Net Income Attributable to RMR Inc. (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Net Income Attributable to RMR Inc.
Income before income tax expense	$ 13,472	$ 17,533	$ 35,981	$ 42,881
RMR Inc. franchise tax expense and interest income	217	107	429	163
Net income before noncontrolling interest	13,689	17,640	36,410	43,044
Net income attributable to noncontrolling interest	(6,539)	(8,453)	(17,395)	(20,628)
Net income attributable to RMR Inc. before income tax expense	7,150	9,187	19,015	22,416
Income tax expense attributable to RMR Inc.	(1,992)	(2,612)	(4,748)	(6,336)
RMR Inc. franchise tax expense and interest income	(217)	(107)	(429)	(163)
Net income attributable to The RMR Group Inc.	$ 4,941	$ 6,468	$ 13,838	$ 15,917