Related Person Transactions - Amount Due from Related Parties (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Sep. 30, 2021
Related Party Transaction [Line Items]
Accounts receivable	$ 27,929	$ 27,470
Reimbursable costs	93,527	75,522
Total	121,456	102,992
Managed Public Real Estate Capital
Related Party Transaction [Line Items]
Accounts receivable	23,727	25,122
Reimbursable costs	72,150	68,659
Total	95,877	93,781
Managed Equity REITs
Related Party Transaction [Line Items]
Accounts receivable	22,608	23,405
Reimbursable costs	71,052	67,479
Total	93,660	90,884
DHC
Related Party Transaction [Line Items]
Accounts receivable	4,815	6,005
Reimbursable costs	20,478	17,866
Total	25,293	23,871
ILPT
Related Party Transaction [Line Items]
Accounts receivable	4,767	2,934
Reimbursable costs	7,362	6,928
Total	12,129	9,862
OPI
Related Party Transaction [Line Items]
Accounts receivable	7,958	8,625
Reimbursable costs	39,539	33,693
Total	47,497	42,318
SVC
Related Party Transaction [Line Items]
Accounts receivable	5,068	5,841
Reimbursable costs	3,673	8,992
Total	8,741	14,833
SEVN
Related Party Transaction [Line Items]
Accounts receivable	1,119	1,717
Reimbursable costs	1,098	1,180
Total	2,217	2,897
Managed Private Real Estate Capital
Related Party Transaction [Line Items]
Accounts receivable	3,763	2,071
Reimbursable costs	6,777	3,448
Total	10,540	5,519
ABP Trust
Related Party Transaction [Line Items]
Accounts receivable	1,116	1,202
Reimbursable costs	2,081	2,678
Total	3,197	3,880
Other private entities
Related Party Transaction [Line Items]
Accounts receivable	2,647	869
Reimbursable costs	4,696	770
Total	7,343	1,639
Managed Operating Companies
Related Party Transaction [Line Items]
Accounts receivable	439	277
Reimbursable costs	14,600	3,415
Total	15,039	3,692
ALR
Related Party Transaction [Line Items]
Accounts receivable	114	136
Reimbursable costs	1,819	422
Total	1,933	558
Sonesta
Related Party Transaction [Line Items]
Accounts receivable	240	17
Reimbursable costs	0	0
Total	240	17
TA
Related Party Transaction [Line Items]
Accounts receivable	85	124
Reimbursable costs	12,781	2,993
Total	$ 12,866	$ 3,117