Per Common Share Amounts - Schedule of EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Numerators:
Net income attributable to The RMR Group Inc.	$ 6,392	$ 4,941	$ 14,434	$ 13,838
Income attributable to unvested participating securities	(62)	(44)	(140)	(121)
Net income attributable to The RMR Group Inc. used in calculating basic EPS	6,330	4,897	14,294	13,717
Add back: income attributable to unvested participating securities	62	44	140	121
Add back: net income attributable to noncontrolling interest	8,197	6,539	18,447	17,395
Add back: income tax expense	2,451	1,992	5,505	4,748
Income tax expense assuming redemption of noncontrolling interest’s Class A units for Class A common shares	(4,894)	(4,113)	(10,909)	(10,547)
Net income used in calculating diluted EPS	$ 12,146	$ 9,359	$ 27,477	$ 25,434
Denominators:
Common shares outstanding (in shares)	16,502	16,413	16,502	16,413
Unvested participating securities (in shares)	(173)	(157)	(175)	(159)
Weighted average common shares outstanding - basic (in shares)	16,329	16,256	16,327	16,254
Assumed redemption of noncontrolling interest’s Class A units for Class A common shares (in shares)	15,000	15,000	15,000	15,000
Add: Incremental unvested shares (in shares)	2	24	1	0
Weighted average common shares outstanding - diluted (in shares)	31,331	31,280	31,328	31,254
Net income attributable to The RMR Group Inc. per common share - basic (in dollars per share)	$ 0.39	$ 0.30	$ 0.88	$ 0.84
Net income attributable to The RMR Group Inc. per common share - diluted (in dollars per share)	$ 0.39	$ 0.30	$ 0.88	$ 0.81
Effective tax rate	28.70%	30.50%	28.40%	29.30%