Note 27 - Financial Statements - Parent Company Only - Statements of Income for Business First Bancshares, Inc. (Parent Company) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Interest Income	$ 103,467	$ 76,195	$ 51,601
Interest Expense	23,269	13,966	7,229
Provision for Income Taxes	6,080	3,279	4,103
Net Income	23,772	14,091	4,848
Parent Company [Member]
Interest Income	45	46
Interest Expense	1,688	237	155
Other Operating Expenses	2,138	3,275	544
Income (Loss) before Income Taxes and Equity in Undistributed Net Income of Subsidiaries	(3,781)	(3,466)	(699)
Provision for Income Taxes	(783)	(623)	233
Income (Loss) before Equity in Undistributed Net Income of Subsidiaries	(2,998)	(2,843)	(932)
Equity in Undistributed Net Income of Subsidiaries	26,770	16,934	5,780
Net Income	$ 23,772	$ 14,091	$ 4,848