Segment Reporting (Details) $ in Thousands	3 Months Ended						9 Months Ended
	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)
Segment Reporting
Number of reportable business segments | segment							1
Revenues:
Revenue	$ 138,649			$ 143,715			$ 439,402	$ 554,124
Expenses:
Compensation and benefits	29,569			28,530			89,888	85,523
Equity based compensation	1,299			1,334			3,183	4,349
Separation costs	0			239			645	7,050
Total compensation and benefits expense	30,868			30,103			93,716	96,922
General and administrative	6,335			7,670			20,678	22,112
Other client company reimbursable expenses	85,650			85,689			267,852	257,088
Transaction and acquisition related costs	427			42			1,596	273
Depreciation and amortization	229			250			731	762
Total expenses	123,509			123,754			384,573	377,157
Operating income (loss)	15,140			19,961			54,829	176,967
Interest and other income	727			2,408			4,102	6,402
Impairment loss on Tremont Mortgage Trust investment	0			(6,213)			0	(6,213)
Equity in earnings of investees	458			174			1,037	318
Unrealized (loss) gain on equity method investment accounted for under the fair value option	1,678			(731)			916	(2,978)
Income (loss) before income tax expense	18,003			15,599			60,884	174,496
Income tax expense	(2,608)			(2,226)			(8,944)	(24,335)
Net income (loss)	15,395	$ 14,921	$ 21,624	13,373	$ 18,708	$ 118,080	51,940	150,161
Total assets	704,723			660,439			704,723	660,439	$ 660,832
Total management and advisory services revenues
Revenues:
Revenue	39,250			44,443			131,473	256,168
Management services
Revenues:
Revenue	38,625			43,641			129,221	133,729
Incentive business management fees
Revenues:
Revenue	0			0			0	120,094
Advisory services
Revenues:
Revenue	625			802			2,252	2,345
Total reimbursable costs
Revenues:
Revenue	99,399			99,272			307,929	297,956
Reimbursable compensation and benefits
Revenues:
Revenue	13,749			13,583			40,077	40,868
Other client company reimbursable expenses
Revenues:
Revenue	85,650			85,689			267,852	257,088
Operating Segments | RMR LLC
Revenues:
Revenue	137,719			142,275			434,940	549,820
Expenses:
Compensation and benefits	28,203			26,864			84,401	80,800
Equity based compensation	1,255			1,310			3,079	4,270
Separation costs				239			645	7,050
Total compensation and benefits expense	29,458			28,413			88,125	92,120
General and administrative	5,292			6,746			17,731	19,298
Other client company reimbursable expenses	85,650			85,689			267,852	257,088
Transaction and acquisition related costs	317			42			366	273
Depreciation and amortization	217			237			696	723
Total expenses	120,934			121,127			374,770	369,502
Operating income (loss)	16,785			21,148			60,170	180,318
Interest and other income	667			2,185			3,753	5,650
Impairment loss on Tremont Mortgage Trust investment				0				0
Equity in earnings of investees	0			0			0	0
Unrealized (loss) gain on equity method investment accounted for under the fair value option	1,678			(731)			916	(2,978)
Income (loss) before income tax expense	19,130			22,602			64,839	182,990
Income tax expense	0			0			0	0
Net income (loss)	19,130			22,602			64,839	182,990
Total assets	655,413			606,164			655,413	606,164
Operating Segments | RMR LLC | Total management and advisory services revenues
Revenues:
Revenue	38,590			43,604			128,404	253,629
Operating Segments | RMR LLC | Management services
Revenues:
Revenue	38,590			43,604			128,404	133,535
Operating Segments | RMR LLC | Incentive business management fees
Revenues:
Revenue								120,094
Operating Segments | RMR LLC | Advisory services
Revenues:
Revenue	0			0			0	0
Operating Segments | RMR LLC | Total reimbursable costs
Revenues:
Revenue	99,129			98,671			306,536	296,191
Operating Segments | RMR LLC | Reimbursable compensation and benefits
Revenues:
Revenue	13,479			12,982			38,684	39,103
Operating Segments | RMR LLC | Other client company reimbursable expenses
Revenues:
Revenue	85,650			85,689			267,852	257,088
All Other Operations
Revenues:
Revenue	930			1,440			4,462	4,304
Expenses:
Compensation and benefits	1,366			1,666			5,487	4,723
Equity based compensation	44			24			104	79
Separation costs				0			0	0
Total compensation and benefits expense	1,410			1,690			5,591	4,802
General and administrative	1,043			924			2,947	2,814
Other client company reimbursable expenses	0			0			0	0
Transaction and acquisition related costs	110			0			1,230	0
Depreciation and amortization	12			13			35	39
Total expenses	2,575			2,627			9,803	7,655
Operating income (loss)	(1,645)			(1,187)			(5,341)	(3,351)
Interest and other income	60			223			349	752
Impairment loss on Tremont Mortgage Trust investment				(6,213)				(6,213)
Equity in earnings of investees	458			174			1,037	318
Unrealized (loss) gain on equity method investment accounted for under the fair value option	0			0			0	0
Income (loss) before income tax expense	(1,127)			(7,003)			(3,955)	(8,494)
Income tax expense	(2,608)			(2,226)			(8,944)	(24,335)
Net income (loss)	(3,735)			(9,229)			(12,899)	(32,829)
Total assets	49,310			54,275			49,310	54,275
All Other Operations | Total management and advisory services revenues
Revenues:
Revenue	660			839			3,069	2,539
All Other Operations | Management services
Revenues:
Revenue	35			37			817	194
All Other Operations | Incentive business management fees
Revenues:
Revenue								0
All Other Operations | Advisory services
Revenues:
Revenue	625			802			2,252	2,345
All Other Operations | Total reimbursable costs
Revenues:
Revenue	270			601			1,393	1,765
All Other Operations | Reimbursable compensation and benefits
Revenues:
Revenue	270			601			1,393	1,765
All Other Operations | Other client company reimbursable expenses
Revenues:
Revenue	0			0			0	0
Intersegment Eliminations
Revenues:
Revenue	$ (988)			$ (909)			$ (4,153)	$ (2,696)