Document And Entity Information - shares	3 Months Ended
	Mar. 31, 2019	May 03, 2019
Document Information [Line Items]
Entity Registrant Name	Business First Bancshares, Inc.
Entity Central Index Key	0001624322
Trading Symbol	bfst
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Current Reporting Status	Yes
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Small Business	false
Entity Common Stock, Shares Outstanding (in shares)		13,361,482
Document Type	10-Q
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Amendment Flag	false