Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Thousands			12 Months Ended
	Oct. 30, 2025	May 15, 2023	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property management fees revenue			$ 69,875	$ 76,444	$ 63,153
Total revenues			$ 182,703	193,920	236,164
Percent of construction supervision			5.00%
Property management fee			$ 9,314	15,641	18,443
Acquisition fee revenue			664
Net investment income			2,447	1,313	0
Rental property revenues			$ 8,273	1,604	0
Tremont Advisors
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Incentive fee percentage condition 1			20.00%
Incentive fee percentage condition 2			7.00%
Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percent of gross collected rents			2.50%
Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percent of gross collected rents			3.50%
Termination and incentive business management fees
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenues		$ 45,282			45,282
Advisory services
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenues			$ 4,475	4,506	4,520
Advisory services | Tremont Advisors
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Management fee percentage pursuant to agreement			1.50%
Total revenues			$ 4,475	4,506	4,520
Termination and incentive fees
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenues			653	1,213	45,942
Termination and incentive fees | Tremont Advisors
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenues			$ 653	1,213	660
Total Managed Equity REITs
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percentage of historical cost of transferred real estate assets			0.50%
Percentage of average invested capital below threshold limit			0.70%
Threshold amount, maximum			$ 250,000
Percentage of average invested capital above threshold limit			0.50%
Threshold limit, minimum			$ 250,000
Percentage of average market capitalization below threshold limit			0.70%
Percentage of average market capitalization above threshold limit			0.50%
Aggregate annual base business management fees			$ 80,030	84,182	85,603
Incentive management fee percentage			12.00%
Aggregate incentive business management fees				0	0	$ 0
Total revenues			$ 123,769	134,882	139,215
Managed Operating Companies
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Management fee percentage pursuant to agreement			0.60%
Managed Public Real Estate Capital
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Aggregate annual business management fees			$ 27,670	27,575	36,815
Total revenues			$ 128,975	$ 140,648	$ 199,617
OPI Management | Subsequent Event
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Agreement term	5 years
Termination term	2 years
Property management fees revenue	$ 14
Percentage of property management fee	3.00%
Percentage of construction supervision fee	0.05