Equity Method Investments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021	Sep. 30, 2021
Schedule of Equity Method Investments [Line Items]
Equity method investments accounted for under the fair value option	$ 35,904		$ 35,904		$ 39,476
Unrealized (loss) gain on equity method investments accounted for under the fair value option	(4,560)	$ (3,402)	(3,364)	$ 4,720
Distributions from equity method investments			208	864
Equity in earnings of investees	0	303	0	727
SEVN
Schedule of Equity Method Investments [Line Items]
Unrealized (loss) gain on equity method investments accounted for under the fair value option	826		884
Distributions from equity method investments	208		208
SEVN | Level 1 | Recurring basis
Schedule of Equity Method Investments [Line Items]
Equity method investments accounted for under the fair value option	9,189		9,189		8,513
TRMT
Schedule of Equity Method Investments [Line Items]
Distributions from equity method investments		847		864
TA
Schedule of Equity Method Investments [Line Items]
Unrealized (loss) gain on equity method investments accounted for under the fair value option	(5,386)	$ (3,402)	(4,248)	$ 4,720
TA | Level 1 | Recurring basis
Schedule of Equity Method Investments [Line Items]
Equity method investments accounted for under the fair value option	$ 26,715		$ 26,715		$ 30,963
Tremont Reality Capital
Schedule of Equity Method Investments [Line Items]
Shares issued per converted share (in shares)	0.516		0.516
Tremont Reality Capital | SEVN
Schedule of Equity Method Investments [Line Items]
Number of shares owned (in shares)	825,651		825,651
Ownership percentage	5.70%		5.70%
Tremont Reality Capital | TRMT
Schedule of Equity Method Investments [Line Items]
Number of shares owned (in shares)	1,600,100		1,600,100
Ownership percentage	19.30%		19.30%
RMR LLC | TA
Schedule of Equity Method Investments [Line Items]
Number of shares owned (in shares)	621,853		621,853
Ownership percentage	4.20%		4.20%