Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) ft² entity property item	Dec. 31, 2017 USD ($) entity	Jun. 30, 2017 USD ($) ft² property	Dec. 31, 2016 USD ($)
Organization
Number of REITs in which the entity holds non-controlling common stock interest | entity	5	6
Number of REITs in which the entity holds non-controlling preferred stock interest | entity	2
Number of Sponsored REITs | entity	5	6
Number of properties in redevelopment | property	1
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs | item	4
Properties
Number of properties | property	34		35
Rentable square feet | ft²	9,760,699		10,084,710
Cash, Cash Equivalents and Restricted Cash
Cash and cash equivalents	$ 10,448		$ 11,537
Restricted cash			86
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Total	$ 10,448	$ 9,819	$ 11,623	$ 9,366
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
Recent Accounting Standards
Increase in assets and liabilities	$ 3,000
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