Document And Entity Information - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Mar. 09, 2018
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	Smaller Reporting Company
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Common Stock, Shares Outstanding (in shares)		10,232,495
Entity Public Float	$ 0
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Amendment Flag	false