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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net income attributable to The RMR Group Inc.	$ 5,849	$ 8,381	$ 66,226	$ 87,857
Income attributable to unvested participating securities	(37)	(45)	(437)	(518)
Net income attributable to The RMR Group Inc. used in calculating basic EPS	$ 5,812	$ 8,336	$ 65,789	$ 87,339
Denominator:
Weighted average common shares outstanding - basic (in shares)	16,137	16,087	16,126	16,072
Net income attributable to RMR Inc. per common share - basic (in dollars per share)	$ 0.36	$ 0.52	$ 4.08	$ 5.43
Numerator:
Net income attributable to The RMR Group Inc.	$ 5,849	$ 8,381	$ 66,226	$ 87,857
Income attributable to unvested participating securities	(37)	(45)	(437)	(518)
Net income attributable to The RMR Group Inc. used in calculating diluted EPS	$ 5,812	$ 8,336	$ 65,789	$ 87,339
Denominator:
Weighted average common shares outstanding - basic (in shares)	16,137	16,087	16,126	16,072
Dilutive effect of incremental unvested shares (in shares)	12	48	16	39
Weighted average common shares outstanding - diluted (in shares)	16,149	16,135	16,142	16,111
Net income attributable to RMR Inc. per common share - diluted (in dollars per share)	$ 0.36	$ 0.52	$ 4.08	$ 5.42