Investments (Details) - USD ($) $ in Thousands			3 Months Ended		9 Months Ended
	Jul. 06, 2020	Oct. 10, 2018	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2019
Schedule of Equity Method Investments [Line Items]
Equity in earnings of investees			$ 458	$ 174	$ 1,037	$ 318
Distributions from equity method investments					721	198
Equity method investment accounted for under the fair value option			4,598		4,598		$ 3,682
Unrealized (loss) gain on equity method investment accounted for under the fair value option			$ 1,678	(731)	$ 916	(2,978)
Tremont Mortgage Trust
Schedule of Equity Method Investments [Line Items]
Ownership percentage			19.50%		19.50%
Equity in earnings of investees					$ 1,037
Distributions from equity method investments			$ 16	$ 132	$ 721	$ 198
TA
Schedule of Equity Method Investments [Line Items]
Ownership percentage			4.00%		4.00%
Tremont Advisors | Tremont Mortgage Trust
Schedule of Equity Method Investments [Line Items]
Number of shares owned (in shares)			1,600,100		1,600,100
Ownership percentage			19.40%		19.40%
RMR LLC | TA
Schedule of Equity Method Investments [Line Items]
Ownership percentage			3.60%		3.60%
Shares post reverse stock split (in shares)					298,538
Purchases from related party		$ 8,382
Subsequent Event | RMR LLC | TA
Schedule of Equity Method Investments [Line Items]
Ownership percentage	3.60%
Shares post reverse stock split (in shares)	218,577
Purchases from related party	$ 3,060