Segment Reporting (Details) $ in Thousands	3 Months Ended						9 Months Ended
	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2021 USD ($) segment	Jun. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)
Segment Reporting
Number of reportable business segments | segment							1
Total revenues	$ 145,244			$ 138,649			$ 433,754	$ 439,402
Compensation and benefits	30,530			29,569			90,610	89,888
Equity based compensation	1,954			1,299			7,267	3,183
Separation costs	0			0			4,159	645
Total compensation and benefits expense	32,484			30,868			102,036	93,716
General and administrative	6,320			6,335			19,684	20,678
Other reimbursable expenses	85,263			85,650			259,856	267,852
Transaction and acquisition related costs	61			427			474	1,596
Depreciation and amortization	245			229			734	731
Total expenses	124,373			123,509			382,784	384,573
Operating income (loss)	20,871			15,140			50,970	54,829
Interest and other income	179			727			614	4,102
Equity in earnings of investees	28			458			755	1,037
Unrealized gain on equity method investment accounted for under the fair value option	1,312			1,678			6,032	916
Income before income tax expense	22,390			18,003			58,371	60,884
Income tax expense	(3,361)			(2,608)			(8,109)	(8,944)
Net income (loss)	19,029	$ 11,480	$ 19,753	15,395	$ 14,921	$ 21,624	50,262	51,940
Total assets	724,589			704,723			724,589	704,723	$ 690,253
Operating Segments | RMR LLC
Segment Reporting
Total revenues	143,602			137,719			428,591	434,940
Compensation and benefits	29,541			28,203			86,517	84,401
Equity based compensation	1,922			1,255			7,160	3,079
Separation costs							4,159	645
Total compensation and benefits expense	31,463			29,458			97,836	88,125
General and administrative	5,335			5,292			16,690	17,731
Other reimbursable expenses	85,263			85,650			259,856	267,852
Transaction and acquisition related costs	45			317			409	366
Depreciation and amortization	234			217			702	696
Total expenses	122,340			120,934			375,493	374,770
Operating income (loss)	21,262			16,785			53,098	60,170
Interest and other income	171			667			585	3,753
Equity in earnings of investees	0			0			0	0
Unrealized gain on equity method investment accounted for under the fair value option	1,312			1,678			6,032	916
Income before income tax expense	22,745			19,130			59,715	64,839
Income tax expense	0			0			0	0
Net income (loss)	22,745			19,130			59,715	64,839
Total assets	674,378			655,413			674,378	655,413
All Other Operations
Segment Reporting
Total revenues	1,642			930			5,163	4,462
Compensation and benefits	989			1,366			4,093	5,487
Equity based compensation	32			44			107	104
Separation costs							0	0
Total compensation and benefits expense	1,021			1,410			4,200	5,591
General and administrative	985			1,043			2,994	2,947
Other reimbursable expenses	0			0			0	0
Transaction and acquisition related costs	16			110			65	1,230
Depreciation and amortization	11			12			32	35
Total expenses	2,033			2,575			7,291	9,803
Operating income (loss)	(391)			(1,645)			(2,128)	(5,341)
Interest and other income	8			60			29	349
Equity in earnings of investees	28			458			755	1,037
Unrealized gain on equity method investment accounted for under the fair value option	0			0			0	0
Income before income tax expense	(355)			(1,127)			(1,344)	(3,955)
Income tax expense	(3,361)			(2,608)			(8,109)	(8,944)
Net income (loss)	(3,716)			(3,735)			(9,453)	(12,899)
Total assets	50,211			49,310			50,211	49,310
Intersegment Eliminations
Segment Reporting
Total revenues	(801)			(988)			(3,003)	(4,153)
Management services
Segment Reporting
Total revenues	44,376			38,625			125,365	129,221
Management services | Operating Segments | RMR LLC
Segment Reporting
Total revenues	44,376			38,590			125,106	128,404
Management services | All Other Operations
Segment Reporting
Total revenues	0			35			259	817
Advisory services
Segment Reporting
Total revenues	1,134			625			2,849	2,252
Advisory services | Operating Segments | RMR LLC
Segment Reporting
Total revenues	0			0			0	0
Advisory services | All Other Operations
Segment Reporting
Total revenues	1,134			625			2,849	2,252
Total management and advisory services revenues
Segment Reporting
Total revenues	45,510			39,250			128,834	131,473
Total management and advisory services revenues | Operating Segments | RMR LLC
Segment Reporting
Total revenues	44,376			38,590			125,106	128,404
Total management and advisory services revenues | All Other Operations
Segment Reporting
Total revenues	1,134			660			3,728	3,069
Reimbursable compensation and benefits
Segment Reporting
Total revenues	13,069			13,013			39,453	38,683
Reimbursable compensation and benefits | Operating Segments | RMR LLC
Segment Reporting
Total revenues	12,561			12,743			38,018	37,290
Reimbursable compensation and benefits | All Other Operations
Segment Reporting
Total revenues	508			270			1,435	1,393
Reimbursable equity based compensation
Segment Reporting
Total revenues	1,402			736			5,611	1,394
Reimbursable equity based compensation | Operating Segments | RMR LLC
Segment Reporting
Total revenues	1,402			736			5,611	1,394
Reimbursable equity based compensation | All Other Operations
Segment Reporting
Total revenues	0			0			0	0
Other reimbursable expenses
Segment Reporting
Total revenues	85,263			85,650			259,856	267,852
Other reimbursable expenses | Operating Segments | RMR LLC
Segment Reporting
Total revenues	85,263			85,650			259,856	267,852
Other reimbursable expenses | All Other Operations
Segment Reporting
Total revenues	0			0			0	0
Total reimbursable costs
Segment Reporting
Total revenues	99,734			99,399			304,920	307,929
Total reimbursable costs | Operating Segments | RMR LLC
Segment Reporting
Total revenues	99,226			99,129			303,485	306,536
Total reimbursable costs | All Other Operations
Segment Reporting
Total revenues	508			270			1,435	1,393
Incentive business management fees
Segment Reporting
Total revenues	$ 0			$ 0			620	$ 0
Incentive business management fees | Operating Segments | RMR LLC
Segment Reporting
Total revenues							0
Incentive business management fees | All Other Operations
Segment Reporting
Total revenues							$ 620