Related Person Transactions - Amount Due from Related Parties (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Related Party Transaction [Line Items]
Due from related parties	$ 90,369	$ 102,759
Managed Equity REITs
Related Party Transaction [Line Items]
Due from related parties	85,459	94,301
DHC
Related Party Transaction [Line Items]
Due from related parties	27,583	25,505
ILPT
Related Party Transaction [Line Items]
Due from related parties	8,880	10,630
OPI
Related Party Transaction [Line Items]
Due from related parties	38,412	39,233
SVC
Related Party Transaction [Line Items]
Due from related parties	10,584	18,933
Managed Operators
Related Party Transaction [Line Items]
Due from related parties	807	565
Five Star
Related Party Transaction [Line Items]
Due from related parties	251	136
Sonesta
Related Party Transaction [Line Items]
Due from related parties	0	37
TA
Related Party Transaction [Line Items]
Due from related parties	556	392
Other Client Companies
Related Party Transaction [Line Items]
Due from related parties	4,103	7,893
ABP Trust
Related Party Transaction [Line Items]
Due from related parties	3,454	2,580
AIC
Related Party Transaction [Line Items]
Due from related parties	7	7
Open End Fund
Related Party Transaction [Line Items]
Due from related parties	0	4,567
RMRM
Related Party Transaction [Line Items]
Due from related parties	0	75
TRMT
Related Party Transaction [Line Items]
Due from related parties	633	664
Centre Street
Related Party Transaction [Line Items]
Due from related parties	$ 9	$ 0