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Note 7 - Fair Value - Quantitative Information for Financial Instruments on Non-recurring Basis (Details) $ in Thousands | Sep. 30, 2023 USD ($)
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Other real estate owned | $ 468
Other real estate owned | 10.0
One-to-Four Family Residential Real Estate [Member] | Fair Value, Nonrecurring [Member] |
Other foreclosed assets | $ 387
One-to-Four Family Residential Real Estate [Member] | Fair Value, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member] |
Other foreclosed assets | $ 387
One-to-Four Family Residential Real Estate [Member] | Fair Value, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] |
Other foreclosed assets | 6.2

X
- Definition Represents fair value of foreclosed assets. + References No definition available. + Details
Name: | bfin_ForeclosedAssetsFairValue
Namespace Prefix: | bfin_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Represents measurement input for foreclosed assets. + References No definition available. + Details
Name: | bfin_ForeclosedAssetsMeasurementInput
Namespace Prefix: | bfin_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

X
- Definition Value of input used to measure other real estate owned. + References No definition available. + Details
Name: | bfin_OtherRealEstateMeasurementInput
Namespace Prefix: | bfin_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

X
- Definition The fair value portion on other real estate owned. + References No definition available. + Details
Name: | bfin_OtherRealEstateOwnedFairValueDisclosure
Namespace Prefix: | bfin_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bfin_OnetoFourFamilyResidentialRealEstateMember
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: |