Segment Reporting (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segment	Jun. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)
Segment Reporting
Number of reportable business segments | segment			1
Revenues:
Total revenues	$ 62,084	$ 55,502	$ 339,906	$ 215,092
Expenses:
Compensation and benefits	28,606	23,731	82,876	68,746
Equity based compensation	2,347	1,038	6,285	3,804
Separation costs	1,739	0	1,875	0
Total compensation and benefits expense	32,692	24,769	91,036	72,550
General and administrative	6,551	6,779	20,281	19,073
Transaction and acquisition related costs	775	1,760	917	2,453
Depreciation and amortization	244	467	996	1,550
Total expenses	40,262	33,775	113,230	95,626
Operating income (loss)	21,822	21,727	226,676	119,466
Interest and other income	1,223	402	3,083	1,059
Tax receivable agreement remeasurement	0	0	24,710	0
Income before income tax expense and equity in earnings (losses) of investee	23,045	22,129	254,469	120,525
Income tax expense	(3,462)	(4,528)	(55,486)	(24,811)
Equity in earnings (losses) of investees	(134)	4	(568)	(161)
Net income (loss)	19,449	17,605	198,415	95,553
Total Assets	527,609	403,988	527,609	403,988	$ 383,719
Intersegment Eliminations
Revenues:
Total revenues	995	221	2,972	872
RMR LLC
Revenues:
Total revenues	60,406	54,483	334,531	212,059
Expenses:
Compensation and benefits	27,047	23,050	78,415	67,038
Equity based compensation	2,333	1,038	6,244	3,804
Separation costs	1,739		1,875
Total compensation and benefits expense	31,119	24,088	86,534	70,842
General and administrative	5,665	6,297	17,343	18,086
Transaction and acquisition related costs	775	1,760	775	2,453
Depreciation and amortization	222	321	931	1,072
Total expenses	37,781	32,466	105,583	92,453
Operating income (loss)	22,625	22,017	228,948	119,606
Interest and other income	1,085	377	2,810	657
Tax receivable agreement remeasurement			0
Income before income tax expense and equity in earnings (losses) of investee	23,710	22,394	231,758	120,263
Income tax expense	0	0	0	0
Equity in earnings (losses) of investees	(2)	0	33	0
Net income (loss)	23,708	22,394	231,791	120,263
Total Assets	460,596	344,738	460,596	344,738
All Other Operations
Revenues:
Total revenues	1,678	1,019	5,375	3,033
Expenses:
Compensation and benefits	1,559	681	4,461	1,708
Equity based compensation	14	0	41	0
Separation costs	0		0
Total compensation and benefits expense	1,573	681	4,502	1,708
General and administrative	886	482	2,938	987
Transaction and acquisition related costs	0	0	142	0
Depreciation and amortization	22	146	65	478
Total expenses	2,481	1,309	7,647	3,173
Operating income (loss)	(803)	(290)	(2,272)	(140)
Interest and other income	138	25	273	402
Tax receivable agreement remeasurement			24,710
Income before income tax expense and equity in earnings (losses) of investee	(665)	(265)	22,711	262
Income tax expense	(3,462)	(4,528)	(55,486)	(24,811)
Equity in earnings (losses) of investees	(132)	4	(601)	(161)
Net income (loss)	(4,259)	(4,789)	(33,376)	(24,710)
Total Assets	67,013	59,250	67,013	59,250
Management services
Revenues:
Revenue	47,328	44,644	142,457	130,629
Management services | RMR LLC
Revenues:
Revenue	47,328	44,644	142,457	130,629
Management services | All Other Operations
Revenues:
Revenue	0	0	0	0
Incentive business management fees
Revenues:
Revenue	0	0	155,881	52,407
Incentive business management fees | RMR LLC
Revenues:
Revenue			155,881	52,407
Incentive business management fees | All Other Operations
Revenues:
Revenue			0	0
Reimbursable payroll related and other costs
Revenues:
Revenue	13,711	9,839	38,076	29,023
Reimbursable payroll related and other costs | RMR LLC
Revenues:
Revenue	13,078	9,839	36,193	29,023
Reimbursable payroll related and other costs | All Other Operations
Revenues:
Revenue	633	0	1,883	0
Advisory services
Revenues:
Revenue	1,045	1,019	3,492	3,033
Advisory services | RMR LLC
Revenues:
Revenue	0	0	0	0
Advisory services | All Other Operations
Revenues:
Revenue	$ 1,045	$ 1,019	$ 3,492	$ 3,033