Investment in unconsolidated Joint Ventures (Narrative) (Details) $ in Thousands					3 Months Ended			9 Months Ended
	Jul. 23, 2020 USD ($) a ft²	Jun. 25, 2020 USD ($) ft² Land_Parcels Vehicles	Jun. 09, 2020 USD ($) ft² yr	Jan. 28, 2020 USD ($) ft² Buildings	Sep. 30, 2020 USD ($) ft²		Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) ft²		Sep. 30, 2019 USD ($)	Sep. 01, 2020 ft² a	Jul. 24, 2020 ft² apartments	Jun. 08, 2020 USD ($)	Feb. 20, 2020 USD ($) a ft²	Dec. 31, 2019 USD ($)
Schedule of Equity Method Investments [Line Items]
Mortgage notes payable, net					$ 2,912,494			$ 2,912,494							$ 2,922,408
Payments to Acquire Equity Method Investments								158,374		$ 65,499
Income (Loss) from Equity Method Investments					$ (6,873)		$ (649)	(5,410)		47,528
Capital distributions from unconsolidated joint ventures								55,123		136,807
Proceeds from sales of real estate								$ 505,679		$ 83,486
Gateway Commons Complex [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage				50.00%	50.00%	[1]		50.00%	[1]
Number of buildings | Buildings				3
Net Rentable Area (in sf) | ft²				768,000
Payments to Acquire Equity Method Investments				$ 350,000
Gateway Commons Complex prior to deferred contribution [Member] [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage				55.00%
Gateway Commons Complex Future Development[Member] [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage				49.00%
Platform 16
Schedule of Equity Method Investments [Line Items]
Ownership Percentage				55.00%	55.00%	[2]		55.00%	[2]
Net Rentable Area (in sf) | ft²				390,000										1,100,000
Finance Lease fixed purchase price														$ 134,800
Prepaid purchase price														$ 15,000
Area of Land | a														5.6
Metropolitan Square Associates Llc [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage			20.00%
Number of extensions			2
Extension Option (in years) | yr			1
Debt instrument, borrowing capacity			$ 325,000
Debt Instrument, Basis Spread on Variable Rate			4.75%
Net Rentable Area (in sf) | ft²			654,000
Debt			$ 288,000										$ 155,900
Debt Instrument, Interest Rate, Stated Percentage													5.75%
Capital distributions from unconsolidated joint ventures			$ 112,700
Derivative, Cap Interest Rate			3.00%
Derivative, Notional Amount			$ 325,000
Annapolis Junction NFM, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		50.00%			50.00%	[3]		50.00%	[3]
Net Rentable Area (in sf) | ft²		300,000
Contractual Sales Price		$ 47,000
Number of Vehicles | Vehicles		511
Parcels of undeveloped land | Land_Parcels		2
land parcel | Land_Parcels		1
Income (Loss) from Equity Method Investments		$ 5,800
Capital distributions from unconsolidated joint ventures		36,800
Proceeds from sales of real estate		$ 45,800
Annapolis Junction Building Eight [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		50.00%
Mortgage notes payable, net		$ 34,500
Debt Instrument, Basis Spread on Variable Rate		2.35%
Net Rentable Area (in sf) | ft²		126,000
Repayments of Debt		$ 16,100
Annapolis Junction Building Seven [Member]
Schedule of Equity Method Investments [Line Items]
Mortgage notes payable, net		$ 18,400
Debt Instrument, Basis Spread on Variable Rate		2.35%
Net Rentable Area (in sf) | ft²		127,000
Beach Cities Media Center [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	50.00%
Net Rentable Area (in sf) | ft²	275,000
Area of Land | a	6.4
Payments to Acquire Equity Method Investments	$ 21,200
1265 Main Office JV LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage					50.00%			50.00%
Market Square North [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage					50.00%			50.00%
Net Rentable Area (in sf) | ft²					418,000			418,000
Debt					$ 114,200			$ 114,200
Debt Instrument, Interest Rate, Stated Percentage					4.85%			4.85%
Agreement with 1265 Main Office JV LLC partner [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage											50.00%
Net Rentable Area (in sf) | ft²											1,200,000
Area of Land | a											41
Joint Venture Partner [Member] | Gateway Commons Complex [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage				50.00%
Number of buildings | Buildings				3
Net Rentable Area (in sf) | ft²				768,000
Payments to Acquire Equity Method Investments				$ 280,800
Future payment to acquire interest in equity method investment				$ 69,200
Joint Venture Partner [Member] | Gateway Commons Complex Future Development[Member] [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage				51.00%
Residential Building | Hub50House [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage												50.00%
Net Rentable Area (in sf) | ft²												320,000
Number of Units in Real Estate Property | apartments												440
Company's Share [Member] | Metropolitan Square Associates Llc [Member]
Schedule of Equity Method Investments [Line Items]
Capital distributions from unconsolidated joint ventures			$ 22,500
Company's Share [Member] | Annapolis Junction NFM, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Parcels of undeveloped land | Land_Parcels		2
Capital distributions from unconsolidated joint ventures		$ 18,400
Base Rate [Member] | Metropolitan Square Associates Llc [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Description of Variable Rate Basis			the greater of (x) LIBOR or (y) 0.65%

[1] As a result of the partner’s deferred contribution, the Company owns an approximately 55% interest in the joint venture at September 30, 2020. Future development projects will be owned 49% by the Company and 51% by its partner.

[2] This entity is a VIE (See Note 2).

[3] The joint venture owns two in-service buildings.