Condensed Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2021	Mar. 31, 2020
Revenues:
Total revenues	$ 131,564		$ 140,861		$ 288,510	$ 300,753
Expenses:
Compensation and benefits	30,586		30,122		60,080	60,319
Equity based compensation	1,752		302		5,313	1,884
Separation costs	0		385		4,159	645
Total compensation and benefits expense	32,338		30,809		69,552	62,848
General and administrative	7,104		7,297		13,364	14,343
Other reimbursable expenses	75,208		84,227		174,593	182,202
Transaction and acquisition related costs	296		373		413	1,169
Depreciation and amortization	251		246		489	502
Total expenses	115,197		122,952		258,411	261,064
Operating income	16,367		17,909		30,099	39,689
Interest and other income	204		1,500		435	3,375
Equity in earnings of investees	303		324		727	579
Unrealized (loss) gain on equity method investment accounted for under the fair value option	(3,402)		(2,200)		4,720	(762)
Income before income tax expense	13,472		17,533		35,981	42,881
Income tax expense	(1,992)		(2,612)		(4,748)	(6,336)
Net income	11,480	$ 19,753	14,921	$ 21,624	31,233	36,545
Net income attributable to noncontrolling interest	(6,539)		(8,453)		(17,395)	(20,628)
Net income attributable to The RMR Group Inc.	$ 4,941		$ 6,468		$ 13,838	$ 15,917
Weighted average common shares outstanding - basic (in shares)	16,256		16,186		16,254	16,181
Weighted average common shares outstanding - diluted (in shares)	31,280		31,186		31,254	31,181
Net income attributable to The RMR Group Inc. per common share - basic (in dollars per share)	$ 0.30		$ 0.40		$ 0.84	$ 0.98
Net income attributable to The RMR Group Inc. per common share - diluted (in dollars per share)	$ 0.30		$ 0.39		$ 0.81	$ 0.96
Total management and advisory services revenues
Revenues:
Total revenues	$ 41,991		$ 44,101		$ 83,324	$ 92,223
Management services
Revenues:
Total revenues	40,242		43,321		80,989	90,596
Incentive business management fees
Revenues:
Total revenues	620		0		620	0
Advisory services
Revenues:
Total revenues	1,129		780		1,715	1,627
Total reimbursable costs
Revenues:
Total revenues	89,573		96,760		205,186	208,530
Reimbursable compensation and benefits
Revenues:
Total revenues	13,159		12,823		26,384	25,670
Reimbursable equity based compensation
Revenues:
Total revenues	1,206		(290)		4,209	658
Other reimbursable expenses
Revenues:
Total revenues	$ 75,208		$ 84,227		$ 174,593	$ 182,202