Investment in unconsolidated Joint Ventures (Narrative) (Details) $ in Thousands			3 Months Ended
	Mar. 18, 2020 USD ($) ft²	Jan. 28, 2020 USD ($) ft² Buildings	Mar. 31, 2020 USD ($)		Mar. 31, 2019 USD ($)	Feb. 20, 2020 USD ($) ft² a	Dec. 31, 2019 USD ($)
Schedule of Equity Method Investments [Line Items]
Mortgage notes payable, net			$ 2,919,157				$ 2,922,408
Payments to Acquire Equity Method Investments			$ 89,997		$ 26,995
Gateway Commons Complex [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		50.00%	50.00%	[1]
number of buildings | Buildings		3
Net Rentable Area (in sf) | ft²		768,000
Payments to Acquire Equity Method Investments		$ 350,000
Platform 16
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		55.00%	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
Annapolis Junction Building Seven and Eight [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	50.00%
Mortgage notes payable, net	$ 34,600
Debt Instrument, Basis Spread on Variable Rate	2.35%
Annapolis Junction Building Seven [Member]
Schedule of Equity Method Investments [Line Items]
Net Rentable Area (in sf) | ft²	127,000
Annapolis Junction Building Eight [Member]
Schedule of Equity Method Investments [Line Items]
Net Rentable Area (in sf) | ft²	126,000
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%
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%

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

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