Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 05, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2020
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	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 523,709,269
Title of 12(b) Security	Common Stock
Trading Symbol	FSP
Security Exchange Name	NYSEAMER
Entity Common Stock, Shares Outstanding		107,328,199
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Central Index Key	0001031316
Amendment Flag	false