Per Common Share Amounts - Schedule of EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Numerators:
Net income attributable to The RMR Group Inc.	$ 6,158	$ 6,717	$ 6,468	$ 9,449	$ 8,354	$ 5,849	$ 8,168	$ 52,209	$ 28,792	$ 74,580	$ 96,041
Income attributable to unvested participating securities									(209)	(482)	(564)
Net income attributable to The RMR Group Inc. used in calculating basic EPS									28,583	74,098	95,477
Net income attributable to noncontrolling interest									37,541	94,464	121,385
Add back: income tax expense									11,552	27,320	58,862
Income tax expense at enacted tax rates assuming redemption of noncontrolling interest’s Class A Units for Class A Common Shares									(23,183)
Net income attributable to The RMR Group Inc. used in calculating diluted EPS									$ 54,493	$ 74,098	$ 95,477
Denominators:
Weighted average common shares outstanding - basic (in shares)									16,194	16,132	16,077
Assumed redemption of noncontrolling interest’s Class A Units for Class A Common Shares (in shares)									15,000	0	0
Incremental unvested shares (in shares)									0	11	43
Weighted average common shares outstanding - diluted (in shares)									31,194	16,143	16,120
Net income attributable to RMR Inc. per common share - basic (in dollars per share)									$ 1.77	$ 4.59	$ 5.94
Net income attributable to The RMR Group Inc. per common share - diluted (in dollars per share)									$ 1.75	$ 4.59	$ 5.92