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Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details)	9 Months Ended
	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.

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- Definition

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- Definition

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Area of a real estate property.

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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

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