Note 6 - Loans and the Allowance for Loan Losses - Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Recorded investment, with an allowance recorded	$ 2,793	$ 2,718
Unpaid principal balance, with an allowance recorded	3,900	3,912
Related allowance	1,355	1,280
Average recorded investment, with an allowance recorded	2,793	1,241
Recorded investment, with no allowance recorded	10,946	9,642
Unpaid principal balance, with no allowance recorded	13,887	12,399
Average recorded investment, with no allowance recorded	13,097	13,583
Recorded investment	13,739	12,360
Unpaid principal balance	17,787	16,311
Average recorded investment	15,890	14,824
Commercial Real Estate Portfolio Segment [Member] \| Construction and Land [Member]
Recorded investment, with an allowance recorded	2
Unpaid principal balance, with an allowance recorded	2
Related allowance	2
Average recorded investment, with an allowance recorded	2	22
Recorded investment, with no allowance recorded	107	32
Unpaid principal balance, with no allowance recorded	133	56
Average recorded investment, with no allowance recorded	94	15
Recorded investment	109	32
Unpaid principal balance	135	56
Average recorded investment	96	37
Commercial Real Estate Portfolio Segment [Member] \| Farmland [Member]
Recorded investment, with an allowance recorded	21	0
Unpaid principal balance, with an allowance recorded	22	0
Related allowance	3	0
Average recorded investment, with an allowance recorded	21	0
Recorded investment, with no allowance recorded	181	112
Unpaid principal balance, with no allowance recorded	265	193
Average recorded investment, with no allowance recorded	196	9
Recorded investment	202	112
Unpaid principal balance	287	193
Average recorded investment	217	9
Commercial Real Estate Portfolio Segment [Member] \| Nonfarm Nonresidential [Member]
Recorded investment, with an allowance recorded	559	447
Unpaid principal balance, with an allowance recorded	568	501
Related allowance	18	47
Average recorded investment, with an allowance recorded	559	367
Recorded investment, with no allowance recorded	3,846	3,708
Unpaid principal balance, with no allowance recorded	4,043	3,833
Average recorded investment, with no allowance recorded	4,321	5,240
Recorded investment	4,405	4,155
Unpaid principal balance	4,611	4,334
Average recorded investment	4,880	5,607
Residential Portfolio Segment [Member] \| Real Estate Residential One- to Four-Family [Member]
Recorded investment, with an allowance recorded	251	363
Unpaid principal balance, with an allowance recorded	288	451
Related allowance	143	96
Average recorded investment, with an allowance recorded	251	303
Recorded investment, with no allowance recorded	2,707	2,365
Unpaid principal balance, with no allowance recorded	4,445	3,975
Average recorded investment, with no allowance recorded	2,818	2,708
Recorded investment	2,958	2,728
Unpaid principal balance	4,733	4,426
Average recorded investment	3,069	3,011
Residential Portfolio Segment [Member] \| Multi-family Residential [Member]
Recorded investment, with an allowance recorded	0	0
Unpaid principal balance, with an allowance recorded	0	0
Related allowance	0	0
Average recorded investment, with an allowance recorded	0	0
Recorded investment, with no allowance recorded	0	0
Unpaid principal balance, with no allowance recorded	0	0
Average recorded investment, with no allowance recorded	0	0
Recorded investment	0	0
Unpaid principal balance	0	0
Average recorded investment	0	0
Commercial Portfolio Segment [Member]
Recorded investment, with an allowance recorded	1,886	1,883
Unpaid principal balance, with an allowance recorded	2,944	2,935
Related allowance	1,115	1,112
Average recorded investment, with an allowance recorded	1,886	547
Recorded investment, with no allowance recorded	3,979	3,325
Unpaid principal balance, with no allowance recorded	4,830	4,198
Average recorded investment, with no allowance recorded	5,506	5,350
Recorded investment	5,865	5,208
Unpaid principal balance	7,774	7,133
Average recorded investment	7,392	5,897
Consumer Portfolio Segment [Member]
Recorded investment, with an allowance recorded	74	25
Unpaid principal balance, with an allowance recorded	76	25
Related allowance	74	25
Average recorded investment, with an allowance recorded	74	2
Recorded investment, with no allowance recorded	126	100
Unpaid principal balance, with no allowance recorded	171	144
Average recorded investment, with no allowance recorded	162	261
Recorded investment	200	125
Unpaid principal balance	247	169
Average recorded investment	$ 236	$ 263

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5212-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5258-111524

+ Details

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

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953550-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRelatedAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5258-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5212-111524

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bfst_FarmlandMember
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_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_MultiFamilyResidentialMember
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: