Related Person Transactions - Amount Due from Related Parties (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Sep. 30, 2023
Related Party Transaction [Line Items]
Accounts receivable	$ 32,477	$ 29,965
Reimbursable costs	90,516	88,619
Total	122,993	118,584
Perpetual Capital
Related Party Transaction [Line Items]
Accounts receivable	22,440	25,168
Reimbursable costs	84,625	81,559
Total	107,065	106,727
Managed Equity REITs
Related Party Transaction [Line Items]
Accounts receivable	20,389	23,505
Reimbursable costs	82,690	79,638
Total	103,079	103,143
DHC
Related Party Transaction [Line Items]
Accounts receivable	4,764	5,953
Reimbursable costs	17,824	13,434
Total	22,588	19,387
ILPT
Related Party Transaction [Line Items]
Accounts receivable	3,658	4,597
Reimbursable costs	6,024	5,869
Total	9,682	10,466
OPI
Related Party Transaction [Line Items]
Accounts receivable	6,565	7,427
Reimbursable costs	50,114	51,912
Total	56,679	59,339
SVC
Related Party Transaction [Line Items]
Accounts receivable	5,402	5,528
Reimbursable costs	8,728	8,423
Total	14,130	13,951
SEVN
Related Party Transaction [Line Items]
Accounts receivable	2,051	1,663
Reimbursable costs	1,935	1,921
Total	3,986	3,584
Private Capital
Related Party Transaction [Line Items]
Accounts receivable	10,037	4,797
Reimbursable costs	5,891	7,060
Total	15,928	11,857
AlerisLife
Related Party Transaction [Line Items]
Accounts receivable	91	74
Reimbursable costs	0	0
Total	91	74
Sonesta
Related Party Transaction [Line Items]
Accounts receivable	87	89
Reimbursable costs	0	0
Total	87	89
RMR Residential
Related Party Transaction [Line Items]
Accounts receivable	6,999	0
Reimbursable costs	0	0
Total	6,999	0
Other private entities
Related Party Transaction [Line Items]
Accounts receivable	2,860	4,634
Reimbursable costs	5,891	7,060
Total	$ 8,751	$ 11,694