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Investment in Unconsolidated Joint Ventures (Narrative) (Details) $ in Thousands |  |  |  |  |  |  |  |  |  | 3 Months Ended | 12 Months Ended |  |  |  |  |  |
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Dec. 14, 2020 USD ($) | Dec. 13, 2020 USD ($) | Nov. 13, 2020 USD ($) ft² | Nov. 12, 2020 USD ($) | Oct. 30, 2020 USD ($) ft² yr | Jul. 23, 2020 USD ($) a ft² | Jun. 25, 2020 USD ($) ft² Land_Parcels Vehicles | Jun. 09, 2020 USD ($) ft² yr | Jan. 28, 2020 USD ($) ft² Buildings | Dec. 31, 2020 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Oct. 01, 2020 ft² | Sep. 30, 2020 USD ($) | Sep. 01, 2020 ft² a | Jul. 24, 2020 ft² apartments | Jun. 08, 2020 USD ($) | Feb. 20, 2020 USD ($) a ft²
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage notes payable, net |  |  |  |  |  |  |  |  |  | $ 2,909,081 |  | $ 2,909,081 |  | $ 2,922,408 |  |  |  |  |  |  |
Payments to Acquire Equity Method Investments |  |  |  |  |  |  |  |  |  |  |  | 172,436 |  | 87,392 | $ 345,717 |  |  |  |  |  |
Income (Loss) from Equity Method Investments |  |  |  |  |  |  |  |  |  |  |  | (85,110) |  | 46,592 | 2,222 |  |  |  |  |  |
Capital distributions from unconsolidated joint ventures |  |  |  |  |  |  |  |  |  |  |  | 55,298 |  | 136,807 | 0 |  |  |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  |  |  |  |  |  |  | $ 519,303 |  | $ 90,824 | $ 455,409 |  |  |  |  |  |
Significant unobservable inputs used in other than temporary impairment analysis |  |  |  |  |  |  |  |  |  | The Company determined that its valuation of the investment was categorized within Level 3 of the fair value hierarchy, as it utilized significant unobservable inputs in its assessment including an exit capitalization rate of 5.25%, a discount rate on the Company’s equity investment (the property is encumbered by mortgage debt) of 8.0% and an average lease commencement on currently vacant space of mid-2023. |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |
Annapolis Junction Building Six [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage notes payable, net |  |  | $ 13,200 | $ 12,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  | 2.50% | 2.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  | 119,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Description of Variable Rate Basis |  |  | bears interest at a variable rate equal to (1) the greater of (x) LIBOR or (y) 0.50%, plus (2) 2.50% per annum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,100,000
Finance Lease fixed purchase price |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 134,800
Prepaid purchase price |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 15,000
Area of Land | a |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.6
Metropolitan Square |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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% |  |  |  |
Mortgage notes payable, net |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 114,200 |  |  |  |
Number of extensions |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Extension Option (in years) | yr |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  |  |  | 2.30% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  | 418,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt |  |  |  |  | $ 125,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.85% |  |  |  |
Debt Instrument, Description of Variable Rate Basis |  |  |  |  | variable rate equal to (1) the greater of (x) LIBOR or (y) 0.50%, plus (2) 2.30% per annum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |
Dock 72 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |  |
Mortgage notes payable, net |  | $ 198,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | 2.85% | 2.25% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 669,000 |  |  |  |  |
Debt Instrument, Description of Variable Rate Basis | initial variable rate equal to (1) the greater of (x) LIBOR or (y) 0.25%, plus (2) 2.85% per annum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Leased percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 33.00% |  |  |  |  |
Debt instrument, total commitment | $ 250,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impairment loss on investment |  |  |  |  |  |  |  |  |  |  |  | $ 60,500 |  |  |  |  |  |  |  |  |
Joint Venture Partner [Member] | Gateway Commons Complex [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |
Number of buildings | Buildings |  |  |  |  |  |  |  |  | 3 |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 December 31, 2020. Future development projects will be owned 49% by the Company and 51% by its partner.
[2] | This entity is a VIE (See Note 1).
[3] | The joint venture owns two in-service buildings.

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