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Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details)	9 Months Ended
	Sep. 30, 2023 ft² property	Sep. 30, 2022 ft² property
Properties
Number of properties	19
Number of owned and consolidated properties	20	22
Rentable square feet | ft²	6,206,460	6,433,954
Number of properties classified as assets held for sale	3
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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Number of real estate properties owned that are classified as assets held for sale.

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

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Area of a real estate property.

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Percentage investment held by members or limited partners of limited liability company (LLC) or limited partnership (LP).

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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

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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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

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