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Business and Property Management Agreements with RMR LLC (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) employee management_agreement	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) employee management_agreement	Sep. 30, 2019 USD ($)	Dec. 31, 2020
Related person transactions
Number of employees | employee	0		0
Property management and construction supervision fees	$ 3,477,000	$ 3,289,000	$ 10,076,000	$ 9,844,000
Property management and construction supervision fees paid	3,571,000	3,419,000	10,433,000	10,145,000
RMR LLC
Related person transactions
Business management fees incurred	5,004,000	6,616,000	15,614,000	20,917,000
Related party transaction business management fees related to subsidiary level management	725,000	725,000	2,175,000	2,175,000
Related party transaction business management agreement, annual incentive fee estimate included in management fees	$ 0	$ 0	$ 0	$ 0
Senior Living Communities | RMR Inc
Related person transactions
Number of management agreements | management_agreement	2		2
Subsequent Event | Scenario, Forecast | RMR LLC
Related person transactions
Period of actual amount of incentive fees					3 years

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