Basis of Presentation	3 Months Ended
Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The RMR Group Inc., or RMR Inc., is a holding company and substantially all of its business is conducted by its majority owned subsidiary The RMR Group LLC, or RMR LLC. RMR Inc. is a Maryland corporation and RMR LLC is a Maryland limited liability company. RMR Inc. serves as the sole managing member of RMR LLC and, in that capacity, operates and controls the business and affairs of RMR LLC. In these financial statements, unless otherwise indicated, "we", "us" and "our" refer to RMR Inc. and its direct and indirect subsidiaries, including RMR LLC. As of December 31, 2017, RMR Inc. owns 15,164,066 class A membership units of RMR LLC, or Class A Units, and 1,000,000 class B membership units of RMR LLC, or Class B Units. The aggregate RMR LLC membership units RMR Inc. owns represent 51.9% of the economic interest of RMR LLC as of December 31, 2017. We refer to economic interest as the right of a holder of a Class A Unit or Class B Unit to share in distributions made by RMR LLC and, upon liquidation, dissolution or winding up of RMR LLC, to share in the assets of RMR LLC after payments to creditors. A wholly owned subsidiary of ABP Trust, a Maryland statutory trust beneficially owned by Barry M. Portnoy and Adam D. Portnoy, our Managing Directors, owns 15,000,000 redeemable Class A Units, representing 48.1% of the economic interest of RMR LLC as of December 31, 2017, which is presented as a noncontrolling interest within the condensed consolidated financial statements. RMR LLC was founded in 1986 to manage public investments in real estate and, as of December 31, 2017, managed a diverse portfolio of publicly owned real estate and real estate related businesses. RMR LLC manages: Government Properties Income Trust, or GOV, a publicly traded real estate investment trust, or REIT, that primarily owns properties that are majority leased to the U.S. Government and other government tenants and office properties located in the metropolitan Washington, D.C. market area. Hospitality Properties Trust, or HPT, a publicly traded REIT that primarily owns hotel and travel center properties; Select Income REIT, or SIR, a publicly traded REIT that primarily owns properties that are leased to single tenants, and Senior Housing Properties Trust, or SNH, a publicly traded REIT that primarily owns healthcare, senior living and medical office buildings. Hereinafter, GOV, HPT, SIR and SNH are collectively referred to as the Managed Equity REITs. RMR LLC also provides management services to other publicly traded and private businesses, including: Five Star Senior Living Inc., or Five Star, a publicly traded operator of senior living communities, many of which are owned by SNH; Sonesta International Hotels Corporation, or Sonesta, a privately owned franchisor and operator of hotels, resorts and cruise ships in the United States, Latin America, the Caribbean and the Middle East, some of whose U.S. hotels are owned by HPT; and TravelCenters of America LLC, or TA, an operator of travel centers along the U.S. Interstate Highway System, many of which are owned by HPT, convenience stores with retail gas stations and restaurants. Hereinafter, Five Star, Sonesta and TA are collectively referred to as the Managed Operators. In addition, RMR LLC also provides management services to certain related private companies, including Affiliates Insurance Company, or AIC, an Indiana insurance company, and ABP Trust and its subsidiaries. Hereinafter, ABP Trust and its subsidiaries are collectively referred to as ABP Trust. RMR Advisors LLC, a Maryland limited liability company, or RMR Advisors, was founded in 2002. RMR Advisors is a wholly owned subsidiary of RMR LLC and is the adviser to RMR Real Estate Income Fund, or RIF. RIF is a closed end investment company focused on investing in real estate securities, including REITs and other dividend paying securities, but excluding our Client Companies, as defined below. Tremont Realty Advisors LLC, or Tremont Advisors, an investment adviser registered with the Securities and Exchange Commission, or SEC, was founded in 2016 in connection with the acquisition of certain assets of Tremont Realty Capital LLC, or the Tremont business. Tremont Advisors is a wholly owned subsidiary of RMR LLC that manages a private fund created for an institutional investor and other separately managed accounts that invest in commercial real estate debt, including secured mortgage debt, mezzanine financings and commercial real estate that may become owned by its clients. Tremont Advisors also manages Tremont Mortgage Trust, or TRMT, a publicly traded mortgage real estate investment trust that completed its initial public offering on September 18, 2017, or the TRMT IPO. TRMT focuses primarily on originating and investing in first mortgage loans secured by middle market and transitional commercial real estate. In these financial statements, we refer to the Managed Equity REITs, the Managed Operators, RIF, TRMT, AIC, ABP Trust and the clients of the Tremont business as our Client Companies. We refer to the Managed Equity REITs and TRMT collectively as the Managed REITs. The accompanying condensed consolidated financial statements of RMR Inc. are unaudited. Certain information and disclosures required by U.S. generally accepted accounting principles, or GAAP, for complete financial statements have been condensed or omitted. We believe the disclosures made are adequate to make the information presented not misleading. However, the accompanying condensed consolidated financial statements should be read in conjunction with the financial statements and notes contained in our Annual Report on Form 10-K for the fiscal year ended September 30, 2017, or our Annual Report. In the opinion of our management, all adjustments, which include only normal recurring adjustments considered necessary for a fair presentation, have been included. All intercompany transactions and balances with or among our consolidated subsidiaries have been eliminated. Our operating results for interim periods are not necessarily indicative of the results that may be expected for the full year. Preparation of these financial statements in conformity with GAAP requires our management to make certain estimates and assumptions that may affect the amounts reported in these financial statements and related notes. The actual results could differ from these estimates.

