Investment in Unconsolidated Joint Ventures (Narrative) (Details) $ in Thousands							3 Months Ended			9 Months Ended
		Sep. 01, 2021 USD ($) ft²	Aug. 31, 2021 USD ($) ft² years loanExtension	Jun. 15, 2021	Apr. 01, 2021 USD ($)	Mar. 30, 2021 USD ($) ft²	Sep. 30, 2021 USD ($)		Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)		Sep. 30, 2020 USD ($)	Jun. 11, 2021 ft²	Feb. 25, 2021 ft²	Dec. 31, 2020 USD ($)
Schedule of Equity Method Investments [Line Items]
Mortgage notes payable, net							$ 2,898,699			$ 2,898,699					$ 2,909,081
Debt Instrument, Basis Spread on Variable Rate				0.775%
Proceeds from Sale of Equity Method Investments										17,789		$ 0
Payments to Acquire Equity Method Investments										95,462		158,374
Income (Loss) from Equity Method Investments							$ (5,597)		$ (6,873)	(1,745)		(5,410)
Capital distributions from unconsolidated joint ventures										122		55,123
Proceeds from sales of real estate										0		505,679
Repayments of Secured Debt										$ 13,261		12,795
Gateway Commons Complex [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage							50.00%	[1]		50.00%	[1]			54.00%	55.00%
Gateway Commons Complex [Member] | Scenario, Plan
Schedule of Equity Method Investments [Line Items]
Ownership Percentage							50.00%			50.00%
Gateway Commons Complex [Member] | Scenario, Plan | 751 Gateway
Schedule of Equity Method Investments [Line Items]
Ownership Percentage							49.00%			49.00%
Gateway Commons Complex prior to deferred contribution [Member] [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage							52.00%			52.00%					55.00%
Platform 16
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	[2]						55.00%			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						17,800
Capital distributions from unconsolidated joint ventures					$ 5,800
Gain on Sale of Investments						10,300				$ 10,300
Repayments of Secured Debt						$ 15,100
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
One Hundred Causeway Street [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage							50.00%			50.00%
One Hundred Causeway Street [Member] | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Ownership Percentage													50.00%
Net Rentable Area (in sf) | ft²													632,000
Safeco Plaza REIT LLC
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		33.67%					33.67%	[4]		33.67%	[4]
Mortgage notes payable, net		$ 250,000
Net Rentable Area (in sf) | ft²		765,000
Payments to Acquire Equity Method Investments		$ 72,600
Debt Instrument, Description of Variable Rate Basis		variable rate equal to the greater of (x) 2.35% or (y) LIBOR plus 2.20% per annum
Aggregate purchase price		$ 465,000
Number of floors		50
number of investors		2
Podium Developer LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage			50.00%				50.00%			50.00%
Mortgage notes payable, net			$ 174,300
Debt Instrument, Basis Spread on Variable Rate			2.25%
Net Rentable Area (in sf) | ft²			382,000
Number of extensions | loanExtension			2
Extension Option (in years) | years			1
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Mortgage notes payable, net							$ 2,995,161			$ 2,995,161					$ 2,637,911
Income (Loss) from Equity Method Investments							(5,597)		(6,873)	(1,745)		(5,410)
Gain on Sale of Investments							$ 0		$ 0	$ 10,257	[5]	$ 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
Company's Share [Member] | Safeco Plaza REIT LLC
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		33.00%
Company's Share [Member] | Safeco Partner Entity One [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		1.00%
Company's Share [Member] | Safeco Partner Entity Two
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		1.00%
Institutional investor number one | Safeco Plaza REIT LLC
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		33.165%
Payments to Acquire Equity Method Investments		$ 71,900
Institutional investor number two | Safeco Plaza REIT LLC
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		33.165%
Payments to Acquire Equity Method Investments		$ 71,900

[1] As a result of the partner’s deferred contribution, the Company owned an approximately 52% and 55% interest in the joint venture at September 30, 2021 and December 31, 2020, respectively. (9)The Company’s ownership includes (1) a 33.0% direct interest in the joint venture, and (2) an additional 1% interest in each of the two entities (each, a “Safeco Partner Entity”) through which each partner owns its interest in the joint venture.

[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] The Company’s ownership includes (1) a 33.0% direct interest in the joint venture, and (2) an additional 1% interest in each of the two entities (each, a “Safeco Partner Entity”) through which each partner owns its interest in the joint venture.

[5] During the nine months ended September 30, 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.