Note 27 - Financial Statements - Parent Company Only - Balance Sheets for Business First Bancshares, Inc. (Parent Company) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 17, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets:
Other Assets	$ 2,699	$ 2,534
Total Assets	2,273,835	2,094,896
Liabilities:
Accrued Interest Payable	1,533	1,374
Subordinated Debt	25,000	25,000	$ 25,000
Other Liabilities	19,206	7,301
Total Liabilities	1,988,738	1,834,838
Shareholders' Equity:
Common Stock	13,279	13,213
Additional Paid-in Capital	212,505	212,332
Retained Earnings	56,700	37,982
Accumulated Other Comprehensive Income (Loss)	2,613	(3,469)
Total Shareholders' Equity	285,097	260,058		$ 179,935	$ 113,559
Total Liabilities and Shareholders' Equity	2,273,835	2,094,896
Parent Company [Member]
Assets:
Cash	6,154	14,643
Investment in Subsidiaries	300,122	267,270
Other Assets	3,843	3,217
Total Assets	310,119	285,130
Liabilities:
Accrued Interest Payable	22	69
Subordinated Debt	25,000	25,000
Other Liabilities		3
Total Liabilities	25,022	25,072
Shareholders' Equity:
Common Stock	13,279	13,213
Additional Paid-in Capital	212,505	212,332
Retained Earnings	56,700	37,982
Accumulated Other Comprehensive Income (Loss)	2,613	(3,469)
Total Shareholders' Equity	285,097	260,058
Total Liabilities and Shareholders' Equity	$ 310,119	$ 285,130