Note 25 - Financial Statements - Parent Company Only - Statements of Income for Business First Bancshares, Inc. (Parent Company) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest	$ 51,601	$ 43,418	$ 38,617
Provision for Income Taxes	4,103	1,613	1,525
Net Income	4,848	5,111	4,096
Parent Company [Member]
Interest			1
Other Operating Expenses	699	1,464	1,437
Income (Loss) before Income Taxes and Equity in Undistributed Net Income of Subsidiaries	(699)	(1,464)	(1,436)
Provision for Income Taxes	233	(498)	(474)
Income (Loss) before Equity in Undistributed Net Income of Subsidiaries	(932)	(966)	(962)
Equity in Undistributed Net Income of Subsidiaries	5,780	6,077	5,058
Net Income	$ 4,848	$ 5,111	$ 4,096