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Investments in Unconsolidated Joint Ventures (Investments in Unconsolidated Joint Ventures) (Details) - USD ($) $ in Thousands |  | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jan. 08, 2024 | Sep. 30, 2024 | Dec. 31, 2023
Schedule of Equity Method Investments [Line Items] |  |  |  |
Other Liabilities |  |  | $ (407,441) | $ (445,947)
Investments in unconsolidated joint ventures |  |  | 1,421,886 | 1,377,319
Investments In Affiliates Subsidiaries Associates And Joint Ventures net | [1] |  | 1,391,149 | 1,337,416
Unconsolidated Joint Ventures [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Other Liabilities |  |  | $ (30,700) | (39,900)
Square 407 Limited Partnership [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | Market Square North |
Ownership Percentage |  |  | 50.00% |
Other Liabilities | [1] |  | $ (12,042) | $ (5,996)
901 New York Avenue LLC [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  |  | 901 New York Avenue
Ownership Percentage | [2] |  |  | 25.00%
Other Liabilities | [1] |  |  | $ (11,764)
901 New York Avenue LLC (economic ownership) [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership Percentage |  |  |  | 50.00%
Payments to Acquire Real Estate and Real Estate Joint Ventures |  | $ 10,000 |  |
901 New York Avenue LLC (economic ownership) [Member] | Joint Venture Partner [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership Percentage |  | 50.00% |  |
WP Project Developer LLC [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | Wisconsin Place Land and Infrastructure |
Ownership Percentage | [3] |  | 33.33% |
Investments in unconsolidated joint ventures | [1] |  | $ 29,969 | $ 30,375
500 North Capitol Venture LLC [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | 500 North Capitol Street, NW |
Ownership Percentage |  |  | 30.00% |
Other Liabilities | [1] |  | $ (11,630) | (10,253)
501 K Street LLC [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | 1001 6th Street |
Ownership Percentage |  |  | 50.00% |
Investments in unconsolidated joint ventures | [1] |  | $ 45,955 | 44,774
Podium Developer LLC [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | The Hub on Causeway - Podium |
Ownership Percentage |  |  | 50.00% |
Investments in unconsolidated joint ventures | [1] |  | $ 42,916 | 45,201
Residential Tower Developer LLC [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | Hub50House |
Ownership Percentage |  |  | 50.00% |
Investments in unconsolidated joint ventures | [1] |  | $ 38,116 | 40,235
Hotel Tower Developer LLC [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | The Hub on Causeway - Hotel Air Rights |
Ownership Percentage |  |  | 50.00% |
Investments in unconsolidated joint ventures | [1] |  | $ 14,076 | 13,494
Office Tower Developer LLC |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | 100 Causeway Street |
Ownership Percentage |  |  | 50.00% |
Investments in unconsolidated joint ventures | [1] |  | $ 56,055 | 57,660
1265 Main Office JV LLC [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | 1265 Main Street |
Ownership Percentage |  |  | 50.00% |
Investments in unconsolidated joint ventures | [1] |  | $ 3,515 | 3,585
BNY Tower Holdings LLC [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | Dock 72 |
Ownership Percentage | [4] |  | 50.00% |
Other Liabilities | [1] |  | $ (7,065) | (11,890)
CA-Colorado Center LLC[Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | Colorado Center |
Ownership Percentage |  |  | 50.00% |
Investments in unconsolidated joint ventures | [1] |  | $ 234,972 | 237,815
7750 Wisconsin Avenue LLC [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | 7750 Wisconsin Avenue |
Ownership Percentage |  |  | 50.00% |
Investments in unconsolidated joint ventures | [1] |  | $ 48,814 | 50,064
BP-M 3HB Venture LLC [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | 3 Hudson Boulevard |
Ownership Percentage |  |  | 25.00% |
Investments in unconsolidated joint ventures | [1] |  | $ 113,288 | 115,103
Platform 16 Holdings LP [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | Platform 16 |
Ownership Percentage |  |  | 55.00% |
Investments in unconsolidated joint ventures | [1] |  | $ 55,381 | 45,564
Gateway Portfolio Holdings LLC [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | Gateway Commons |
Ownership Percentage |  |  | 50.00% |
Investments in unconsolidated joint ventures | [1] |  | $ 393,679 | 376,834
Rosecrans-Sepulveda Partners 4, LLC |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | Beach Cities Media Campus |
Ownership Percentage |  |  | 50.00% |
Investments in unconsolidated joint ventures | [1] |  | $ 27,051 | 27,034
Safeco Plaza REIT LLC |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | Safeco Plaza |
Ownership Percentage | [5] |  | 33.67% |
Investments in unconsolidated joint ventures | [1] |  | $ 47,510 | 44,734
Safeco Plaza REIT LLC | Company's Share [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership Percentage |  |  | 33.00% |
360 PAS Holdco LLC |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | 360 Park Avenue South |
Ownership Percentage | [6] |  | 71.11% |
Investments in unconsolidated joint ventures | [1] |  | $ 69,538 | 42,988
360 PAS Holdco LLC | Company's Share [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership Percentage |  |  | 35.79% |
PR II BXP Reston Gateway LLC |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | Skymark - Reston Next Residential |
Ownership Percentage |  |  | 20.00% |
Investments in unconsolidated joint ventures | [1] |  | $ 15,359 | 15,184
751 Gateway Holdings LLC |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | 751 Gateway |
Ownership Percentage |  |  | 49.00% |
Investments in unconsolidated joint ventures | [1] |  | $ 98,882 | 93,411
200 Fifth Avenue JV LLC |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | 200 Fifth Avenue |
Ownership Percentage |  |  | 26.69% |
Investments in unconsolidated joint ventures | [1] |  | $ 68,563 | 75,718
ABXP Worldgate Investments LLC |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Properties |  |  | 13100 and 13150 Worldgate Drive |
Ownership Percentage |  |  | 50.00% |
Investments in unconsolidated joint ventures | [1] |  | $ 18,247 | $ 17,546
Entity Owning Land And Infrastructure Of Project [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership Percentage |  |  | 33.33% |
Safeco Partner Entity One [Member] | Company's Share [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership Percentage |  |  | 1.00% |
Safeco Partner Entity Two | Company's Share [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership Percentage |  |  | 1.00% |
360 Park Avenue South Partners Entity | Company's Share [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership Percentage |  |  | 1.00% |
360 PAS Holdco LLC (indirect ownership) | Company's Share [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership Percentage |  |  | 35.02% |
[1] | Investments with deficit balances aggregating approximately $30.7 million and $39.9 million at September 30, 2024 and December 31, 2023, respectively, are included within Other Liabilities in the Company’s Consolidated Balance Sheets.
[2] | 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.
[3] | The Company’s wholly-owned subsidiary that owns Wisconsin Place Office also owns a 33.33% interest in the joint venture entity that owns the land, parking garage and infrastructure of the project.
[4] | This property includes net equity balances from the amenity joint venture.
[5] | 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 through which each partner owns its interest in the joint venture.
[6] | 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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