Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details)	3 Months Ended
	Mar. 31, 2020 entity	Mar. 31, 2020 ft²	Mar. 31, 2020	Mar. 31, 2020 property	Mar. 31, 2019 ft² property
Organization
Number of REITs in which the entity holds non-controlling common stock interest | entity	2
Number of Sponsored REITs	2			2
Number of properties in redevelopment				3
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs				1
Properties
Number of properties				32	32
Rentable square feet | ft²		9,506,513			9,495,118
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%