Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details) - $ in Thousands	Mar. 31, 2016 USD ($) ft² entity property item	Dec. 31, 2015 USD ($) entity	Mar. 31, 2015 ft² property
Properties
Number of properties | property	35		36
Rentable square feet | ft²	9,325,249		9,310,131
Number of REITs in which the entity holds non-controlling common stock interest | entity	8	9
Number of REITs in which the entity holds non-controlling preferred stock interest | entity	2
Number of properties in redevelopment | property	1
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs | item	5
Recently Issued Accounting Standards
Debt issuance costs | $	$ 2,120	$ 2,353
Accounting Standards Update 2015-03: Simplifying the Presentation of Debt Issuance Costs | Other assets
Recently Issued Accounting Standards
Debt issuance costs | $		(2,400)
Accounting Standards Update 2015-03: Simplifying the Presentation of Debt Issuance Costs | Contra term loans
Recently Issued Accounting Standards
Debt issuance costs | $		$ 2,400
Mortgage loan secured by property
Properties
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs | item	2
Secured revolving lines of credit
Properties
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs | item	3
FSP Investments LLC
Properties
Ownership interest (as a percent)	100.00%
FSP Property Management LLC
Properties
Ownership interest (as a percent)	100.00%
FSP Holdings LLC
Properties
Ownership interest (as a percent)	100.00%
FSP Protective TRS Corp.
Properties
Ownership interest (as a percent)	100.00%