other real estate owned at carrying value before valuation allowance

No definition available.

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

- Definition

Real estate acquired through foreclosure fair value.

+ References

No definition available.

+ Details

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

- Definition

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), without single maturity date and not allocated over maturity grouping.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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

- References

No definition available.

+ Details

Name:	us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of the sensitivity of the fair value measurements to changes in unobservable inputs and the inter-relationships between those inputs as well as how those inter-relationships might magnify or mitigate the effect of changes in the unobservable inputs on the fair value measurements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementsSensitivityAnalysisDescription
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of the valuation processes in place for fair value measurements categorized within Level 3 of the fair value hierarchy.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementsValuationProcessesDescription
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

+ Details

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

- Definition

Value of input used to measure real estate owned by financial institution by financial institution which is not directly related to its business.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_OtherRealEstateOwnedMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

For each period for which an income statement is required, disclosure of the changes in the allowance, including balances at end of period.

+ References

+ Details

Name:	us-gaap_RealEstateOwnedValuationAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of gain recognized in the income statement for an increase in valuation of real estate owned (REO).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(4))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 35
-Paragraph 40
-URI http://asc.fasb.org/extlink&oid=29634951&loc=d3e1720-110224

+ Details

Name:	us-gaap_RealEstateOwnedValuationAllowanceValuationIncrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_RealEstatePropertiesDomain
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=us-gaap_LandMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Fair Value - Financial Assets Accounted for at Fair Value (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	$ 0		$ 27,000
Schedule of Company's financial instruments measured at fair values
Total Fair value	83,240,000		88,179,000
Real Estate Acquired Through Foreclosure Fair Value			1,000
Real Estate Owned, Valuation Allowance, Valuation Increase	0	$ 25,000
Other real estate owned at carrying value before valuation allowance	921,000		1,249,000
Real Estate Owned, Valuation Allowance	0	$ 321,000	23,000	$ 305,000
Certificates of deposit [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	69,011,000		73,507,000
Equity Funds [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	0		0
Residential Mortgage Backed Securities [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	10,207,000		10,478,000
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	10,207,000
Collateralized Debt Obligations [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	3,511,000		3,685,000
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	3,511,000
Municipal Bonds [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	511,000		509,000
Nonrecurring [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	83,240,000		88,179,000
Nonrecurring [Member] \| Certificates of deposit [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	69,011,000		73,507,000
Nonrecurring [Member] \| Mortgage-backed securities - residential [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	10,207,000		10,478,000
Nonrecurring [Member] \| Collateralized mortgage obligations - residential [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	3,511,000		3,685,000
Nonrecurring [Member] \| Municipal Securities [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	511,000		509,000
Nonrecurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	0		0
Nonrecurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Certificates of deposit [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	0		0
Nonrecurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Mortgage-backed securities - residential [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	0		0
Nonrecurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Collateralized mortgage obligations - residential [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	0		0
Nonrecurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Municipal Securities [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	0		0
Nonrecurring [Member] \| Significant Observable Inputs (Level 2) [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	83,240,000		88,179,000
Nonrecurring [Member] \| Significant Observable Inputs (Level 2) [Member] \| Certificates of deposit [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	69,011,000
Nonrecurring [Member] \| Significant Observable Inputs (Level 2) [Member] \| Mortgage-backed securities - residential [Member]
Schedule of Company's financial instruments measured at fair values
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	10,207,000
Nonrecurring [Member] \| Significant Observable Inputs (Level 2) [Member] \| Collateralized mortgage obligations - residential [Member]
Schedule of Company's financial instruments measured at fair values
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	3,511,000
Nonrecurring [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	0		0
Nonrecurring [Member] \| Significant Unobservable Inputs (Level 3) [Member] \| Certificates of deposit [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	0		0
Nonrecurring [Member] \| Significant Unobservable Inputs (Level 3) [Member] \| Mortgage-backed securities - residential [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	0		0
Nonrecurring [Member] \| Significant Unobservable Inputs (Level 3) [Member] \| Collateralized mortgage obligations - residential [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	0		0
Nonrecurring [Member] \| Significant Unobservable Inputs (Level 3) [Member] \| Municipal Securities [Member]
Schedule of Company's financial instruments measured at fair values
Total Fair value	0		0
Commercial Real Estate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	$ 0		27,000
Impaired Financing Receivable, with Related Allowance, Recorded Investment			270,000
Land Loans [Member]
Schedule of Company's financial instruments measured at fair values
Real Estate Acquired Through Foreclosure Fair Value			1,000
Land Loans [Member] \| Nonrecurring [Member]
Schedule of Company's financial instruments measured at fair values
Real Estate Acquired Through Foreclosure Fair Value			1,000
Land Loans [Member] \| Nonrecurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Schedule of Company's financial instruments measured at fair values
Real Estate Acquired Through Foreclosure Fair Value			0
Land Loans [Member] \| Nonrecurring [Member] \| Significant Observable Inputs (Level 2) [Member]
Schedule of Company's financial instruments measured at fair values
Real Estate Acquired Through Foreclosure Fair Value			0
Land Loans [Member] \| Nonrecurring [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Schedule of Company's financial instruments measured at fair values
Real Estate Acquired Through Foreclosure Fair Value			$ 1,000
Sales Comparison Valuation Technique [Member] \| Land Loans [Member]
Schedule of Company's financial instruments measured at fair values
Fair Value Measurements, Valuation Processes, Description			Sales comparison
Fair Value Measurements, Sensitivity Analysis, Description			Discount applied to valuation
Real estate owned, measurement input	0.123
Measurement Input, Comparability Adjustment [Member] \| Maximum [Member] \| Real Estate Properties [Domain] \| Nonresidential Real Estate [Member]
Schedule of Company's financial instruments measured at fair values
Real estate owned, measurement input			0.1522
Measurement Input, Comparability Adjustment [Member] \| Weighted Average [Member] \| Real Estate Properties [Domain] \| Nonresidential Real Estate [Member]
Schedule of Company's financial instruments measured at fair values
Real estate owned, measurement input			0.1100
Land [Member]
Schedule of Company's financial instruments measured at fair values
Other real estate owned at carrying value before valuation allowance	$ 0		$ 24,000
Real Estate Owned, Valuation Allowance	$ 0		$ 23,000