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, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Numerators:
Net income attributable to The RMR Group Inc.	$ 4,941	$ 6,468	$ 13,838	$ 15,917
Income attributable to unvested participating securities	(44)	(47)	(121)	(119)
Net income attributable to The RMR Group Inc. used in calculating basic EPS	4,897	6,421	13,717	15,798
Add back: income attributable to unvested participating securities	44	47	121	119
Add back: net income attributable to noncontrolling interest	6,539	8,453	17,395	20,628
Add back: income tax expense	1,992	2,612	4,748	6,336
Income tax expense assuming redemption of noncontrolling interest’s Class A Units for Class A Common Shares	(4,113)	(5,317)	(10,547)	(12,858)
Net income used in calculating diluted EPS	$ 9,359	$ 12,216	$ 25,434	$ 30,023
Denominators:
Common stock shares outstanding (in shares)	16,413	16,314	16,413	16,314
Unvested participating securities (in shares)	(157)	(128)	(159)	(133)
Weighted average common shares outstanding - basic (in shares)	16,256	16,186	16,254	16,181
Assumed redemption of noncontrolling interest’s Class A Units for Class A Common Shares (in shares)	15,000	15,000	15,000	15,000
Incremental unvested shares (in shares)	24	0	0	0
Weighted average common shares outstanding - diluted (in shares)	31,280	31,186	31,254	31,181
Net income attributable to The RMR Group Inc. per common share - basic (in dollars per share)	$ 0.30	$ 0.40	$ 0.84	$ 0.98
Net income attributable to The RMR Group Inc. per common share - diluted (in dollars per share)	$ 0.30	$ 0.39	$ 0.81	$ 0.96