Segment Reporting (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Segment Reporting
Number of segments | segment									1
Revenues:
Total revenues	$ 159,244	$ 143,715	$ 130,096	$ 280,313	$ 65,073	$ 62,084	$ 59,281	$ 218,541	$ 713,368	$ 404,979	$ 271,728
Expenses:
Compensation and benefits									114,529	108,763	92,625
Equity based compensation									9,040	10,423	7,128
Separation costs									7,050	3,730	0
Total compensation and benefits expense									130,619	122,916	99,753
General and administrative									28,706	27,149	25,189
Reimbursed Expense									354,540	0	0
Transaction and acquisition related costs									698	1,697	9,187
Depreciation and amortization									1,017	1,248	2,038
Total expenses									515,580	153,010	136,167
Operating income (loss)									197,788	251,969	135,561
Interest and other income									8,770	4,546	1,565
Tax receivable agreement remeasurement									0	24,710	0
Impairment loss on Tremont Mortgage Trust investment									(6,213)	(4,359)	0
Equity in losses of investees									719	(578)	(206)
Unrealized loss on equity method investment accounted for under the fair value option									(4,700)	0	0
Income (loss) before income tax expense									196,364	276,288	136,920
Income tax expense									(27,320)	(58,862)	(28,251)
Net income (loss)	18,883	$ 13,373	$ 18,708	118,080	19,011	$ 19,449	$ 19,642	159,324	169,044	217,426	108,669
Total assets	667,872				504,428				667,872	504,428	383,719
Operating segments | RMR LLC
Revenues:
Total revenues									707,525	398,139	267,573
Expenses:
Compensation and benefits									107,562	102,736	89,688
Equity based compensation									8,862	10,310	7,128
Separation costs									7,050	2,946
Total compensation and benefits expense									123,474	115,992	96,816
General and administrative									25,026	23,397	23,538
Reimbursed Expense									354,540
Transaction and acquisition related costs									698	1,555	337
Depreciation and amortization									966	1,161	1,415
Total expenses									504,704	142,105	122,106
Operating income (loss)									202,821	256,034	145,467
Interest and other income									7,831	4,170	1,130
Tax receivable agreement remeasurement										0
Impairment loss on Tremont Mortgage Trust investment									0	0
Equity in losses of investees									0	33	0
Unrealized loss on equity method investment accounted for under the fair value option									(4,700)
Income (loss) before income tax expense									205,952	260,237	146,597
Income tax expense									0	0	0
Net income (loss)									205,952	260,237	146,597
Total assets	606,844				443,211				606,844	443,211	308,018
Operating segments | All other operations
Revenues:
Total revenues									5,843	6,840	4,155
Expenses:
Compensation and benefits									6,967	6,027	2,937
Equity based compensation									178	113	0
Separation costs									0	784
Total compensation and benefits expense									7,145	6,924	2,937
General and administrative									3,680	3,752	1,651
Reimbursed Expense									0
Transaction and acquisition related costs									0	142	8,850
Depreciation and amortization									51	87	623
Total expenses									10,876	10,905	14,061
Operating income (loss)									(5,033)	(4,065)	(9,906)
Interest and other income									939	376	435
Tax receivable agreement remeasurement										24,710
Impairment loss on Tremont Mortgage Trust investment									(6,213)	(4,359)
Equity in losses of investees									719	(611)	(206)
Unrealized loss on equity method investment accounted for under the fair value option									0
Income (loss) before income tax expense									(9,588)	16,051	(9,677)
Income tax expense									(27,320)	(58,862)	(28,251)
Net income (loss)									(36,908)	(42,811)	(37,928)
Total assets	$ 61,028				$ 61,217				61,028	61,217	75,701
Intersegment Eliminations
Revenues:
Total revenues									3,975	4,002	738
Total management and advisory services revenues
Revenues:
Revenues									301,338	351,827	231,396
Total management and advisory services revenues | Operating segments | RMR LLC
Revenues:
Revenues									298,169	347,475	227,294
Total management and advisory services revenues | Operating segments | All other operations
Revenues:
Revenues									3,169	4,352	4,102
Management services
Revenues:
Revenues									178,075	191,594	174,887
Management services | Operating segments | RMR LLC
Revenues:
Revenues									178,075	191,594	174,887
Management services | Operating segments | All other operations
Revenues:
Revenues									0	0	0
Incentive business management fees
Revenues:
Revenues				$ 120,094				$ 155,881	120,094	155,881	52,407
Incentive business management fees | Operating segments | RMR LLC
Revenues:
Revenues									120,094	155,881	52,407
Incentive business management fees | Operating segments | All other operations
Revenues:
Revenues									0	0	0
Advisory services
Revenues:
Revenues									3,169	4,352	4,102
Advisory services | Operating segments | RMR LLC
Revenues:
Revenues									0	0	0
Advisory services | Operating segments | All other operations
Revenues:
Revenues									3,169	4,352	4,102
Total reimbursable costs
Revenues:
Revenues									412,030	53,152	40,332
Total reimbursable costs | Operating segments | RMR LLC
Revenues:
Revenues									409,356	50,664	40,279
Total reimbursable costs | Operating segments | All other operations
Revenues:
Revenues									2,674	2,488	53
Other client company reimbursable expenses
Revenues:
Revenues									354,540	0	0
Other client company reimbursable expenses | Operating segments | RMR LLC
Revenues:
Revenues									354,540
Other client company reimbursable expenses | Operating segments | All other operations
Revenues:
Revenues									0
Reimbursable compensation and benefits
Revenues:
Revenues									57,490	53,152	40,332
Reimbursable compensation and benefits | Operating segments | RMR LLC
Revenues:
Revenues									54,816	50,664	40,279
Reimbursable compensation and benefits | Operating segments | All other operations
Revenues:
Revenues									$ 2,674	$ 2,488	$ 53