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Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Detail)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
USD ($)
ft²
entity
property
item
Dec. 31, 2017
USD ($)
entity
Mar. 31, 2017
USD ($)
ft²
property
Dec. 31, 2016
USD ($)
Organization        
Number of REITs in which the entity holds non-controlling common stock interest | entity 6 6    
Number of REITs in which the entity holds non-controlling preferred stock interest | entity 2      
Number of Sponsored REITs | entity 6 6    
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 34   35  
Rentable square feet | ft² 9,760,657   10,118,212  
Cash, Cash Equivalents and Restricted Cash        
Cash and cash equivalents $ 14,355   $ 11,143  
Restricted cash     31  
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Total $ 14,355 $ 9,819 $ 11,174 $ 9,366
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%      
ASU 2016-02        
Recent Accounting Standards        
Increase in assets and 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