Investment in Unconsolidated Joint Ventures (Narrative) (Details) $ in Thousands				3 Months Ended
		Apr. 01, 2021 USD ($)	Mar. 30, 2021 USD ($) ft²	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)		Mar. 31, 2020 USD ($)	Feb. 25, 2021 ft²	Dec. 31, 2020 USD ($)
Schedule of Equity Method Investments [Line Items]
Mortgage notes payable, net					$ 2,904,672				$ 2,909,081
Proceeds from Sale of Equity Method Investments					17,589		$ 0
Payments to Acquire Equity Method Investments					16,684		89,997
Income (Loss) from Equity Method Investments					5,225		(369)
Capital distributions from unconsolidated joint ventures					122		0
Proceeds from sales of real estate					0		259,489
Repayments of Secured Debt					$ 5,374		4,212
Gateway Commons Complex [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage					50.00%	[1]		54.00%	55.00%
Gateway Commons Complex [Member] | Scenario, Plan
Schedule of Equity Method Investments [Line Items]
Ownership Percentage					50.00%
Gateway Commons Complex [Member] | Scenario, Plan | 751 Gateway
Schedule of Equity Method Investments [Line Items]
Ownership Percentage					49.00%
Gateway Commons Complex prior to deferred contribution [Member] [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage					54.00%				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	[2]				55.00%
Annapolis Junction NFM, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage			50.00%		50.00%	[3]
Contractual Sales Price			$ 65,900
Proceeds from Sale of Equity Method Investments			17,600
Gain on Sale of Investments			10,300		$ 10,300
Repayments of Secured Debt			$ 15,100
Annapolis Junction NFM, LLC [Member] | Subsequent Event [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds from Sale of Equity Method Investments				$ 200
Capital distributions from unconsolidated joint ventures		$ 5,800
Annapolis Junction NFM, LLC [Member] | Annapolis Junction Building Six and Seven | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Net Rentable Area (in sf) | ft²			247,000
Joint Venture Partner [Member] | Gateway Commons Complex Future Development[Member] [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage					51.00%
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Mortgage notes payable, net					$ 2,662,616				$ 2,637,911
Income (Loss) from Equity Method Investments					5,225		(369)
Gain on Sale of Investments					$ 10,257	[4]	$ 0
Construction in Progress | Gateway Commons Complex [Member] | 751 Gateway | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Net Rentable Area (in sf) | ft²								229,000

[1] As a result of the partner’s deferred contribution, the Company owned an approximately 54% and 55% interest in the joint venture at March 31, 2021 and December 31, 2020, respectively. 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] On March 30, 2021, the Company sold its interest in the joint venture to the partner. See below for additional details.

[4] During the three months ended March 31, 2021, the Company completed the sale of its 50% ownership interest in Annapolis Junction NFM LLC. The Company recognized a gain on sale of investment of approximately $10.3 million.