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Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details) | 9 Months Ended |  |  |
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Sep. 30, 2021 | Sep. 30, 2021 entity | Sep. 30, 2021 property | Sep. 30, 2021 ft² | Dec. 31, 2020 entity | Sep. 30, 2020 ft² property
Organization |  |  |  |  |  |
Number of REITs in which the entity holds non-controlling common stock interest | entity |  | 2 |  |  | 2 |
Number of Sponsored REITs |  | 2 | 2 |  |  |
Number of properties in redevelopment |  |  | 1 |  |  | 3
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs |  |  | 1 |  |  |
Properties |  |  |  |  |  |
Number of properties |  |  | 26 |  |  | 32
Rentable square feet | ft² |  |  |  | 7,671,275 |  | 9,526,822
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 Represents the number of promissory notes secured by mortgages on real estate owned by Sponsored REITs. + References No definition available. + Details
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- Definition Area of a real estate property. + References No definition available. + Details
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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 -Publisher FASB -Name Accounting Standards Codification -Topic 272 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055 + Details
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- Details
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