Note 7 - Loans and the Allowance for Loan Losses - Troubled Debt Restructurings (Details) $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Number of contracts	10	6
Pre-modification outstanding recorded investment	$ 8,326	$ 5,201
Post-modification outstanding recorded investment	$ 5,820	$ 3,060
Residential Portfolio Segment [Member] \| Real Estate Residential One- to Four-Family [Member]
Number of contracts	1	2
Pre-modification outstanding recorded investment		$ 703
Post-modification outstanding recorded investment		$ 455
Residential Portfolio Segment [Member] \| Nonfarm Nonresidential [Member]
Number of contracts	3
Pre-modification outstanding recorded investment	$ 2,412
Post-modification outstanding recorded investment	$ 2,308
Consumer Portfolio Segment [Member]
Number of contracts		4
Pre-modification outstanding recorded investment		$ 4,498
Post-modification outstanding recorded investment		$ 2,605
Commercial 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

+ Details

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

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

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

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ 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_ConsumerPortfolioSegmentMember
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: