Related Party Transactions (Details) ft² in Thousands			1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 18, 2017	Aug. 12, 2014 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($) ft²	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($) ft²	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($) ft²	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Aug. 31, 2014 USD ($) ft²
Related Party Transaction [Line Items]
Receivable from Vornado Realty Trust			$ 76,738,000		$ 76,738,000		$ 76,738,000		$ 75,062,000
Universal Buildings, Washington DC
Related Party Transaction [Line Items]
Area of real estate property (in square feet) | ft²											687
Bowen Building, Washington DC
Related Party Transaction [Line Items]
Area of real estate property (in square feet) | ft²				231		231		231
Vornado's revolving credit facility
Related Party Transaction [Line Items]
Proceeds from Vornado's revolving credit facility			$ 115,600,000
Financing of the Universal Buildings
Related Party Transaction [Line Items]
Debt instrument, face amount											$ 185,000,000.0
Affiliate | Vornado | Vornado's revolving credit facility
Related Party Transaction [Line Items]
Interest expense to Vornado					625,000	$ 145,000	1,200,000	$ 145,000
Proceeds from Vornado's revolving credit facility				$ 115,600,000
Affiliate | Vornado | Mortgage loan secured by Bowen Building
Related Party Transaction [Line Items]
Repayment of mortgage Loan				115,000,000
Accrued interest on mortgage loan				$ 608,000
Affiliate | Allocations of centralized corporate costs | Vornado
Related Party Transaction [Line Items]
Related party transaction amount					$ 5,400,000	4,600,000	$ 12,200,000	10,700,000
Affiliate | Financing transactions | Vornado | Financing of the Universal Buildings
Related Party Transaction [Line Items]
Loan to Vornado		$ 86,000,000
Effective interest rate			4.40%		4.40%		4.40%
Repayment of loan receivable by Vornado									4,000,000	$ 7,000,000
Interest Income on loan receivable from Vornado					$ 843,000	746,000	$ 1,700,000	1,500,000
Affiliate | Financing transactions | Vornado | Financing of the Universal Buildings | LIBOR
Related Party Transaction [Line Items]
Mortgage loan interest rate							2.90%
Affiliate | Financing transactions | Vornado | Note agreement for development of Bartlett
Related Party Transaction [Line Items]
Debt instrument, face amount			$ 170,000,000		$ 170,000,000		$ 170,000,000
Effective interest rate			4.70%		4.70%		4.70%
Amount outstanding on note agreements with Vornado			$ 174,300,000		$ 174,300,000		$ 174,300,000		$ 166,500,000
Interest expense to Vornado					2,000,000	1,100,000	$ 3,700,000	1,800,000
Affiliate | Financing transactions | Vornado | Note agreement for development of Bartlett | LIBOR
Related Party Transaction [Line Items]
Mortgage loan interest rate							2.90%
Affiliate | Supervise cleaning, engineering and security services | BMS
Related Party Transaction [Line Items]
Related party transaction amount					$ 3,200,000	$ 3,200,000	$ 6,300,000	$ 6,300,000
Affiliate | Separation and Combination transaction | Vornado | Subsequent Event
Related Party Transaction [Line Items]
Transaction services, initial period	2 years
Trustee | Consulting agreement
Related Party Transaction [Line Items]
Transaction services, initial period							24 months
Related party transaction, monthly amount							$ 166,667