Segment Reporting (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Segment Reporting
Number of segments | segment									1
Revenues:
Total revenues	$ 150,103	$ 138,649	$ 140,861	$ 159,892	$ 159,244	$ 143,715	$ 130,096	$ 280,313	$ 589,505	$ 713,368	$ 404,979
Expenses:
Compensation and benefits									121,386	114,529	108,763
Equity based compensation									7,828	9,040	10,423
Separation costs									1,881	7,050	3,730
Compensation and benefits expense									131,095	130,619	122,916
General and administrative									26,514	28,706	27,149
Other client company reimbursable expenses									360,572	354,540	0
Transaction and acquisition related costs									1,618	698	1,697
Depreciation and amortization									968	1,017	1,248
Total expenses									520,767	515,580	153,010
Operating income (loss)									68,738	197,788	251,969
Interest and other income									4,451	8,770	4,546
Tax receivable agreement remeasurement									0	0	24,710
Impairment loss on Tremont Mortgage Trust investment									0	(6,213)	(4,359)
Equity in losses of investees									1,545	719	(578)
Unrealized gain (loss) on equity method investment accounted for under the fair value option									3,151	(4,700)	0
Income (loss) before income tax expense									77,885	196,364	276,288
Income tax expense									(11,552)	(27,320)	(58,862)
Net income (loss)	14,393	$ 15,395	$ 14,921	$ 21,624	18,883	$ 13,373	$ 18,708	$ 118,080	66,333	169,044	217,426
Total assets	690,253				660,832				690,253	660,832	499,039
Operating segments | RMR LLC
Revenues:
Total revenues									584,169	707,331	397,084
Expenses:
Compensation and benefits									113,758	107,562	102,736
Equity based compensation									7,680	8,862	10,310
Separation costs									1,881	7,050	2,946
Compensation and benefits expense									123,319	123,474	115,992
General and administrative									22,843	25,026	23,397
Other client company reimbursable expenses									360,572	354,540
Transaction and acquisition related costs									485	698	1,555
Depreciation and amortization									921	966	1,161
Total expenses									508,140	504,704	142,105
Operating income (loss)									76,029	202,627	254,979
Interest and other income									4,079	7,831	4,170
Tax receivable agreement remeasurement											0
Impairment loss on Tremont Mortgage Trust investment										0	0
Equity in losses of investees									0	0	33
Unrealized gain (loss) on equity method investment accounted for under the fair value option									3,151	(4,700)
Income (loss) before income tax expense									83,259	205,758	259,182
Income tax expense									0	0	0
Net income (loss)									83,259	205,758	259,182
Total assets	643,318				606,650				643,318	606,650	442,156
Operating segments | All other operations
Revenues:
Total revenues									5,336	6,037	7,895
Expenses:
Compensation and benefits									7,628	6,967	6,027
Equity based compensation									148	178	113
Separation costs									0	0	784
Compensation and benefits expense									7,776	7,145	6,924
General and administrative									3,671	3,680	3,752
Other client company reimbursable expenses									0	0
Transaction and acquisition related costs									1,133	0	142
Depreciation and amortization									47	51	87
Total expenses									12,627	10,876	10,905
Operating income (loss)									(7,291)	(4,839)	(3,010)
Interest and other income									372	939	376
Tax receivable agreement remeasurement											24,710
Impairment loss on Tremont Mortgage Trust investment										(6,213)	(4,359)
Equity in losses of investees									1,545	719	(611)
Unrealized gain (loss) on equity method investment accounted for under the fair value option									0	0
Income (loss) before income tax expense									(5,374)	(9,394)	17,106
Income tax expense									(11,552)	(27,320)	(58,862)
Net income (loss)									(16,926)	(36,714)	(41,756)
Total assets	$ 46,935				$ 54,182				46,935	54,182	56,883
Intersegment Eliminations
Revenues:
Revenues									5,091	3,975
Total revenues											4,002
Total management and advisory services revenues
Revenues:
Revenues									171,677	301,338	351,827
Total management and advisory services revenues | Operating segments | RMR LLC
Revenues:
Revenues									167,949	297,975	346,420
Total management and advisory services revenues | Operating segments | All other operations
Revenues:
Revenues									3,728	3,363	5,407
Management services
Revenues:
Revenues									168,766	178,075	191,594
Management services | Operating segments | RMR LLC
Revenues:
Revenues									167,949	177,881	190,539
Management services | Operating segments | All other operations
Revenues:
Revenues									817	194	1,055
Incentive business management fees
Revenues:
Revenues									0	120,094	155,881
Incentive business management fees | Operating segments | RMR LLC
Revenues:
Revenues										120,094	155,881
Incentive business management fees | Operating segments | All other operations
Revenues:
Revenues										0	0
Advisory services
Revenues:
Revenues									2,911	3,169	4,352
Advisory services | Operating segments | RMR LLC
Revenues:
Revenues									0	0	0
Advisory services | Operating segments | All other operations
Revenues:
Revenues									2,911	3,169	4,352
Total reimbursable costs
Revenues:
Revenues									417,828	412,030	53,152
Total reimbursable costs | Operating segments | RMR LLC
Revenues:
Revenues									416,220	409,356	50,664
Total reimbursable costs | Operating segments | All other operations
Revenues:
Revenues									1,608	2,674	2,488
Reimbursable compensation and benefits
Revenues:
Revenues									57,256	57,490	53,152
Reimbursable compensation and benefits | Operating segments | RMR LLC
Revenues:
Revenues									55,648	54,816	50,664
Reimbursable compensation and benefits | Operating segments | All other operations
Revenues:
Revenues									1,608	2,674	2,488
Other client company reimbursable expenses
Revenues:
Revenues									360,572	354,540	$ 0
Other client company reimbursable expenses | Operating segments | RMR LLC
Revenues:
Revenues									360,572	354,540
Other client company reimbursable expenses | Operating segments | All other operations
Revenues:
Revenues									$ 0	$ 0