Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details) $ in Millions	Jun. 30, 2015 USD ($) ft² property item entity	Dec. 31, 2014 USD ($) entity	Jun. 30, 2014 ft² property entity
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	5
Number of REITs in which the entity holds non-controlling common stock interest | entity	9
Number of REITs in which the entity holds non-controlling preferred stock interest | entity	2
Number of Sponsored REITs | entity	9	10	14
Properties
Number of properties | property	36		39
Rentable square feet | ft²	9,639,740		9,686,215
Recent Accounting Standards
Reclassification of debt issuance costs from assets to contra liabilities | $	$ 6.6	$ 7.6
Mortgage loan secured by property
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	1
Secured construction loan
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	1
Secured revolving lines of credit
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	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%