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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net income attributable to The RMR Group Inc.	$ 6,717	$ 5,849	$ 22,634	$ 66,226
Income attributable to unvested participating securities	(48)	(37)	(166)	(437)
Net income attributable to The RMR Group Inc. used in calculating basic EPS	6,669	5,812	22,468	65,789
Add back: net income attributable to noncontrolling interest	8,678	7,524	29,306	83,935
Add back: income tax expense	2,608	2,226	8,944	24,335
Income tax expense at enacted tax rates assuming redemption of noncontrolling interest’s Class A Units for Class A Common Shares	(5,313)		(18,114)
Net income attributable to The RMR Group Inc. used in calculating diluted EPS	$ 12,642	$ 5,812	$ 42,604	$ 65,789
Denominators:
Weighted average common shares outstanding - basic (in shares)	16,198	16,137	16,187	16,126
Assumed redemption of noncontrolling interest’s Class A Units for Class A Common Shares (in shares)	15,000	0	15,000	0
Incremental unvested shares (in shares)	0	12	0	16
Weighted average common shares outstanding - diluted (in shares)	31,198	16,149	31,187	16,142
Net income attributable to RMR Inc. per common share - basic (in dollars per share)	$ 0.41	$ 0.36	$ 1.39	$ 4.08
Net income attributable to RMR Inc. per common share - diluted (in dollars per share)	$ 0.41	$ 0.36	$ 1.37	$ 4.08