Shareholders’ Equity - Distributions (Details) - USD ($) $ / shares in Units, $ in Thousands						6 Months Ended
	Apr. 15, 2021	Feb. 18, 2021	Nov. 19, 2020	Feb. 20, 2020	Nov. 14, 2019	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Class A and B-1 common shares
Class of Stock [Line Items]
Dividends paid (in dollars per share)		$ 0.38	$ 0.38	$ 0.38	$ 0.38	$ 0.76	$ 0.76
Value of dividends		$ 6,230	$ 6,230	$ 6,194	$ 6,195	$ 12,460	$ 12,389
Class A and B-1 common shares | Subsequent Event
Class of Stock [Line Items]
Dividends paid (in dollars per share)	$ 0.38
Value of dividends	$ 6,237
Common stock shares outstanding (in shares)	16,413,191
Class A Common Stock
Class of Stock [Line Items]
Common stock shares outstanding (in shares)						15,395,641		15,395,641
ABP Trust | Class A Common Stock
Class of Stock [Line Items]
Value of dividends		4,500	4,500	4,500	4,500	$ 9,000	9,000
Common stock shares outstanding (in shares)						15,000,000
ABP Trust | Class A Common Stock | Subsequent Event
Class of Stock [Line Items]
Value of dividends	$ 4,500
RMR LLC | Class A and B-1 common shares
Class of Stock [Line Items]
Value of dividends		$ 4,919	$ 4,919	$ 4,890	$ 4,891	$ 9,838	$ 9,781
RMR LLC | Class A and B-1 common shares | Subsequent Event
Class of Stock [Line Items]
Dividends paid (in dollars per share)	$ 0.30
Value of dividends	$ 4,924
RMR LLC | RMR LLC
Class of Stock [Line Items]
Dividends paid (in dollars per share)		$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.60	$ 0.60
Value of dividends		$ 9,419	$ 9,419	$ 9,390	$ 9,391	$ 18,838	$ 18,781
RMR LLC | RMR LLC | Subsequent Event
Class of Stock [Line Items]
Value of dividends	$ 9,424