Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details)	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020 entity	Jun. 30, 2020 property	Jun. 30, 2020 ft²	Dec. 31, 2019 entity	Jun. 30, 2019 ft² property
Organization
Number of REITs in which the entity holds non-controlling common stock interest | entity		2			2
Number of Sponsored REITs		2	2
Number of properties in redevelopment			3			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				9,508,226		9,498,858
FSP Investments 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%