Related Party Transactions - Amount Due from Related Parties (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Related Party Transaction [Line Items]
Due from related parties	$ 37,029	$ 32,712
Managed Equity REITs
Related Party Transaction [Line Items]
Due from related parties	34,545	31,238
GOV
Related Party Transaction [Line Items]
Due from related parties	7,870	6,369
HPT
Related Party Transaction [Line Items]
Due from related parties	8,391	7,968
ILPT
Related Party Transaction [Line Items]
Due from related parties	2,692	0
SIR
Related Party Transaction [Line Items]
Due from related parties	5,887	7,351
SNH
Related Party Transaction [Line Items]
Due from related parties	9,705	9,550
Managed Operators
Related Party Transaction [Line Items]
Due from related parties	910	750
Five Star
Related Party Transaction [Line Items]
Due from related parties	281	305
Sonesta
Related Party Transaction [Line Items]
Due from related parties	30	1
TA
Related Party Transaction [Line Items]
Due from related parties	599	444
Other
Related Party Transaction [Line Items]
Due from related parties	1,574	724
ABP Trust
Related Party Transaction [Line Items]
Due from related parties	383	551
ABP Trust
Related Party Transaction [Line Items]
Due from related parties	20	22
Private Fund
Related Party Transaction [Line Items]
Due from related parties	608	0
AIC
Related Party Transaction [Line Items]
Due from related parties	31	36
TRMT
Related Party Transaction [Line Items]
Due from related parties	$ 532	$ 115