| Document And Entity Information | Mar. 22, 2021 |
| Document Information [Line Items] | |
| Entity, Registrant Name | BUSINESS FIRST BANCSHARES, INC. |
| Document, Type | 8-K |
| Document, Period End Date | Mar. 22, 2021 |
| Entity, Incorporation, State or Country Code | LA |
| Entity, File Number | 001-38447 |
| Entity, Tax Identification Number | 20-5340628 |
| Entity, Address, Address Line One | 500 Laurel Street, Suite 101 |
| Entity, Address, City or Town | Baton Rouge |
| Entity, Address, State or Province | LA |
| Entity, Address, Postal Zip Code | 70801 |
| City Area Code | 225 |
| Local Phone Number | 248-7600 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | BFST |
| Security Exchange Name | NASDAQ |
| Entity, Emerging Growth Company | false |
| Amendment Flag | false |
| Entity, Central Index Key | 0001624322 |
