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Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details) | 9 Months Ended |
--- | --- | ---
Sep. 30, 2025 ft² property | Sep. 30, 2024 ft² property
Properties |  |
Number of operating properties | 14 |
Number of properties | 14 | 16
Rentable square feet | ft² | 4,807,663 | 5,180,158
Number of properties classified as assets held for sale |  | 1
FSP Investments LLC |  |
Organization |  |
Ownership interest (as a percent) | 100.00% |
FSP Property Management LLC |  |
Organization |  |
Ownership interest (as a percent) | 100.00% |
FSP Holdings LLC |  |
Organization |  |
Ownership interest (as a percent) | 100.00% |
FSP Protective TRS Corp. |  |
Organization |  |
Ownership interest (as a percent) | 100.00% |

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- Definition Percentage investment held by members or limited partners of limited liability company (LLC) or limited partnership (LP). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 272 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482987/272-10-50-3 + Details
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