Shareholders’ Equity - Distributions (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2020	Jun. 30, 2019
Class A and B-1 common shares
Class of Stock [Line Items]
Dividends paid (in dollars per share)	$ 0.38	$ 0.38	$ 0.38	$ 0.35	$ 0.35	$ 0.35	$ 1.14	$ 1.05
Value of dividends	$ 6,200	$ 6,194	$ 6,195	$ 5,684	$ 5,680	$ 5,680	$ 18,589	$ 17,044
ABP Trust | Class A Common Stock
Class of Stock [Line Items]
Value of dividends	4,500	4,500	4,500	4,500	4,500	4,500	13,500	13,500
RMR LLC | Class A and B-1 common shares
Class of Stock [Line Items]
Value of dividends	$ 4,894	$ 4,890	$ 4,891	$ 4,872	$ 4,869	$ 4,869	$ 14,675	$ 14,610
RMR LLC | RMR LLC
Class of Stock [Line Items]
Dividends paid (in dollars per share)	$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.90	$ 0.90
Value of dividends	$ 9,394	$ 9,390	$ 9,391	$ 9,372	$ 9,369	$ 9,369	$ 28,175	$ 28,110