| Document and Entity Information | Nov. 19, 2020 |
| Document And Entity Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Nov. 19, 2020 |
| Entity Registrant Name | F-STAR THERAPEUTICS, INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-37718 |
| Entity Tax Identification Number | 52-2386345 |
| Entity Address, Address Line One | Eddeva B920 Baraham Research Campus |
| Entity Address, City or Town | Cambridge |
| Entity Address, Country | GB |
| Entity Address, Postal Zip Code | CB22 3AT |
| Country Region | 44 |
| City Area Code | 1223 |
| Local Phone Number | 497400 |
| Entity Information, Former Legal or Registered Name | Spring Bank Pharmaceuticals, Inc. |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Current Fiscal Year End Date | --12-31 |
| Amendment Flag | false |
| Entity Central Index Key | 0001566373 |
| Former Address [Member] | |
| Document And Entity Information [Line Items] | |
| Entity Address, Address Line One | 35 Parkwood Drive |
| Entity Address, Address Line Two | Suite 210 |
| Entity Address, City or Town | Hopkinton |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 01748 |
