Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details) $ in Thousands	Dec. 31, 2019 USD ($)	Jun. 30, 2017 USD ($) ft² entity item property	Dec. 31, 2016 USD ($) entity	Jun. 30, 2016 ft² property
Organization
Number of REITs in which the entity holds non-controlling common stock interest | entity		6	7
Number of REITs in which the entity holds non-controlling preferred stock interest | entity		2
Number of Sponsored REITs | entity		6	7
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		35		35
Rentable square feet | ft²		10,084,710		9,523,054
Recently Issued Accounting Standards
Assets		$ 2,042,567	$ 2,088,133
Liabilities		$ 1,134,723	$ 1,126,089
Accounting Standards Update 2016-02 | Scenario Forecast Adjustment
Recently Issued Accounting Standards
Assets	$ 3,000
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
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%