Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details) $ in Thousands	Dec. 31, 2019 USD ($)	Mar. 31, 2017 USD ($) ft² entity item property	Dec. 31, 2016 USD ($) entity	Mar. 31, 2016 ft² property
Properties
Rentable square feet | ft²		10,118,112		9,325,249
Number of REITs in which the entity holds non-controlling common stock interest | entity		7	7
Number of REITs in which the entity holds non-controlling preferred stock interest | entity		2
Number of properties | property		35		35
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
Assets		$ 2,078,286	$ 2,088,133
Liabilities		$ 1,130,263	$ 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
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%