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

No definition available.

Name:	bfin_CollateralizedLoanReceivablePercentageOfCollateralLocatedOutsidePrimaryMarketArea
Namespace Prefix:	bfin_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents maturity of commercial leases.

+ References

No definition available.

+ Details

Name:	bfin_CommercialLeasesMaturity
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

Name:	bfin_LoanAmountToAppraisedValueOfPropertySecuringLoanPercentage
Namespace Prefix:	bfin_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents outstanding credit exposure of loans and lease receivables.

+ References

No definition available.

+ Details

Name:	bfin_LoansAndLeaseReceivablesOutstandingCreditExposure
Namespace Prefix:	bfin_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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
Balance Type:	na
Period Type:	duration

- Definition

Represents balloon maturity 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

Represents maturity of mortgage loans on real estate.

+ References

No definition available.

+ Details

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

- Definition

Represents loan maturity for national or contractual credit extension commitments.

+ References

No definition available.

+ Details

Name:	bfin_NotionalOrContractualCreditExtensionCommitmentsLoanMaturity
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

Name:	bfin_PercentThresholdOnLoanToValueRatiosRequiringPrivateMortgageInsurance
Namespace Prefix:	bfin_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	bfin_ProjectedCashFlowToLoanDebtServiceRequirement
Namespace Prefix:	bfin_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents amortization period of loans and leases receivable.

+ References

No definition available.

+ Details

Name:	bfin_RendLeasesReceivableAmortizationPeriod
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

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

- Definition

Amount of contractual principal due at origination of mortgage loan within investment in mortgage loan on real estate by entity with substantial portion of business acquiring and holding investment real estate or interest in real estate.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 12
-Paragraph Column F
-Subsection 29

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29(Column F))
-URI http://asc.fasb.org/extlink&oid=120402547&loc=d3e617274-123014

+ Details

Name:	srt_MortgageLoansOnRealEstateFaceAmountOfMortgages
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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=121611835&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
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=121611835&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
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=121593590&loc=d3e4428-111522

+ Details

Name:	us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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

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

- Definition

The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=bfin_PaycheckProtectionProgramCaresActMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis=us-gaap_FirstMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=bfin_AdjustableRateAndFixedRateResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=bfin_FixedRateNonresidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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

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Name:	us-gaap_ExternalCreditRatingByGroupingAxis=bfin_NoPublicRatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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Name:	srt_MortgageLoansOnRealEstateLoanTypeAxis=bfin_CollateralizedLoanMember
Namespace Prefix:
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Note 4 - Loans Receivable (Details Textual)	12 Months Ended
Note 4 - Loans Receivable (Details Textual)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2017 USD ($)
Loans and Leases Receivable, Gross, Total	$ 1,009,958,000	$ 1,174,728,000
Reserve for Uncollected Loan Interest	133,000	81,000
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	0
Paycheck Protection Program CARES Act [Member]
Proceeds from Issuance of Long-term Debt, Total	$ 11,000,000
First Mortgage [Member]
Percent Threshold On Loan To Value Ratios Requiring Private Mortgage Insurance			80.00%
Residential Mortgage [Member] \| Adjustable Rate and Fixed Rate Residential Mortgage [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Face Amount of Mortgages			$ 2,500,000
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
Commercial leases [Member]
Loans And Lease Receivables, Outstanding Credit Exposure	$ 10,000,000.0
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] \| Residential Mortgage [Member] \| Adjustable Rate and Fixed Rate Residential Mortgage [Member]
Notional Or Contractual Credit Extension Commitments, Loan Maturity (Year)			30 years
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] \| Commercial leases [Member] \| No Public Rating [Member]
Commercial Leases Maturity (Year)	5 years
Loans And Lease Receivables, Outstanding Credit Exposure	$ 10,000,000.0
Collateralized Loan [Member] \| Multi Family Mortgage Loans [Member]
Collateralized Loan Receivable, Percentage Of Collateral Located Outside Primary Market Area	51.40%
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	5,000,000.0
Collateralized Loan [Member] \| Maximum [Member] \| Nonresidential Real Estate Loans [Member]
Loans and Leases Receivable, Gross, Total	$ 5,000,000.0