Basis of Presentation

The RMR Group Inc., or RMR Inc., is a holding company and substantially all of its business is conducted by its majority owned subsidiary The RMR Group LLC, or RMR LLC. RMR Inc. is a Maryland corporation and RMR LLC is a Maryland limited liability company. RMR Inc. serves as the sole managing member of RMR LLC and, in that capacity, operates and controls the business and affairs of RMR LLC. In these financial statements, unless otherwise indicated, "we", "us" and "our" refer to RMR Inc. and its direct and indirect subsidiaries, including RMR LLC.

As of December 31, 2017, RMR Inc. owns 15,164,066 class A membership units of RMR LLC, or Class A Units, and 1,000,000 class B membership units of RMR LLC, or Class B Units. The aggregate RMR LLC membership units RMR Inc. owns represent 51.9% of the economic interest of RMR LLC as of December 31, 2017. We refer to economic interest as the right of a holder of a Class A Unit or Class B Unit to share in distributions made by RMR LLC and, upon liquidation, dissolution or winding up of RMR LLC, to share in the assets of RMR LLC after payments to creditors. A wholly owned subsidiary of ABP Trust, a Maryland statutory trust beneficially owned by Barry M. Portnoy and Adam D. Portnoy, our Managing Directors, owns 15,000,000 redeemable Class A Units, representing 48.1% of the economic interest of RMR LLC as of December 31, 2017, which is presented as a noncontrolling interest within the condensed consolidated financial statements.

RMR LLC was founded in 1986 to manage public investments in real estate and, as of December 31, 2017, managed a diverse portfolio of publicly owned real estate and real estate related businesses. RMR LLC manages: Government Properties Income Trust, or GOV, a publicly traded real estate investment trust, or REIT, that primarily owns properties that are majority leased to the U.S. Government and other government tenants and office properties located in the metropolitan Washington, D.C. market area. Hospitality Properties Trust, or HPT, a publicly traded REIT that primarily owns hotel and travel center properties; Select Income REIT, or SIR, a publicly traded REIT that primarily owns properties that are leased to single tenants, and Senior Housing Properties Trust, or SNH, a publicly traded REIT that primarily owns healthcare, senior living and medical office buildings. Hereinafter, GOV, HPT, SIR and SNH are collectively referred to as the Managed Equity REITs. RMR LLC also provides management services to other publicly traded and private businesses, including: Five Star Senior Living Inc., or Five Star, a publicly traded operator of senior living communities, many of which are owned by SNH; Sonesta International Hotels Corporation, or Sonesta, a privately owned franchisor and operator of hotels, resorts and cruise ships in the United States, Latin America, the Caribbean and the Middle East, some of whose U.S. hotels are owned by HPT; and TravelCenters of America LLC, or TA, an operator of travel centers along the U.S. Interstate Highway System, many of which are owned by HPT, convenience stores with retail gas stations and restaurants. Hereinafter, Five Star, Sonesta and TA are collectively referred to as the Managed Operators. In addition, RMR LLC also provides management services to certain related private companies, including Affiliates Insurance Company, or AIC, an Indiana insurance company, and ABP Trust and its subsidiaries. Hereinafter, ABP Trust and its subsidiaries are collectively referred to as ABP Trust.

RMR Advisors LLC, a Maryland limited liability company, or RMR Advisors, was founded in 2002. RMR Advisors is a wholly owned subsidiary of RMR LLC and is the adviser to RMR Real Estate Income Fund, or RIF. RIF is a closed end investment company focused on investing in real estate securities, including REITs and other dividend paying securities, but excluding our Client Companies, as defined below.

Tremont Realty Advisors LLC, or Tremont Advisors, an investment adviser registered with the Securities and Exchange Commission, or SEC, was founded in 2016 in connection with the acquisition of certain assets of Tremont Realty Capital LLC, or the Tremont business. Tremont Advisors is a wholly owned subsidiary of RMR LLC that manages a private fund created for an institutional investor and other separately managed accounts that invest in commercial real estate debt, including secured mortgage debt, mezzanine financings and commercial real estate that may become owned by its clients. Tremont Advisors also manages Tremont Mortgage Trust, or TRMT, a publicly traded mortgage real estate investment trust that completed its initial public offering on September 18, 2017, or the TRMT IPO. TRMT focuses primarily on originating and investing in first mortgage loans secured by middle market and transitional commercial real estate.

In these financial statements, we refer to the Managed Equity REITs, the Managed Operators, RIF, TRMT, AIC, ABP Trust and the clients of the Tremont business as our Client Companies. We refer to the Managed Equity REITs and TRMT collectively as the Managed REITs.

The accompanying condensed consolidated financial statements of RMR Inc. are unaudited. Certain information and disclosures required by U.S. generally accepted accounting principles, or GAAP, for complete financial statements have been condensed or omitted. We believe the disclosures made are adequate to make the information presented not misleading. However, the accompanying condensed consolidated financial statements should be read in conjunction with the financial statements and notes contained in our Annual Report on Form 10-K for the fiscal year ended September 30, 2017, or our Annual Report. In the opinion of our management, all adjustments, which include only normal recurring adjustments considered necessary for a fair presentation, have been included. All intercompany transactions and balances with or among our consolidated subsidiaries have been eliminated. Our operating results for interim periods are not necessarily indicative of the results that may be expected for the full year.

Preparation of these financial statements in conformity with GAAP requires our management to make certain estimates and assumptions that may affect the amounts reported in these financial statements and related notes. The actual results could differ from these estimates.