Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details)	6 Months Ended
Jun. 30, 2026 ft² property	Jun. 30, 2025 ft² property
Properties
Number of operating properties	14
Number of properties	14	14
Rentable square feet \| ft²	4,809,487	4,807,663
Number of properties classified as assets held for sale	1
FSP Investments LLC
Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards
Ownership interest (as a percent)	100.00%
FSP Property Management LLC
Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards
Ownership interest (as a percent)	100.00%
FSP Holdings LLC
Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards
Ownership interest (as a percent)	100.00%
FSP Protective TRS Corp.
Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards
Ownership interest (as a percent)	100.00%

Number of real estate properties owned that are classified as assets held for sale.

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- Definition

Number of real estate properties owned and consolidated by the entity.

+ References

No definition available.

+ Details

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- References

No definition available.

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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
-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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- Definition

The number of real estate properties owned as of the balance sheet date.

+ References

No definition available.

+ Details

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- References

No definition available.

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