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, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net income attributable to The RMR Group Inc.	$ 6,468	$ 8,168	$ 15,917	$ 60,377
Income attributable to unvested participating securities	(47)	(55)	(119)	(409)
Net income attributable to The RMR Group Inc. used in calculating basic EPS	6,421	8,113	15,798	59,968
Add back: net income attributable to noncontrolling interest	8,453	10,540	20,628	76,411
Add back: income tax expense	2,612	3,139	6,336	22,109
Income attributable to unvested participating securities	(5,317)		(12,858)
Net income attributable to The RMR Group Inc. used in calculating diluted EPS	$ 12,169	$ 8,113	$ 29,904	$ 59,968
Denominators:
Weighted average common shares outstanding - basic (in shares)	16,186	16,120	16,181	16,120
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	27	0	20
Weighted average common shares outstanding - diluted (in shares)	31,186	16,147	31,181	16,140
Net income attributable to RMR Inc. per common share - basic (in dollars per share)	$ 0.40	$ 0.50	$ 0.98	$ 3.72
Net income attributable to RMR Inc. per common share - diluted (in dollars per share)	$ 0.39	$ 0.50	$ 0.96	$ 3.72