Related Person Transactions - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended
	Jun. 30, 2020	Feb. 13, 2020
Five Star
Related Party Transaction [Line Items]
Ownership percentage	6.40%
SVC
Related Party Transaction [Line Items]
Ownership percentage	1.10%
ILPT
Related Party Transaction [Line Items]
Ownership percentage	1.20%
OPI
Related Party Transaction [Line Items]
Ownership percentage	1.50%
DHC
Related Party Transaction [Line Items]
Ownership percentage	1.10%
TA
Related Party Transaction [Line Items]
Ownership percentage	4.00%
RMRM
Related Party Transaction [Line Items]
Ownership percentage	2.30%
TRMT
Related Party Transaction [Line Items]
Ownership percentage	19.50%
ABP Trust | Centre Street
Related Party Transaction [Line Items]
Ownership percentage	100.00%
ABP Trust | Open End Fund
Related Party Transaction [Line Items]
Ownership percentage	100.00%
Shares owned (in shares)	206,300
ABP Trust | AIC
Related Party Transaction [Line Items]
Ownership percentage		14.30%
ABP Trust | Class A Common Stock
Related Party Transaction [Line Items]
Common stock shares outstanding (in shares)	147,502
ABP Trust | Class A Units
Related Party Transaction [Line Items]
Common stock shares outstanding (in shares)	15,000,000
RMR LLC | TA
Related Party Transaction [Line Items]
Ownership percentage	3.60%
RMR LLC | Open End Fund
Related Party Transaction [Line Items]
Shares owned (in shares)	0
RMR LLC | Open End Fund
Related Party Transaction [Line Items]
Committed capital	$ 100,000