Summary of Significant Accounting Policies - Additional Information (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021 USD ($) joint_venture property segment $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) shares	Jun. 05, 2015 USD ($) shares
Schedule of Equity Method Investments [Line Items]
Number of joint ventures | joint_venture	2
Number of properties (in properties) | property	178
Unamortized discount (premium), net	$ 15,232
Common shares of beneficial interest, par value (in dollars per share) | $ / shares	$ 0.01	$ 0.01
Antidilutive securities (in shares) | shares	34,000	14,000	12,000
Number of business segments | segment	1
Joint Venture Property 1
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	51.00%
Joint Venture Property 2
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	50.00%
RMR Inc
Schedule of Equity Method Investments [Line Items]
Equity securities, FV-NI (in shares) | shares				1,541,201
Equity securities, FV-NI, cost				$ 17,462
Unconsolidated Joint Ventures
Schedule of Equity Method Investments [Line Items]
Number of joint ventures | joint_venture	2
Number of properties (in properties) | property	3
Mortgage indebtedness	$ 82,000
Unconsolidated Joint Ventures
Schedule of Equity Method Investments [Line Items]
Number of joint ventures | joint_venture	2
Number of properties (in properties) | property	3
Unconsolidated Joint Ventures | Joint Venture Property 1
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	51.00%
Unconsolidated Joint Ventures | Joint Venture Property 2
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	50.00%
RMR Inc
Schedule of Equity Method Investments [Line Items]
Equity securities, FV-NI				$ 39,833
Amortization of liability	$ 1,087	$ 1,087	$ 1,087
Amortization of liability, year one	1,087
Amortization of liability, year two	1,087
Amortization of liability, year three	1,087
Amortization of liability, year four	1,087
Amortization of liability, year five	1,087
Amortization of liability, thereafter	$ 9,797