Summary of Significant Accounting Policies - Schedule of Cash Flow, Supplemental Disclosures (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total cash and cash equivalents shown in the consolidated statements of cash flows	$ 62,297	$ 141,599	$ 267,989	$ 189,088
Parent Company
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total cash and cash equivalents shown in the consolidated statements of cash flows	19,478	23,189	26,802
RMR LLC
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total cash and cash equivalents shown in the consolidated statements of cash flows	$ 42,819	$ 118,410	$ 241,187