Business and Property Management Agreements with RMR LLC (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Jan. 31, 2018 USD ($)	Sep. 30, 2018 USD ($) employee agreement	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) employee agreement	Sep. 30, 2017 USD ($)
Real Estate Properties [Line Items]
Number of employees | employee		0		0
RMR LLC | Internal audit expense
Real Estate Properties [Line Items]
Related party costs and expenses		$ 49	$ 67	$ 173	$ 202
RMR LLC | Amended and restate business management agreement
Real Estate Properties [Line Items]
Number of management service agreements | agreement		2		2
Business management fees incurred		$ 10,430	10,865	$ 30,048	68,526
Estimated incentive fees recorded			873		38,243
Related Party Transaction, Estimated Incentive Fee Calculation Period				3 years
Incentive fees paid	$ 74,573
Related party property management and construction management fee		18	12	$ 43	33
Related party reimbursement expenses		$ 56	$ 40	$ 167	$ 131