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Note 8 - Fair Value - Quantitative Information for Financial Instruments on Non-recurring Basis (Details) - Real Estate Portfolio [Member] - Nonresidential Real Estate [Member] - Fair Value, Nonrecurring [Member] $ in Thousands	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Impaired loans - nonresidential real estate, fair value	$ 268	$ 268
Fair Value, Inputs, Level 3 [Member]
Impaired loans - nonresidential real estate, fair value	$ 268	$ 268
Fair Value, Inputs, Level 3 [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member]
Imapired loans, measurement input	0.220	0.220

X

- Definition

Fair value portion of impaired financing receivable.

+ References

No definition available.

+ Details

Name:	bfin_ImpairedFinancingReceivableFairValue
Namespace Prefix:	bfin_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Value of input used to measure impaired financing receivable.

+ References

No definition available.

+ Details

Name:	bfin_ImpairedFinancingReceivableMeasurementInput
Namespace Prefix:	bfin_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

X

- Details

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

X

- Details

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

X

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_MarketApproachValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: