Shareholders’ Equity - Distributions (Details) - USD ($) $ / shares in Units, $ in Thousands														3 Months Ended												12 Months Ended
	Oct. 09, 2025	Aug. 14, 2025	May 15, 2025	Feb. 20, 2025	Nov. 14, 2024	Aug. 15, 2024	May 16, 2024	Feb. 15, 2024	Nov. 16, 2023	Aug. 17, 2023	May 18, 2023	Feb. 16, 2023	Nov. 17, 2022	Sep. 30, 2025	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Common class A and B1
Class of Stock [Line Items]
Dividends paid (in dollars per share)														$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.40	$ 0.40	$ 0.40	$ 0.40	$ 0.40	$ 0.40	$ 1.80	$ 1.70	$ 1.60
Value of dividends														$ 7,591	$ 7,595	$ 7,580	$ 7,581	$ 7,526	$ 7,529	$ 6,684	$ 6,684	$ 6,645	$ 6,648	$ 6,641	$ 6,642	$ 30,347	$ 28,423	$ 26,576
Common class A and B1 | Subsequent Event
Class of Stock [Line Items]
Value of dividends	$ 7,679
Common distributions declared (in dollars per share)	$ 0.45
Membership Units | Subsequent Event | RMR LLC
Class of Stock [Line Items]
Shares owned (in shares)	17,063,495,000
Membership Units | RMR LLC
Class of Stock [Line Items]
Dividends paid (in dollars per share)		$ 0.32	$ 0.32	$ 0.32	$ 0.32	$ 0.32	$ 0.32	$ 0.32	$ 0.32	$ 0.32	$ 0.32	$ 0.32	$ 0.32													$ 1.28	$ 1.28	$ 1.28
Value of dividends		$ 10,198	$ 10,201	$ 10,190	$ 10,191	$ 10,152	$ 10,154	$ 10,147	$ 10,148	$ 10,116	$ 10,118	$ 10,113	$ 10,114													$ 40,780	$ 40,601	$ 40,461
Membership Units | RMR LLC | Subsequent Event
Class of Stock [Line Items]
Value of dividends	$ 10,260
Common distributions declared (in dollars per share)	$ 0.32
Membership Units | RMR LLC | Abp Trust
Class of Stock [Line Items]
Value of dividends		4,800	4,800	4,800	4,800	4,800	4,800	4,800	4,800	4,800	4,800	4,800	4,800													19,200	19,200	19,200
Membership Units | RMR LLC | Abp Trust | Subsequent Event
Class of Stock [Line Items]
Value of dividends	$ 4,800
Membership Units | RMR LLC | RMR, Inc
Class of Stock [Line Items]
Value of dividends		$ 5,398	$ 5,401	$ 5,390	$ 5,391	$ 5,352	$ 5,354	$ 5,347	$ 5,348	$ 5,316	$ 5,318	$ 5,313	$ 5,314													$ 21,580	$ 21,401	$ 21,261
Membership Units | RMR LLC | RMR, Inc | Subsequent Event
Class of Stock [Line Items]
Value of dividends	$ 5,460
Capital Unit Redeemable Class A Units | Abp Trust
Class of Stock [Line Items]
Membership units (in shares)														15,000,000												15,000,000