Basis of Presentation (Details)	6 Months Ended
	Mar. 31, 2021 real_estate_investment_trust company subsidiary shares
Related Party Transaction [Line Items]
Number of managed trusts | real_estate_investment_trust	4
Number of real estate operating companies | company	3
Wholly owned subsidiaries | subsidiary	2
RMR LLC
Related Party Transaction [Line Items]
Ownership percentage	52.20%
Class B membership units
Related Party Transaction [Line Items]
Membership units (in units)	1,000,000
Tremont Advisors
Related Party Transaction [Line Items]
Number of managed trusts | real_estate_investment_trust	2
Class A Common Stock | Class A membership units
Related Party Transaction [Line Items]
Membership units (in units)	15,413,191
Capital Unit Redeemable Class A Units | ABP Trust
Related Party Transaction [Line Items]
Membership units (in units)	15,000,000
Capital Unit Redeemable Class A Units | ABP Trust | RMR LLC
Related Party Transaction [Line Items]
Ownership percentage	47.80%