Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2023	Feb. 20, 2024	Jun. 30, 2023
Document Information [Line Items]
Entity Central Index Key	0001624322
Entity Registrant Name	Business First Bancshares, Inc.
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2023
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2023
Document Transition Report	false
Entity File Number	001-38447
Entity Incorporation, State or Country Code	LA
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
Title of 12(b) Security	Common Stock, par value $1.00 per share
Trading Symbol	BFST
Security Exchange Name	NASDAQ
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
Document Financial Statement Error Correction [Flag]	false
Entity Shell Company	false
Entity Public Float			$ 357,400,000
Entity Common Stock, Shares Outstanding		25,356,393
Auditor Name	FORVIS, LLP
Auditor Firm ID	686
Auditor Location	Fort Worth, TX