Per Common Share Amounts - Schedule of EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Numerators:
Net income attributable to The RMR Group Inc.	$ 8,232	$ 6,717	$ 22,070	$ 22,634
Income attributable to unvested participating securities	(72)	(48)	(194)	(166)
Net income attributable to The RMR Group Inc. used in calculating basic EPS	8,160	6,669	21,876	22,468
Add back: income attributable to unvested participating securities	72	48	194	166
Add back: net income attributable to noncontrolling interest	10,797	8,678	28,192	29,306
Add back: income tax expense	3,361	2,608	8,109	8,944
Income tax expense assuming redemption of noncontrolling interest’s Class A Units for Class A Common Shares (1)	(6,686)	(5,313)	(17,346)	(18,114)
Net income used in calculating diluted EPS	$ 15,704	$ 12,690	$ 41,025	$ 42,770
Denominators:
Common stock shares outstanding (in shares)	16,408	16,315	16,408	16,315
Unvested participating securities (in shares)	(139)	(117)	(149)	(128)
Weighted average common shares outstanding - basic (in shares)	16,269	16,198	16,259	16,187
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)	39	0	12	0
Weighted average common shares outstanding - diluted (in shares)	31,308	31,198	31,271	31,187
Net income attributable to The RMR Group Inc. per common share - basic (in dollars per share)	$ 0.50	$ 0.41	$ 1.35	$ 1.39
Net income attributable to The RMR Group Inc. per common share - diluted (in dollars per share)	$ 0.50	$ 0.41	$ 1.31	$ 1.37