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Real Estate (Narrative) (Details) $ in Thousands				3 Months Ended		9 Months Ended
		Mar. 31, 2024 USD ($)	Jan. 08, 2024 USD ($) ft²	Sep. 30, 2024 USD ($) ft² Real_Estate_Properties	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) ft² Real_Estate_Properties	Sep. 30, 2024 USD ($) ft² Real_Estate_Properties	Sep. 30, 2023 USD ($)	Sep. 26, 2024 ft²	Jul. 22, 2024 USD ($)	Jul. 17, 2024 ft²	May 07, 2024 USD ($) ft²	Apr. 05, 2024 ft²	Feb. 12, 2024 ft² apartments	Jan. 07, 2024 USD ($)	Dec. 31, 2023 USD ($)
Real Estate Properties [Line Items]
Total revenue				$ 859,227	$ 824,283		$ 2,549,148	$ 2,444,636
Net Income (Loss)				83,628	(111,826)		243,126	70,290
Carying value of the Company's investment in unconsolidated joint ventures				1,421,886		$ 1,421,886	1,421,886									$ 1,377,319
Impairment loss				$ 0	$ 0		$ 13,615	$ 0
901 New York Avenue LLC [Member]
Real Estate Properties [Line Items]
Ownership Percentage	[1]															25.00%
Construction in Progress
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²				2,700,000		2,700,000	2,700,000
Number of real estate properties | Real_Estate_Properties				9		9	9
901 New York Avenue
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²			524,000
Payments to Acquire Investments			$ 10,000
Net Working Capital, Including Cash Acquired from Acquisition			7,100
Gain on sale / consolidation			21,800
Total revenue						$ 24,700
Net Income (Loss)						$ (6,900)
Asset Acquisition, Consideration Transferred			20,000
901 New York Avenue | Unconsolidated Properties
Real Estate Properties [Line Items]
Gain on sale / consolidation			21,800
901 New York Avenue | 901 New York Avenue LLC [Member]
Real Estate Properties [Line Items]
Carying value of the Company's investment in unconsolidated joint ventures															$ (11,800)
Investments in Affiliates, Subsidiaries, Associates, and Joint Ventures, Fair Value Disclosure			10,000
901 New York Avenue | 901 New York Avenue LLC [Member] | Unconsolidated Properties
Real Estate Properties [Line Items]
Ownership Percentage															50.00%
901 New York Avenue | Mortgages [Member]
Real Estate Properties [Line Items]
Debt			207,100
Assumption of mortgage notes payable			$ 207,100
901 New York Avenue | Joint Venture Partner [Member] | 901 New York Avenue LLC [Member]
Real Estate Properties [Line Items]
Ownership Percentage			50.00%
901 New York Avenue | Joint Venture Partner [Member] | 901 New York Avenue LLC [Member] | Unconsolidated Properties
Real Estate Properties [Line Items]
Ownership Percentage			50.00%
121 Broadway | Construction in Progress | Residential Properties [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²														492,000
Number of Units in Real Estate Property | apartments														439
760 Boylston Street
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²													118,000
Reston Next Office Phase II
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²											90,000
180 CityPoint [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²									329,000
Shady Grove Innovation District [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²												143,000
Impairment loss		$ 13,600					$ 13,600
Number of real estate properties												2
Contractual Sales Price										$ 24,700		$ 24,800
[1] At December 31, 2023, the Company’s economic ownership was approximately 50%. On January 8, 2024, the Company completed the acquisition of its joint venture partner’s 50% economic ownership interest for a gross purchase price of $10.0 million, as described in Note 3 and this Note 5. Since then, the Company accounts for its assets, liabilities and operations on a consolidated basis.

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