Segment Reporting (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2018 USD ($) segment	Mar. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)
Segment Reporting
Number of reportable business segments | segment			1
Revenues:
Management services	$ 46,559	$ 43,258	$ 95,129	$ 85,985
Incentive business management fees	0	0	155,881	52,407
Reimbursable payroll related and other costs	11,657	10,034	24,365	19,184
Advisory services	1,065	1,004	2,447	2,014
Total revenues	59,281	54,296	277,822	159,590
Expenses:
Compensation and benefits	28,073	22,983	54,270	45,287
Equity based compensation	1,217	1,566	3,938	2,494
Separation costs	136	0	136	0
Total compensation and benefits expense	29,426	24,549	58,344	47,781
General and administrative	7,024	6,453	13,730	12,294
Transaction and acquisition related costs	0	693	142	693
Depreciation and amortization	372	528	752	1,083
Total expenses	36,822	32,223	72,968	61,851
Operating income (loss)	22,459	22,073	204,854	97,739
Interest and other income	1,076	450	1,860	657
Tax receivable agreement remeasurement	0	0	24,710	0
Income before income tax expense and equity in earnings (losses) of investee	23,535	22,523	231,424	98,396
Income tax expense	(3,681)	(4,610)	(52,024)	(20,283)
Equity in earnings (losses) of investees	(212)	(165)	(434)	(165)
Net income (loss)	19,642	17,748	178,966	77,948
Total Assets	524,961	395,589	524,961	395,589	$ 383,719
Intersegment Eliminations
Revenues:
Total revenues	954	120	1,907	651
RMR LLC
Revenues:
Management services	46,559	43,258	95,129	85,985
Incentive business management fees			155,881	52,407
Reimbursable payroll related and other costs	11,026	10,034	23,115	19,184
Advisory services	0	0	0	0
Total revenues	57,585	53,292	274,125	157,576
Expenses:
Compensation and benefits	26,620	22,500	51,368	44,260
Equity based compensation	1,204	1,566	3,911	2,494
Separation costs	136		136
Total compensation and benefits expense	27,960	24,066	55,415	46,754
General and administrative	6,022	6,099	11,678	11,789
Transaction and acquisition related costs		693	0	693
Depreciation and amortization	350	382	708	751
Total expenses	34,332	31,240	67,801	59,987
Operating income (loss)	23,253	22,052	206,324	97,589
Interest and other income	999	181	1,725	280
Tax receivable agreement remeasurement			0
Income before income tax expense and equity in earnings (losses) of investee	24,252	22,233	208,049	97,869
Income tax expense	0	0	0	0
Equity in earnings (losses) of investees	13	0	34	0
Net income (loss)	24,265	22,233	208,083	97,869
Total Assets	463,931	335,364	463,931	335,364
All Other Operations
Revenues:
Management services	0	0	0	0
Incentive business management fees			0	0
Reimbursable payroll related and other costs	631	0	1,250	0
Advisory services	1,065	1,004	2,447	2,014
Total revenues	1,696	1,004	3,697	2,014
Expenses:
Compensation and benefits	1,453	483	2,902	1,027
Equity based compensation	13	0	27	0
Separation costs	0		0
Total compensation and benefits expense	1,466	483	2,929	1,027
General and administrative	1,002	354	2,052	505
Transaction and acquisition related costs		0	142	0
Depreciation and amortization	22	146	44	332
Total expenses	2,490	983	5,167	1,864
Operating income (loss)	(794)	21	(1,470)	150
Interest and other income	77	269	135	377
Tax receivable agreement remeasurement			24,710
Income before income tax expense and equity in earnings (losses) of investee	(717)	290	23,375	527
Income tax expense	(3,681)	(4,610)	(52,024)	(20,283)
Equity in earnings (losses) of investees	(225)	(165)	(468)	(165)
Net income (loss)	(4,623)	(4,485)	(29,117)	(19,921)
Total Assets	$ 61,030	$ 60,225	$ 61,030	$ 60,225