Note 4 - Loans Receivable (Details Textual)	12 Months Ended
Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Loans and Leases Receivable, Gross, Total	$ 1,050,638,000	$ 1,009,958,000
Reserve for Uncollected Loan Interest	140,000	133,000
Financing Receivable, Troubled Debt Restructuring, Commitment to Lend	$ 0	$ 0
Financing Receivable, Modifications, Number of Contracts	0	0
Financing Receivable, Troubled Debt Restructuring, Subsequent Default, Number of Contracts	0
Residential Mortgage [Member] \| First Mortgage [Member]
Percent Threshold On Loan To Value Ratios Requiring Private Mortgage Insurance	80.00%
Nonresidential Real Estate Loans [Member]
Projected Cash Flow To Loan Debt Service Requirement	120.00%
Nonresidential Real Estate Loans [Member] \| Fixed Rate Nonresidential Mortgage [Member]
Loans and Leases Receivable, Amortization Period (Year)	15 years
Equipment Finance [Member]
Loans and Lease Receivables Outstanding Credit Exposure, Average Amount	$ 1,100,000
Commercial leases [Member]
Loans and Lease Receivables Outstanding Credit Exposure, Average Amount	$ 1,100,000
Minimum [Member] \| Nonresidential Real Estate Loans [Member]
Mortgage Loans on Real Estate, Maturity (Year)	3 years
Loans and Leases Receivable, Balloon Maturity (Year)	3 years
Loans and Leases Receivable, Amortization Period (Year)	20 years
Maximum [Member] \| Multi Family Mortgage Loans [Member]
Loan Amount To Appraised Value Of Property Securing Loan, Percentage	80.00%
Maximum [Member] \| Nonresidential Real Estate Loans [Member]
Mortgage Loans on Real Estate, Maturity (Year)	5 years
Loans and Leases Receivable, Balloon Maturity (Year)	5 years
Loans and Leases Receivable, Amortization Period (Year)	30 years
Maximum [Member] \| Equipment Finance [Member]
Commercial Leases Maturity (Year)	5 years
Loans And Lease Receivables, Outstanding Credit Exposure	$ 10,000,000
Maximum [Member] \| Commercial leases [Member]
Loans And Lease Receivables, Outstanding Credit Exposure	$ 15,000,000
Collateralized Loan [Member] \| Multi Family Mortgage Loans [Member]
Collateralized Loan Receivable, Percentage Of Collateral Located Outside Primary Market Area	45.00%
Projected Cash Flow To Loan Debt Service Requirement	120.00%
Collateralized Loan [Member] \| Minimum [Member] \| Multi Family Mortgage Loans [Member]
Loans and Leases Receivable, Gross, Total	$ 500,000
Collateralized Loan [Member] \| Minimum [Member] \| Nonresidential Real Estate Loans [Member]
Loans and Leases Receivable, Gross, Total	500,000
Collateralized Loan [Member] \| Maximum [Member] \| Multi Family Mortgage Loans [Member]
Loans and Leases Receivable, Gross, Total	6,000,000.0
Collateralized Loan [Member] \| Maximum [Member] \| Nonresidential Real Estate Loans [Member]
Loans and Leases Receivable, Gross, Total	$ 6,000,000.0

Percentage of collateral located outside primary market area for collateralized loan receivable.

No definition available.

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Namespace Prefix:	bfin_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents maturity of commercial leases.

+ References

No definition available.

+ Details

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Namespace Prefix:	bfin_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents percentage of loan amount to appraised value of property securing loan.

+ References

No definition available.

+ Details

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Namespace Prefix:	bfin_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents outstanding credit exposure of loans and lease receivables.

+ References

No definition available.

+ Details

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Namespace Prefix:	bfin_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the average amount of of an equipment finance transaction.

+ References

No definition available.

+ Details

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Namespace Prefix:	bfin_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- Definition

Represents amortization period of loans and leases receivable.

+ References

No definition available.

+ Details

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Namespace Prefix:	bfin_
Data Type:	xbrli:durationItemType
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Period Type:	duration

- Definition

Represents balloon maturity of loans and leases receivable.

+ References

No definition available.

+ Details

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- Definition

Represents maturity of mortgage loans on real estate.

+ References

No definition available.

+ Details

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Namespace Prefix:	bfin_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents percentage threshold on loan to value ratios requiring private mortgage insurance.

+ References

No definition available.

+ Details

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Namespace Prefix:	bfin_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents percentage of projected cash flow to loan debt service requirement.

+ References

No definition available.

+ Details

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Namespace Prefix:	bfin_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents amortization period of loans and leases receivable.

+ References

No definition available.

+ Details

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Namespace Prefix:	bfin_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of reserve for uncollected loan interest.

+ References

No definition available.

+ Details

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Namespace Prefix:	bfin_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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

+ References

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=123577603&loc=SL6953676-111524

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Balance Type:	na
Period Type:	duration

- Definition

Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953682-111524

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Period Type:	duration

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=124259787&loc=d3e4428-111522

+ Details

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Balance Type:	debit
Period Type:	instant

- Definition

Amount of unfunded portion of contractual agreement to lend funds to borrower for financing receivable modified as troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18794-111554

+ Details

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

- Details

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Namespace Prefix:
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Balance Type:
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- Details

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- Details

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Namespace Prefix:
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- Details

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- Details

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- Details

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