, 2017 through the issuance of these consolidated financial statements. Any material subsequent events that occurred during this time have been properly recognized or disclosed in the consolidated financial statements and accompanying notes. Distribution On January 25, 2018, the Board declared a cash distribution of $0.33300 per limited partner unit for the three months ended December 31, 2017. The distribution was paid on February 14, 2018 to unitholders of record as of February 5, 2018. Equity Issuances On February 6, 2018, we completed the sale of 25,000,000 common units in a registered public offering for approximately $673.5 million net proceeds ($680.0 million gross proceeds, or $27.20 per common unit, less $6.0 million of underwriter's fees and $0.5 million of transaction fees). In connection with the issuance of common units, we issued 510,204 general partner units to our general partner for $13.9 million in order to maintain its 2% general partner interest in us. On February 6, 2018, we also completed the sale of 11,029,412 common units in a private placement with Shell Midstream LP Holdings LLC, an indirect subsidiary of Shell, for an aggregate purchase price of $300.0 million, or $27.20 per common unit. In connection with the issuance of the common units, we issued 225,091 general partner units to the general partner for $6.1 million in order to maintain its 2% general partner interest in us. We used the net proceeds from these sales of common units and from our general partner’s proportionate capital contribution to repay borrowings outstanding under the Five Year Revolver due October 2019 and the Five Year Revolver due December 2022, as well as for general partnership purposes. As part of the registered public offering, the underwriters received a 30-day option to purchase up to an additional 3,750,000 common units from us on the same terms as in the registered public offering. Tax Sharing Agreement We have entered into a tax sharing agreement with Shell. Pursuant to this agreement, we have agreed to reimburse Shell for state and local income and franchise taxes attributable to any activity of our operating subsidiaries, and reported on Shell’s state or local income or franchise tax returns filed on a combined or unitary basis. Reimbursements under this agreement equal the amount of tax our applicable operating subsidiaries would be required to pay with respect to such activity, if such subsidiaries were to file a combined or unitary tax return separate from Shell. Shell will compute and invoice us for the tax reimbursement amount within 15 days of Shell filing its combined or unitary tax return on which such activity is included. We may be required to make prepayments toward the tax reimbursement amount to the extent that Shell is required to make estimated tax payments during the relevant tax year. The tax sharing agreement currently in place is effective for all taxable periods ending on or after December 31, 2017. The current agreement replaced a similar tax sharing agreement between Zydeco and Shell, which was effective for all tax periods ending before December 31, 2017. Partnership Agreement On February 26, 2018, Shell Midstream Partners GP LLC, the general partner of the Partnership, executed Amendment No. 1 to the Partnership’s Amended and Restated Agreement of Limited Partnership dated November 3, 2014, in response to changes to the Internal Revenue Code enacted by the Bipartisan Budget Act of 2015 relating to partnership audit and adjustment procedures.

Subsequent Event(s)

We have evaluated events that occurred after December 31, 2017 through the issuance of these consolidated financial statements. Any material subsequent events that occurred during this time have been properly recognized or disclosed in the consolidated financial statements and accompanying notes.

Distribution

On January 25, 2018, the Board declared a cash distribution of $0.33300 per limited partner unit for the three months ended December 31, 2017. The distribution was paid on February 14, 2018 to unitholders of record as of February 5, 2018.

Equity Issuances

On February 6, 2018, we completed the sale of 25,000,000 common units in a registered public offering for approximately $673.5 million net proceeds ($680.0 million gross proceeds, or $27.20 per common unit, less $6.0 million of underwriter's fees and $0.5 million of transaction fees). In connection with the issuance of common units, we issued 510,204 general partner units to our general partner for $13.9 million in order to maintain its 2% general partner interest in us. On February 6, 2018, we also completed the sale of 11,029,412 common units in a private placement with Shell Midstream LP Holdings LLC, an indirect subsidiary of Shell, for an aggregate purchase price of $300.0 million, or $27.20 per common unit. In connection with the issuance of the common units, we issued 225,091 general partner units to the general partner for $6.1 million in order to maintain its 2% general partner interest in us.

We used the net proceeds from these sales of common units and from our general partner’s proportionate capital contribution to repay borrowings outstanding under the Five Year Revolver due October 2019 and the Five Year Revolver due December 2022, as well as for general partnership purposes. As part of the registered public offering, the underwriters received a 30-day option to purchase up to an additional 3,750,000 common units from us on the same terms as in the registered public offering.

Tax Sharing Agreement

We have entered into a tax sharing agreement with Shell. Pursuant to this agreement, we have agreed to reimburse Shell for state and local income and franchise taxes attributable to any activity of our operating subsidiaries, and reported on Shell’s state or local income or franchise tax returns filed on a combined or unitary basis. Reimbursements under this agreement equal the amount of tax our applicable operating subsidiaries would be required to pay with respect to such activity, if such subsidiaries were to file a combined or unitary tax return separate from Shell. Shell will compute and invoice us for the tax reimbursement amount within 15 days of Shell filing its combined or unitary tax return on which such activity is included. We may be required to make prepayments toward the tax reimbursement amount to the extent that Shell is required to make estimated tax payments during the relevant tax year. The tax sharing agreement currently in place is effective for all taxable periods ending on or after December 31, 2017. The current agreement replaced a similar tax sharing agreement between Zydeco and Shell, which was effective for all tax periods ending before December 31, 2017.

Partnership Agreement

On February 26, 2018, Shell Midstream Partners GP LLC, the general partner of the Partnership, executed Amendment No. 1 to the Partnership’s Amended and Restated Agreement of Limited Partnership dated November 3, 2014, in response to changes to the Internal Revenue Code enacted by the Bipartisan Budget Act of 2015 relating to partnership audit and adjustment procedures.