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Investment in Unconsolidated Joint Ventures (Narrative) (Details) $ in Thousands |  |  |  |  |  |  | 3 Months Ended | 9 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jul. 18, 2024 ft² | Jul. 17, 2024 USD ($) | Jun. 07, 2024 | Feb. 09, 2024 USD ($) ft² | Feb. 06, 2024 USD ($) | Jan. 08, 2024 USD ($) ft² | Sep. 30, 2024 USD ($) ft² | Sep. 30, 2023 USD ($) | Sep. 30, 2024 USD ($) ft² | Sep. 30, 2023 USD ($) | Aug. 07, 2024 ft² | Jul. 12, 2024 ft² apartments | May 08, 2024 | Jan. 07, 2024 | Jan. 02, 2024 ft² | Dec. 31, 2023
Construction in Progress |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  | 2,700,000 |  | 2,700,000 |  |  |  |  |  |  |
Gateway Commons Complex [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  |  | 50.00% |  | 50.00% |  |  |  |  |  |  |
901 New York Avenue LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage | [1] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25.00%
3 Hudson Boulevard [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  |  | 25.00% |  | 25.00% |  |  |  |  |  |  |
7750 Wisconsin Avenue LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  |  | 50.00% |  | 50.00% |  |  |  |  |  |  |
PR II BXP Reston Gateway LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  |  | 20.00% |  | 20.00% |  |  |  |  |  |  |
Office Tower Developer LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  |  | 50.00% |  | 50.00% |  |  |  |  |  |  |
360 PAS Holdco LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage | [2] |  |  |  |  |  |  | 71.11% |  | 71.11% |  |  |  |  |  |  |
Unconsolidated Joint Ventures [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain on sale / consolidation |  |  |  |  |  |  |  | $ 0 | $ 0 | $ 21,696 | $ 0 |  |  |  |  |  |
Unconsolidated Properties | Gateway Commons Complex [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |
Unconsolidated Properties | 651 Gateway | Office and Life Sciences Building [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 327,000 |
901 New York Avenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  | 524,000 |  |  |  |  |  |  |  |  |  |
Payments to Acquire Investments |  |  |  |  |  |  | $ 10,000 |  |  |  |  |  |  |  |  |  |
Gain on sale / consolidation |  |  |  |  |  |  | 21,800 |  |  |  |  |  |  |  |  |  |
901 New York Avenue | Mortgages [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt |  |  |  |  |  |  | $ 207,100 |  |  |  |  |  |  |  |  |  |
Debt Instrument, Maturity Date |  |  |  |  |  |  | Jan. 05, 2025 |  |  |  |  |  |  |  |  |  |
901 New York Avenue | 901 New York Avenue LLC [Member] | Joint Venture Partner [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |
901 New York Avenue | Unconsolidated Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain on sale / consolidation |  |  |  |  |  |  | $ 21,800 |  |  |  |  |  |  |  |  |  |
901 New York Avenue | Unconsolidated Properties | 901 New York Avenue LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |  |
901 New York Avenue | Unconsolidated Properties | 901 New York Avenue LLC [Member] | Joint Venture Partner [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |
3 Hudson Boulevard [Member] | Extended Maturity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Maturity Date |  |  |  | Aug. 07, 2024 |  | May 09, 2024 |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt, Term |  |  |  | 60 days |  |  |  |  |  |  |  |  |  | 30 days |  |  |
3 Hudson Boulevard [Member] | Unconsolidated Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  |  | 3.61% |  |  |  |  |  |  |  |  |  |  |  |  |
Debt |  |  |  |  |  | $ 80,000 |  |  |  |  |  |  |  |  |  |  |
Long Term Debt, Including Accrued Interest |  |  |  |  |  |  |  | $ 116,000 |  | $ 116,000 |  |  |  |  |  |  |
3 Hudson Boulevard [Member] | Unconsolidated Properties | 3 Hudson Boulevard [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  | 25.00% |  |  |  |  |  |  |  |  |  |  |
7750 Wisconsin Avenue [Member] | Extended Maturity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Maturity Date |  |  |  |  | Apr. 26, 2025 |  |  |  |  |  |  |  |  |  |  |  |
7750 Wisconsin Avenue [Member] | Unconsolidated Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  | 736,000 |  |  |  |  |  |  |  |  |  |  |  |
Construction Loan |  |  |  |  | $ 252,600 |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  |  |  | 1.35% |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  |  |  | $ 251,600 |  |  |  |  |  |  |  |  |  |  |  |
7750 Wisconsin Avenue [Member] | Unconsolidated Properties | 7750 Wisconsin Avenue LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |
Skymark | Unconsolidated Properties | PR II BXP Reston Gateway LLC | Residential Building | Construction in Progress |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  |  |  |  |  |  |  | 20.00% |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  | 417,000 |  |  |  |
Number of Units in Real Estate Property | apartments |  |  |  |  |  |  |  |  |  |  |  |  | 508 |  |  |  |
Low Rise Property |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of floors |  |  |  |  |  |  |  |  |  |  |  |  | 5 |  |  |  |
Tower Property |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of floors |  |  |  |  |  |  |  |  |  |  |  |  | 39 |  |  |  |
100 Causeway Street [Member] | Unconsolidated Properties | Office Tower Developer LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  | 634,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  | 1.48% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt |  |  | $ 333,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |
100 Causeway Street [Member] | Unconsolidated Properties | Extended Maturity | Office Tower Developer LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 1.48% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Maturity Date |  | Sep. 05, 2025 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt, Term |  | 1 year |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
360 Park Avenue South | Unconsolidated Properties | 360 PAS Holdco LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  |  |  |  |  |  | 71.11% |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  | 450,000 |  |  |  |  |
[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.
[2] | The Company’s ownership includes (1) a 35.79% direct interest in the joint venture, (2) an additional 35.02% indirect ownership in the joint venture, and (3) an additional 1% interest in the entity through which the partner owns its interest in the joint venture.

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