| Document and Entity Information | May 04, 2021 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | May 4, 2021 |
| 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 |
| Entity Address, Adress Line Two | Suite 200 |
| Entity Address, State or Province | MA |
| Entity Address, City or Town | Wakefield |
| Entity Address, Postal Zip Code | 01880 |
| City Area Code | 781 |
| Local Phone Number | 557-1300 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $.0001 par value per share |
| Trading Symbol | FSP |
| Security Exchange Name | NYSEAMER |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001031316 |
| Amendment Flag | false |
