Business and Basis of Presentation	3 Months Ended
Business and Basis of Presentation	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Business and Basis of Presentation	Business and Basis of Presentation BP Midstream Partners LP (either individually or together with its subsidiaries, as the context requires, the “Partnership”) is a Delaware limited partnership formed on May 22, 2017 by BP Pipelines (North America) Inc. (“BP Pipelines”), an indirect wholly owned subsidiary of BP p.l.c. (“BP”), a “foreign private issuer” within the meaning of the Securities Exchange Act of 1934, as amended. Unless otherwise stated or the context otherwise indicates, all references to “we,” “our,” “us,” or similar expressions refer to the legal entity BP Midstream Partners LP. The term “our Parent” refers to BP Pipelines; any entity that wholly owns BP Pipelines, indirectly or directly, including BP and BP America Inc. (“BPA”), an indirect wholly owned subsidiary of BP; and any entity that is wholly owned by the aforementioned entities, excluding BP Midstream Partners LP. Business BP Midstream Partners LP is a master limited partnership formed by BP Pipelines to own, operate, develop and acquire pipelines and other midstream assets. The Partnership's assets consist of interests in entities that own crude oil, natural gas, refined products and diluent pipelines and refined product terminals serving as key infrastructure for BP and other customers to transport onshore crude oil production to BP’s refinery in Whiting, Indiana (the “Whiting Refinery”) and offshore crude oil and natural gas production to key refining markets and trading and distribution hubs. Certain assets deliver refined products and diluent from the Whiting Refinery and other U.S. supply hubs to major demand centers. As of March 31, 2021, the Partnership's assets consisted of the following: •BP Two Pipeline Company LLC, which owns the BP#2 crude oil pipeline system (“BP2”). •BP River Rouge Pipeline Company LLC, which owns the Whiting to River Rouge refined products pipeline system (“River Rouge”). •BP D-B Pipeline Company LLC, which owns the Diamondback diluent pipeline system (“Diamondback”). BP2, River Rouge, and Diamondback, together, are referred to as the "Wholly Owned Assets". •28.5% ownership interest in Mars Oil Pipeline Company, LLC (“Mars”), which owns a major corridor crude oil pipeline system in the Gulf of Mexico. •65% ownership interest and 100% managing member interest in Mardi Gras Transportation System Company, LLC ("Mardi Gras"), which holds the following investments in joint ventures located in the Gulf of Mexico: •56% ownership interest in Caesar Oil Pipeline Company, LLC (“Caesar”), •53% ownership interest in Cleopatra Gas Gathering Company, LLC (“Cleopatra”), •65% ownership interest in Proteus Oil Pipeline Company, LLC (“Proteus”), and, •65% ownership interest in Endymion Oil Pipeline Company, LLC (“Endymion”). •Together Endymion, Caesar, Cleopatra and Proteus are referred to as the “Mardi Gras Joint Ventures.” •22.7% ownership interest in Ursa Oil Pipeline Company, LLC ("Ursa"). •25% ownership interest in KM Phoenix Holdings, LLC ("KM Phoenix"). We generate a majority of revenue by charging fees for the transportation of crude oil, refined products and diluent through our pipelines under agreements with minimum volume commitments ("MVC"). We do not engage in the marketing and trading of any commodities. All operations are conducted in the United States, and all long-lived assets are located in the United States. Partnership operations consist of one reportable segment. Certain Partnership businesses are subject to regulation by various authorities including, but not limited to the Federal Energy Regulatory Commission ("FERC"). Regulatory bodies exercise statutory authority over matters such as common carrier tariffs, construction, rates and ratemaking and agreements with customers. Basis of Presentation Condensed consolidated financial statements have been prepared under the rules and regulations of the Securities and Exchange Commission (“SEC”). These rules and regulations conform to the accounting principles contained in the Financial Accounting Standards Board’s (“FASB”) Accounting Standards Codification, the single source of accounting principles generally accepted in the United States (“GAAP”). Certain information and footnote disclosures normally included in the annual consolidated financial statements have been condensed or omitted from these condensed consolidated financial statements. The condensed consolidated financial statements as of March 31, 2021, and for the three months ended March 31, 2021 and 2020, included herein, are unaudited. These financial statements include all known accruals and adjustments necessary, in the opinion of management, for a fair presentation of our condensed consolidated financial position, results of operations and cash flows. Unless otherwise specified, all such adjustments are of a normal and recurring nature. The unaudited results of operations for the interim periods reported are not necessarily indicative of results to be expected for the full year. These unaudited condensed consolidated financial statements should be read in conjunction with our audited consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2020 (the "Partnership's 2020 10-K"). Partnership financial position, results of operations and cash flows consist of consolidated BP Midstream Partners LP activities and balances. All intercompany accounts and transactions within the financial statements have been eliminated for all periods presented. Subordinated Unit Conversion On February 11, 2021, we paid a cash distribution of $0.3475 per limited partner unit to unitholders of record on January 28, 2021, for the three months ended December 31, 2020. Following payment of the distribution, the board of directors of our General Partner confirmed and approved that the financial tests required for conversion of our subordinated units had been met under the terms of the partnership agreement. As a result, on February 12, 2021, all of the Partnership's subordinated units were converted into common units on a one-for-one basis and the subordination period was terminated. Refer to Note 8 - Net Income Per Limited Partner Unit for additional information. As a result of the completion of this conversion event, common units are now presented on a combined basis in the Condensed Consolidated Balance Sheets and the Condensed Consolidated Statements of Changes in Equity as of and for the period ended March 31, 2021. Prior periods have been reclassified in the comparative periods to conform to current period presentation in the Condensed Consolidated Balance Sheets and the Condensed Consolidated Statements of Changes in Equity. Summary of Significant Accounting Policies There have been no significant changes to accounting policies as disclosed in Note 2 - Summary of Significant Accounting Policies in the Partnership's 2020 10-K.