Related Person Transactions - Schedule of Revenues from Related Parties (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Related Party Transaction [Line Items]
Total revenues	$ 44,068	$ 49,266	$ 136,965	$ 145,430
Total reimbursable costs	107,950	156,213	396,958	539,469
Income from loan investments, net	677	0	1,869	0
Rental property revenues	2,033	191	5,080	415
Total revenues	154,728	205,670	540,872	685,314
Total revenues from related parties
Related Party Transaction [Line Items]
Total revenues	44,068	49,266	136,965	145,430
Total reimbursable costs	107,950	156,213	396,958	539,469
Total revenues	152,018	205,479	533,923	684,899
Perpetual Capital
Related Party Transaction [Line Items]
Total revenues	31,189	34,168	96,275	106,436
Total reimbursable costs	91,106	133,990	333,066	478,851
Total revenues	122,295	168,158	429,341	585,287
Total Managed Equity REITs
Related Party Transaction [Line Items]
Total revenues	29,829	32,657	92,541	102,297
Total reimbursable costs	89,842	132,589	329,003	474,438
Total revenues	119,671	165,246	421,544	576,735
DHC
Related Party Transaction [Line Items]
Total revenues	5,292	5,847	17,318	18,257
Total reimbursable costs	21,453	24,175	86,747	96,180
Total revenues	26,745	30,022	104,065	114,437
ILPT
Related Party Transaction [Line Items]
Total revenues	9,135	9,073	27,503	27,403
Total reimbursable costs	10,727	8,338	29,349	27,953
Total revenues	19,862	17,411	56,852	55,356
OPI
Related Party Transaction [Line Items]
Total revenues	5,781	7,307	18,188	23,494
Total reimbursable costs	38,762	52,386	120,061	168,677
Total revenues	44,543	59,693	138,249	192,171
SVC
Related Party Transaction [Line Items]
Total revenues	9,621	10,430	29,532	33,143
Total reimbursable costs	18,900	47,690	92,846	181,628
Total revenues	28,521	58,120	122,378	214,771
SEVN
Related Party Transaction [Line Items]
Total revenues	1,360	1,511	3,734	4,139
Total reimbursable costs	1,264	1,401	4,063	4,413
Total revenues	2,624	2,912	7,797	8,552
Private Capital
Related Party Transaction [Line Items]
Total revenues	12,879	15,098	40,690	38,994
Total reimbursable costs	16,844	22,223	63,892	60,618
Total revenues	29,723	37,321	104,582	99,612
AlerisLife
Related Party Transaction [Line Items]
Total revenues	1,452	1,442	4,273	4,275
Total reimbursable costs	0	0	0	0
Total revenues	1,452	1,442	4,273	4,275
Sonesta
Related Party Transaction [Line Items]
Total revenues	2,628	2,624	6,873	6,847
Total reimbursable costs	0	0	0	0
Total revenues	2,628	2,624	6,873	6,847
RMR Residential
Related Party Transaction [Line Items]
Total revenues	3,454	5,688	13,878	11,864
Total reimbursable costs	5,110	7,924	18,499	16,249
Total revenues	8,564	13,612	32,377	28,113
Other private entities
Related Party Transaction [Line Items]
Total revenues	5,345	5,344	15,666	16,008
Total reimbursable costs	11,734	14,299	45,393	44,369
Total revenues	17,079	19,643	61,059	60,377
Total revenues from unrelated parties
Related Party Transaction [Line Items]
Total revenues	0	0	0	0
Total reimbursable costs	0	0	0	0
Total revenues	$ 2,710	$ 191	$ 6,949	$ 415

RMR GROUP INC. 10-Q