Revenue Recognition - Advisory Agreements (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Related Party Transaction [Line Items]
Total revenues	$ 131,564	$ 140,861	$ 288,510	$ 300,753
Tremont Advisors
Related Party Transaction [Line Items]
Business management fee percent based on management agreements			1.50%
Total revenues	$ 620		$ 620
Incentive fee percentage condition 1	20.00%
Incentive fee percentage condition 2	7.00%
Minimum | Tremont Advisors
Related Party Transaction [Line Items]
Advisory fee percentage based on aggregate principal amounts of loan originate			0.50%
Maximum | Tremont Advisors
Related Party Transaction [Line Items]
Advisory fee percentage based on aggregate principal amounts of loan originate				1.00%
Advisory services
Related Party Transaction [Line Items]
Total revenues	$ 1,129	780	$ 1,715	$ 1,627
Management services
Related Party Transaction [Line Items]
Total revenues	$ 40,242	43,321	80,989	90,596
RMRM
Related Party Transaction [Line Items]
Advisory fee percent	0.85%
RMR Advisors and Tremont Advisors | Advisory services
Related Party Transaction [Line Items]
Total revenues	$ 751	743	1,300	1,554
Tremont Advisors | Advisory services
Related Party Transaction [Line Items]
Total revenues	378	37	415	73
Tremont Advisors | Management services
Related Party Transaction [Line Items]
Total revenues	$ 0	$ 102	$ 259	$ 782