Summary of Significant Accounting Policies - Additional Information (Details) $ / shares in Units, $ in Thousands			12 Months Ended
	Oct. 02, 2017 property joint_venture	Jun. 05, 2015 USD ($) shares	Dec. 31, 2019 USD ($) segment property joint_venture $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares
Schedule of Equity Method Investments [Line Items]
Number of joint ventures | joint_venture			2
Unamortized discount (premium), net			$ 17,406
Future amortization of liability			1,087
Amortization of liability thereafter			11,971
Increase in rental income to record revenue on straight line basis			27,507	$ 10,164	$ 5,582
Rental income			91,076	45,261	30,998
Tenant reimbursements received			$ 86,353	$ 40,764	$ 27,146
Common shares of beneficial interest, par value (in dollars per share) | $ / shares			$ 0.01	$ 0.01
Antidilutive securities (in shares) | shares			12	4	5
Number of business segments | segment			1
First Potomac Realty Trust
Schedule of Equity Method Investments [Line Items]
Number of joint ventures | joint_venture	2		2
Number of properties | property	3		3
Joint Venture Property 1
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest			50.00%
Joint Venture Property 1 | First Potomac Realty Trust
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	50.00%		51.00%
Joint Venture Property 2
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest			51.00%
Joint Venture Property 2 | First Potomac Realty Trust
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	51.00%		50.00%
Unconsolidated Joint Ventures
Schedule of Equity Method Investments [Line Items]
Number of properties | property			3
Mortgage indebtedness			$ 82,000
Continuing Operations
Schedule of Equity Method Investments [Line Items]
Number of properties | property			189
Increase in rental income to record revenue on straight line basis			$ 27,507	$ 10,164	$ 5,582
Straight line rent receivables			54,837	34,006
RMR Inc
Schedule of Equity Method Investments [Line Items]
Equity investment shares acquired (in shares) | shares		1,541,201
Payments to acquire equity securities		$ 17,462
Equity securities		$ 39,833
Amortization of liability			1,087	$ 1,087	$ 1,087
Amortization of liability, year two			1,087
Related Party Transaction Amortization Of Liability, Year Three			1,087
Related Party Transaction Amortization Of Liability, Year Four			1,087
Related Party Transaction Amortization Of Liability, Year Five			$ 1,087
Tenant Concentration
Schedule of Equity Method Investments [Line Items]
2020			4.80%
2021			1.60%
2022			2.70%
2023			1.10%
2024			2.00%
2025			5.20%
2026			2.60%
2027			1.10%
2028			2.60%
2030			0.80%
2034			0.20%
Government Tenants, All Other | Tenant Concentration
Schedule of Equity Method Investments [Line Items]
Concentration risk, percentage			6.70%
13 Government Tenants | Tenant Concentration
Schedule of Equity Method Investments [Line Items]
Concentration risk, percentage			6.40%