Note 25 - Financial Statements - Parent Company Only - Balance Sheets for Business First Bancshares, Inc. (Parent Company) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other Assets	$ 5,206	$ 8,159
Total Assets	1,321,256	1,105,841
Short Term Borrowings	862	862
Accrued Interest Payable	890	920
Long Term Borrowings	2,700	3,000
Other Liabilities	4,397	4,921
Total Liabilities	1,141,321	992,282
Common Stock	10,232	6,917
Additional Paid-in Capital	144,172	85,133
Retained Earnings	27,175	23,839
Total Shareholders' Equity	179,935	113,559	$ 112,449	$ 78,845
Total Liabilities and Shareholders' Equity	1,321,256	1,105,841
Parent Company [Member]
Cash	62,727	54
Investment in Subsidiaries	119,176	116,031
Other Assets	2,121	2,354
Total Assets	184,024	118,439
Short Term Borrowings	862	862
Accrued Interest Payable	7	6
Long Term Borrowings	2,700	3,000
Other Liabilities	520	1,012
Total Liabilities	4,089	4,880
Common Stock	10,232	6,917
Additional Paid-in Capital	144,172	85,133
Retained Earnings	25,531	21,509
Total Shareholders' Equity	179,935	113,559
Total Liabilities and Shareholders' Equity	$ 184,024	$ 118,439