Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Quarterly Financial Data [Line Items]
Total revenues	$ 150,103	$ 138,649	$ 140,861	$ 159,892	$ 159,244	$ 143,715	$ 130,096	$ 280,313	$ 589,505	$ 713,368	$ 404,979
Net income	14,393	15,395	14,921	21,624	18,883	13,373	18,708	118,080	66,333	169,044	217,426
Net income attributable to The RMR Group Inc.	$ 6,158	$ 6,717	$ 6,468	$ 9,449	$ 8,354	$ 5,849	$ 8,168	$ 52,209	28,792	74,580	96,041
Net income attributable to The RMR Group Inc. per common share - diluted (in dollars per share)	$ 0.38	$ 0.41	$ 0.39	$ 0.58	$ 0.51	$ 0.36	$ 0.50	$ 3.22
Common distributions declared (in dollars per share)	$ 0.38	$ 0.38	$ 0.38	$ 0.38	$ 0.35	$ 0.35	$ 0.35	$ 0.35
Incentive business management fees
Quarterly Financial Data [Line Items]
Revenues									$ 0	$ 120,094	$ 155,881