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Loans Receivable - Troubled Debt Restructuring (Details) - USD ($) | Dec. 31, 2019 | Dec. 31, 2018
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Troubled Debt Restructurings |  |
Financing receivable modifications | $ 0 | $ 17,000

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- Definition Troubled debt restructurings. + References No definition available. + Details
Name: | bfin_TroubledDebtRestructuringsAbstract
Namespace Prefix: | bfin_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 40 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554 + Details
Name: | us-gaap_FinancingReceivableModificationsRecordedInvestment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant