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Real Estate (Narrative) (Details) $ in Thousands |  | 3 Months Ended | 6 Months Ended | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 27, 2025 USD ($) ft² | Sep. 30, 2025 USD ($) | Sep. 30, 2024 USD ($) | Jun. 26, 2025 USD ($) | Sep. 30, 2025 USD ($) | Sep. 30, 2024 USD ($) | Jul. 31, 2025 ft² floor | Jul. 17, 2025 ft² | Jul. 01, 2025 ft² | Mar. 31, 2025 ft² | Jan. 16, 2025 ft²
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  | $ 1,932 | $ 517 |  | $ 20,322 | $ 517 |  |  |  |  |
Net Income (Loss) |  | (116,840) | 108,452 |  | 89,206 | 322,005 |  |  |  |  |
Impairment loss |  | 68,901 | 0 |  | 68,901 | 13,615 |  |  |  |  |
Wholly Owned Properties |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |
Impairment loss |  | 68,900 |  |  |  |  |  |  |  |  |
Boston Properties Limited Partnership |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  | 1,932 | 517 |  | 20,421 | 517 |  |  |  |  |
Net Income (Loss) |  | (112,224) | 110,156 |  | 97,327 | 327,109 |  |  |  |  |
Impairment loss |  | 65,988 | 0 |  | 65,988 | 13,615 |  |  |  |  |
Boston Properties Limited Partnership | Wholly Owned Properties |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |
Impairment loss |  | $ 66,000 |  |  |  |  |  |  |  |  |
Reston Next Retail |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  | 30,000
1050 Winter Street |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  | 162,000 | 162,000 |
17 Hartwell Avenue |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² | 30,000 |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  |  |  | 18,400 |  |  |  |  |  |
Proceeds from Sale, Real Estate, Held-for-Investment | $ 21,800 |  |  |  |  |  |  |  |  |  |
Net Income (Loss) |  |  | $ (100) | $ (300) |  | $ 200 |  |  |  |  |
17 Hartwell Avenue | Boston Properties Limited Partnership |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |
Gains on sales of real estate |  |  |  |  | $ 18,500 |  |  |  |  |  |
Reston Next Office Phase II |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  | 87,000 |  |  |
343 Madison Avenue | Construction in Progress |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  | 930,000 |  |  |  |
Number of floors | floor |  |  |  |  |  |  | 46 |  |  |  |

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