Note 4 - Loans Receivable - Allowance for Loan Losses and Loans Receivable (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Allowance for loan losses, individually evaluated for impairment	$ 20		$ 0
Allowance for loan losses, collectively evaluated for impairment	7,991		7,632
Allowance for loan losses	8,011	$ 8,156	7,632	$ 7,603	$ 7,824	$ 8,470
Loan balances, individually evaluated for impairment	4,367		2,743
Loan balances, collectively evaluated for impairment	1,069,169		1,171,985
Loans receivable	1,073,536		1,174,728
Net deferred loan origination costs	367		912
Allowance for loan losses	(8,011)	(8,156)	(7,632)	(7,603)	(7,824)	(8,470)
Loans receivable, net of allowance for loan losses: September 30, 2020, $8,011 and December 31, 2019, $7,632	1,065,892		1,168,008
Nonresidential Real Estate [Member]
Allowance for loan losses, individually evaluated for impairment			0
Real Estate Portfolio [Member] \| One-to-Four Family Residential Real Estate [Member]
Allowance for loan losses, individually evaluated for impairment	0		0
Allowance for loan losses, collectively evaluated for impairment	623		675
Allowance for loan losses	623	665	675	580	563	699
Loan balances, individually evaluated for impairment	1,746		1,835
Loan balances, collectively evaluated for impairment	43,066		53,915
Loans receivable	44,812		55,750
Allowance for loan losses	(623)	(665)	(675)	(580)	(563)	(699)
Real Estate Portfolio [Member] \| Multi-family Mortgage [Member]
Allowance for loan losses, individually evaluated for impairment	0		0
Allowance for loan losses, collectively evaluated for impairment	4,143		3,676
Allowance for loan losses	4,143	4,185	3,676	3,728	3,988	3,991
Loan balances, individually evaluated for impairment	596		620
Loan balances, collectively evaluated for impairment	522,229		563,130
Loans receivable	522,825		563,750
Allowance for loan losses	(4,143)	(4,185)	(3,676)	(3,728)	(3,988)	(3,991)
Real Estate Portfolio [Member] \| Nonresidential Real Estate [Member]
Allowance for loan losses, individually evaluated for impairment	20		0
Allowance for loan losses, collectively evaluated for impairment	1,697		1,176
Allowance for loan losses	1,717	1,602	1,176	1,217	1,195	1,476
Loan balances, individually evaluated for impairment	1,870		288
Loan balances, collectively evaluated for impairment	122,607		134,386
Loans receivable	124,477		134,674
Allowance for loan losses	(1,717)	(1,602)	(1,176)	(1,217)	(1,195)	(1,476)
Commercial Portfolio Segment [Member]
Allowance for loan losses, individually evaluated for impairment	0		0
Allowance for loan losses, collectively evaluated for impairment	1,480		2,065
Allowance for loan losses	1,480	1,658	2,065	2,040	2,044	2,272
Loan balances, individually evaluated for impairment	155		0
Loan balances, collectively evaluated for impairment	379,483		418,343
Loans receivable	379,638		418,343
Allowance for loan losses	(1,480)	(1,658)	(2,065)	(2,040)	(2,044)	(2,272)
Consumer Portfolio Segment [Member]
Allowance for loan losses, individually evaluated for impairment	0		0
Allowance for loan losses, collectively evaluated for impairment	48		40
Allowance for loan losses	48	46	40	36	31	28
Loan balances, individually evaluated for impairment	0		0
Loan balances, collectively evaluated for impairment	1,784		2,211
Loans receivable	1,784		2,211
Allowance for loan losses	$ (48)	$ (46)	$ (40)	$ (36)	$ (31)	$ (28)

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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

- Definition

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

+ References

+ Details

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

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

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

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for 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 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=121614798&loc=d3e15032-111544

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

+ Details

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

- Definition

Amount of deferred interest and fee income, unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of acquired loans. Excludes amounts for 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 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=121568338&loc=d3e45377-112738

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5074-111524

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159169&loc=d3e10152-111534

+ Details

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

- 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

+ Details

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

- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

+ 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=121593590&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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