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Organization (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) ft² entity property item	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($) entity	Mar. 31, 2018 USD ($) ft² property	Dec. 31, 2017 USD ($)
Organization
Number of REITs in which the entity holds non-controlling common stock interest | entity	3		3
Number of Sponsored REITs | entity	3		3
Number of properties in redevelopment | property	3			1
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs | item	4
Properties
Number of properties | property	32			34
Rentable square feet | ft²	9,495,118			9,760,657
Cash, Cash Equivalents and Restricted Cash
Cash and cash equivalents	$ 8,832,000			$ 14,355,000
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Total	8,832,000		$ 11,177,000	$ 14,355,000	$ 9,819,000
Right to use asset	2,059,192
Lease liability	$ 2,141,000
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%
ASU 2016-02 | Adjustment
Cash, Cash Equivalents and Restricted Cash
Right to use asset		$ 2,100,000
Lease liability		$ 2,200,000
Mortgage loan secured by property
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs | item	2
Secured revolving lines of credit
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs | item	2

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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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