XML 58 R1.htm IDEA: XBRL DOCUMENT v3.19.3.a.u2
Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2019
Feb. 06, 2020
Jun. 30, 2019
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Transition Report false    
Document Period End Date Dec. 31, 2019    
Entity File Number 001-32470    
Entity Registrant Name FRANKLIN STREET PROPERTIES CORP.    
Entity Incorporation, State or Country Code MD    
Entity Tax Identification Number 04-3578653    
Entity Address, Address Line One 401 Edgewater Place, Suite 200    
Entity Address, City or Town Wakefield    
Entity Address, State or Province MA    
Entity Address, Postal Zip Code 01880    
City Area Code 781    
Local Phone Number 557-1300    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 759,493,045
Title of 12(b) Security Common Stock    
Trading Symbol FSP    
Security Exchange Name NYSEAMER    
Entity Common Stock, Shares Outstanding   107,269,201  
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Entity Central Index Key 0001031316    
Amendment Flag false