Note 6 - Loans and the Allowance for Loan Losses - Troubled Debt Restructurings (Details) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Number of contracts	10	10
Pre-modification outstanding recorded investment	$ 8,325	$ 8,326
Post-modification outstanding recorded investment	$ 5,733	$ 5,820
Residential Portfolio Segment [Member] \| Real Estate Residential One- to Four-Family [Member]
Number of contracts	1	1
Pre-modification outstanding recorded investment
Post-modification outstanding recorded investment
Residential Portfolio Segment [Member] \| Nonfarm Nonresidential [Member]
Number of contracts	3	3
Pre-modification outstanding recorded investment	$ 2,411	$ 2,412
Post-modification outstanding recorded investment	$ 2,300	$ 2,308
Commercial Portfolio Segment [Member]
Number of contracts	6
Pre-modification outstanding recorded investment	$ 5,914
Post-modification outstanding recorded investment	$ 3,433
Consumer Portfolio Segment [Member]
Number of contracts		6
Pre-modification outstanding recorded investment		$ 5,914
Post-modification outstanding recorded investment		$ 3,512

Number of financing receivables that have been modified by troubled debt restructurings.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bfst_RealEstateResidentialOneToFourFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bfst_NonfarmNonresidentialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: