Fair Value of Financial Instruments - Schedule of Valuation Techniques (Details) $ in Thousands	Dec. 31, 2025 USD ($)	Sep. 30, 2025 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Earnout liability		$ 3,639
MF Fund VII
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments	$ 2,708	3,156
Joint Venture
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments	$ 11,134	$ 11,134
Discount rates \| Monte Carlo
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Earnout Liability Measurement Input		0.0584
Discount rates \| Minimum \| Discounted cash flow \| MF Fund VII
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Measurement Input	0.0650	0.0650
Discount rates \| Maximum \| Discounted cash flow \| MF Fund VII
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Measurement Input	0.0700	0.0700
Exit capitalization rates \| Minimum \| Discounted cash flow \| MF Fund VII
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Measurement Input	0.0500	0.0500
Exit capitalization rates \| Minimum \| Discounted cash flow \| Joint Venture
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Measurement Input	0.0497	0.0497
Exit capitalization rates \| Maximum \| Discounted cash flow \| MF Fund VII
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Measurement Input	0.0550	0.0550
Exit capitalization rates \| Maximum \| Discounted cash flow \| Joint Venture
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Measurement Input	0.0515	0.0515
Holding period \| Discounted cash flow \| MF Fund VII
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Measurement Input	10,000	10,000
Holding period \| Discounted cash flow \| Joint Venture
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Measurement Input	3,000	3,000
Unlevered IRR \| Minimum \| Discounted cash flow \| Joint Venture
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Measurement Input	0.1202	0.1202
Unlevered IRR \| Maximum \| Discounted cash flow \| Joint Venture
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity Measurement Input	0.1237	0.1237
Capital deployment volatility \| Monte Carlo
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Earnout Liability Measurement Input		0.1500

RMR GROUP INC. 10-Q