Loans Receivable - Loan Origination and Risk Management (Details) - USD ($)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, amount	$ 1,174,728,000	$ 1,331,194,000
Maximum [Member] \| Finance Leases Financing Receivable [Member] \| Investment Grade [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding credit exposure	20,000,000
Multi-family mortgage loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, amount	$ 563,750,000	619,870,000
Multi-family mortgage loans [Member] \| Collateralized Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collateralized Loan Receivable, Percentage Of Collateral Located Outside Primary Market Area	57.00%
Multi-family mortgage loans [Member] \| Collateralized Loan [Member] \| Mortgage loans above $400,000 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Projected Cash Flow To Loan Debt Service Requirement	120.00%
Multi-family mortgage loans [Member] \| Minimum [Member] \| Collateralized Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, amount	$ 500,000
Multi-family mortgage loans [Member] \| Maximum [Member] \| Collateralized Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, amount	$ 5,000,000.0
Loan amount as a percentage to value of property collateralized	0.8
Nonresidential real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, amount	$ 134,674,000	152,442,000
Nonresidential real estate loans [Member] \| Collateralized Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Projected Cash Flow To Loan Debt Service Requirement	120.00%
Nonresidential real estate loans [Member] \| Collateralized Loan [Member] \| Fixed Rate Nonresidential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortization period	15 years
Nonresidential real estate loans [Member] \| Minimum [Member] \| Collateralized Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, amount	$ 500,000
Maturity period	3 years
Balloon maturity	3 years
Amortization period	20 years
Nonresidential real estate loans [Member] \| Maximum [Member] \| Collateralized Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, amount	$ 5,000,000.0
Loan amount as a percentage to value of property collateralized	0.8
Maturity period	5 years
Balloon maturity	5 years
Amortization period	30 years
Residential mortgage [Member] \| Maximum [Member] \| Adjustable Rate And Fixed Rate Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Maturity period	30 years
Residential mortgage [Member] \| Maximum [Member] \| $2,500,000 [Member] \| Adjustable Rate And Fixed Rate Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Face Amount of Mortgages	$ 2,500,000
Finance Leases Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases, amount	$ 272,629,000	$ 299,394,000
Finance Leases Financing Receivable [Member] \| Maximum [Member] \| No Public Rating [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Leases, Maturity	5 years
Outstanding credit exposure	$ 10,000,000
Finance Leases Financing Receivable [Member] \| Maximum [Member] \| Investment Grade [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding credit exposure	$ 10,000,000

Collateralized Loan Receivable, Percentage Of Collateral Located Outside Primary Market Area

No definition available.

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

- Definition

Commercial Leases, Maturity

+ References

No definition available.

+ Details

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

- Definition

Loan Amount To Appraised Value Of Property Securing Loan, Percentage

+ References

No definition available.

+ Details

Name:	bfin_LoanAmountToAppraisedValueOfPropertySecuringLoanPercentage
Namespace Prefix:	bfin_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Loans And Lease Receivables, Outstanding Credit Exposure

+ References

No definition available.

+ Details

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

- Definition

Loans And Leases Receivable, Amortization Period

+ References

No definition available.

+ Details

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

- Definition

Loans And Leases Receivable, Balloon Maturity

+ References

No definition available.

+ Details

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

- Definition

Mortage Loans on Real Estate, Maturity

+ References

No definition available.

+ Details

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

- Definition

Notional Or Contractual Credit Extension Commitments, Loan Maturity

+ References

No definition available.

+ Details

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

- Definition

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

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=6589523&loc=d3e617274-123014

+ Details

Name:	srt_MortgageLoansOnRealEstateFaceAmountOfMortgages
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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=82891408&loc=d3e4428-111522

+ Details

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

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	bfin_LoanTypeAxis=bfin_CollateralizedLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MortgageLoansOnRealEstateOriginalLoanAmountAxis=bfin_MortgageLoansAbove400000Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
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:

- 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:	srt_MortgageLoansOnRealEstateOriginalLoanAmountAxis=bfin_TwoMillionFiveHundredThousandMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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