Segment Reporting (Details) $ in Thousands	3 Months Ended				6 Months Ended
	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2021 USD ($) segment	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)
Segment Reporting
Number of reportable business segments | segment					1
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
Total assets	701,629		685,924		701,629	685,924	$ 690,253
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
Operating Segments | RMR LLC
Revenues:
Total revenues	129,324		139,949		284,989	297,221
Expenses:
Compensation and benefits	29,107		28,923		56,976	56,198
Equity based compensation	1,721		257		5,238	1,824
Separation costs			385		4,159	645
Total compensation and benefits expense	30,828		29,565		66,373	58,667
General and administrative	6,102		6,314		11,355	12,439
Other reimbursable expenses	75,208		84,227		174,593	182,202
Transaction and acquisition related costs	296		0		364	49
Depreciation and amortization	241		235		468	479
Total expenses	112,675		120,341		253,153	253,836
Operating income	16,649		19,608		31,836	43,385
Interest and other income	196		1,366		414	3,086
Equity in earnings of investees	0		0		0	0
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	13,443		18,774		36,970	45,709
Income tax expense	0		0		0	0
Net income	13,443		18,774		36,970	45,709
Total assets	650,539		636,994		650,539	636,994
Operating Segments | RMR LLC | Total management and advisory services revenues
Revenues:
Total revenues	40,242		43,219		80,730	89,814
Operating Segments | RMR LLC | Management services
Revenues:
Total revenues	40,242		43,219		80,730	89,814
Operating Segments | RMR LLC | Incentive business management fees
Revenues:
Total revenues	0				0
Operating Segments | RMR LLC | Advisory services
Revenues:
Total revenues	0		0		0	0
Operating Segments | RMR LLC | Total reimbursable costs
Revenues:
Total revenues	89,082		96,730		204,259	207,407
Operating Segments | RMR LLC | Reimbursable compensation and benefits
Revenues:
Total revenues	12,668		12,793		25,457	24,547
Operating Segments | RMR LLC | Reimbursable equity based compensation
Revenues:
Total revenues	1,206		(290)		4,209	658
Operating Segments | RMR LLC | Other reimbursable expenses
Revenues:
Total revenues	75,208		84,227		174,593	182,202
All Other Operations
Revenues:
Total revenues	2,240		912		3,521	3,532
Expenses:
Compensation and benefits	1,479		1,199		3,104	4,121
Equity based compensation	31		45		75	60
Separation costs			0		0	0
Total compensation and benefits expense	1,510		1,244		3,179	4,181
General and administrative	1,002		983		2,009	1,904
Other reimbursable expenses	0		0		0	0
Transaction and acquisition related costs	0		373		49	1,120
Depreciation and amortization	10		11		21	23
Total expenses	2,522		2,611		5,258	7,228
Operating income	(282)		(1,699)		(1,737)	(3,696)
Interest and other income	8		134		21	289
Equity in earnings of investees	303		324		727	579
Unrealized (loss) gain on equity method investment accounted for under the fair value option	0		0		0	0
Income before income tax expense	29		(1,241)		(989)	(2,828)
Income tax expense	(1,992)		(2,612)		(4,748)	(6,336)
Net income	(1,963)		(3,853)		(5,737)	(9,164)
Total assets	51,090		48,930		51,090	48,930
All Other Operations | Total management and advisory services revenues
Revenues:
Total revenues	1,749		882		2,594	2,409
All Other Operations | Management services
Revenues:
Total revenues	0		102		259	782
All Other Operations | Incentive business management fees
Revenues:
Total revenues	620				620
All Other Operations | Advisory services
Revenues:
Total revenues	1,129		780		1,715	1,627
All Other Operations | Total reimbursable costs
Revenues:
Total revenues	491		30		927	1,123
All Other Operations | Reimbursable compensation and benefits
Revenues:
Total revenues	491		30		927	1,123
All Other Operations | Reimbursable equity based compensation
Revenues:
Total revenues	0		0		0	0
All Other Operations | Other reimbursable expenses
Revenues:
Total revenues	0		0		0	0
Intersegment Eliminations
Revenues:
Total revenues	$ (1,060)		$ (1,090)		$ (2,202)	$ (3,165)