Segment Reporting (Details) $ in Thousands	3 Months Ended				6 Months Ended
	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2022 USD ($) segment	Mar. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)
Segment Reporting
Number of reportable business segments | segment					1
Total revenues	$ 197,654		$ 131,564		$ 379,222	$ 288,510
Compensation and benefits	31,710		30,586		63,501	60,080
Equity based compensation	1,988		1,752		4,207	5,313
Separation costs	217		0		217	4,159
Total compensation and benefits expense	33,915		32,338		67,925	69,552
General and administrative	8,470		7,104		16,141	13,364
Other reimbursable expenses	133,493		75,208		253,051	174,593
Transaction and acquisition related costs	0		296		0	413
Depreciation and amortization	242		251		478	489
Total expenses	176,120		115,197		337,595	258,411
Operating income (loss)	21,534		16,367		41,627	30,099
Interest and other income	66		204		123	435
Equity in earnings of investees	0		303		0	727
Unrealized (loss) gain on equity method investments accounted for under the fair value option	(4,560)		(3,402)		(3,364)	4,720
Income before income tax expense	17,040		13,472		38,386	35,981
Income tax expense	(2,451)		(1,992)		(5,505)	(4,748)
Net income (loss)	14,589	$ 18,292	11,480	$ 19,753	32,881	31,233
Total assets	528,579		701,629		528,579	701,629	$ 497,911
Operating Segments | RMR LLC
Segment Reporting
Total revenues	195,928		129,324		375,680	284,989
Compensation and benefits	30,507		29,107		61,163	56,976
Equity based compensation	1,852		1,721		3,935	5,238
Separation costs	217				217	4,159
Total compensation and benefits expense	32,576		30,828		65,315	66,373
General and administrative	7,396		6,102		14,070	11,355
Other reimbursable expenses	133,493		75,208		253,051	174,593
Transaction and acquisition related costs			296			364
Depreciation and amortization	233		241		460	468
Total expenses	173,698		112,675		332,896	253,153
Operating income (loss)	22,230		16,649		42,784	31,836
Interest and other income	59		196		108	414
Equity in earnings of investees			0			0
Unrealized (loss) gain on equity method investments accounted for under the fair value option	(5,386)		(3,402)		(4,248)	4,720
Income before income tax expense	16,903		13,443		38,644	36,970
Income tax expense	0		0		0	0
Net income (loss)	16,903		13,443		38,644	36,970
Total assets	484,347		650,539		484,347	650,539
All Other Operations
Segment Reporting
Total revenues	1,726		2,240		3,542	3,521
Compensation and benefits	1,203		1,479		2,338	3,104
Equity based compensation	136		31		272	75
Separation costs	0				0	0
Total compensation and benefits expense	1,339		1,510		2,610	3,179
General and administrative	1,074		1,002		2,071	2,009
Other reimbursable expenses	0		0		0	0
Transaction and acquisition related costs			0			49
Depreciation and amortization	9		10		18	21
Total expenses	2,422		2,522		4,699	5,258
Operating income (loss)	(696)		(282)		(1,157)	(1,737)
Interest and other income	7		8		15	21
Equity in earnings of investees			303			727
Unrealized (loss) gain on equity method investments accounted for under the fair value option	826		0		884	0
Income before income tax expense	137		29		(258)	(989)
Income tax expense	(2,451)		(1,992)		(5,505)	(4,748)
Net income (loss)	(2,314)		(1,963)		(5,763)	(5,737)
Total assets	44,232		51,090		44,232	51,090
Intersegment Eliminations
Segment Reporting
Total revenues	1,081		1,060		2,082	2,202
Management services
Segment Reporting
Total revenues	48,151		40,242		93,048	80,989
Management services | Operating Segments | RMR LLC
Segment Reporting
Total revenues	48,151		40,242		92,995	80,730
Management services | All Other Operations
Segment Reporting
Total revenues	0		0		53	259
Advisory services
Segment Reporting
Total revenues	1,137		1,129		2,255	1,715
Advisory services | Operating Segments | RMR LLC
Segment Reporting
Total revenues	0		0		0	0
Advisory services | All Other Operations
Segment Reporting
Total revenues	1,137		1,129		2,255	1,715
Total management and advisory services revenues
Segment Reporting
Total revenues	49,288		41,991		95,303	83,324
Total management and advisory services revenues | Operating Segments | RMR LLC
Segment Reporting
Total revenues	48,151		40,242		92,995	80,730
Total management and advisory services revenues | All Other Operations
Segment Reporting
Total revenues	1,137		1,749		2,308	2,594
Reimbursable compensation and benefits
Segment Reporting
Total revenues	13,506		13,159		27,903	26,384
Reimbursable compensation and benefits | Operating Segments | RMR LLC
Segment Reporting
Total revenues	12,917		12,668		26,669	25,457
Reimbursable compensation and benefits | All Other Operations
Segment Reporting
Total revenues	589		491		1,234	927
Reimbursable equity based compensation
Segment Reporting
Total revenues	1,367		1,206		2,965	4,209
Reimbursable equity based compensation | Operating Segments | RMR LLC
Segment Reporting
Total revenues	1,367		1,206		2,965	4,209
Reimbursable equity based compensation | All Other Operations
Segment Reporting
Total revenues	0		0		0	0
Other reimbursable expenses
Segment Reporting
Total revenues	133,493		75,208		253,051	174,593
Other reimbursable expenses | Operating Segments | RMR LLC
Segment Reporting
Total revenues	133,493		75,208		253,051	174,593
Other reimbursable expenses | All Other Operations
Segment Reporting
Total revenues	0		0		0	0
Total reimbursable costs
Segment Reporting
Total revenues	148,366		89,573		283,919	205,186
Total reimbursable costs | Operating Segments | RMR LLC
Segment Reporting
Total revenues	147,777		89,082		282,685	204,259
Total reimbursable costs | All Other Operations
Segment Reporting
Total revenues	589		491		1,234	927
Incentive business management fees
Segment Reporting
Total revenues	$ 0		620		$ 0	620
Incentive business management fees | Operating Segments | RMR LLC
Segment Reporting
Total revenues			0			0
Incentive business management fees | All Other Operations
Segment Reporting
Total revenues			$ 620			$ 620