Net Income Attributable to RMR Inc. (Details) - USD ($) $ 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
Net Income Attributable to RMR Inc.
Income before income tax expense									$ 77,885	$ 196,364	$ 276,288
RMR Inc. franchise tax expense and interest income									481	329	488
Tax receivable agreement remeasurement									0	0	(24,710)
Fees from services provided prior to our initial public offering									0	0	(127)
Net income before noncontrolling interest									78,366	196,693	251,939
Net income attributable to noncontrolling interest									(37,541)	(94,464)	(121,258)
Net income attributable to RMR Inc. before income tax expense									40,825	102,229	130,681
Tax receivable agreement remeasurement									0	0	24,710
Income tax expense attributable to RMR Inc.									(11,552)	(27,320)	(58,862)
RMR Inc. franchise tax expense and interest income									(481)	(329)	(488)
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