Note 7 - Fair Value - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Securities	$ 239,549	$ 153,203
Fair Value, Inputs, Level 1 [Member]
Securities	92,709	112,508
Fair Value, Inputs, Level 2 [Member]
Securities	146,840	40,695
Fair Value, Inputs, Level 3 [Member]
Securities	0	0
Reported Value Measurement [Member]
Cash and cash equivalents and time deposits in other financial institutions	163,188	207,997
Securities	239,549	153,203
Loans receivable, net of allowance for credit losses	1,007,980	1,050,761
FHLB and FRB stock	7,490	7,490
Accrued interest receivable	7,709	7,542
Certificates of deposit	224,362	222,391
Borrowings	25,000	25,000
Subordinated notes	18,705	19,678
Estimate of Fair Value Measurement [Member]
Cash and cash equivalents and time deposits in other financial institutions	163,188	207,997
Securities	239,549	153,203
Loans receivable, net of allowance for credit losses	958,233	997,897
Accrued interest receivable	7,709	7,542
Certificates of deposit	222,221	220,222
Borrowings	24,830	24,960
Subordinated notes	16,934	17,698
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents and time deposits in other financial institutions	123,542	177,169
Securities	92,709	112,508
Loans receivable, net of allowance for credit losses	0	0
FHLB and FRB stock	0	0
Accrued interest receivable	342	475
Certificates of deposit	0	0
Borrowings	0	0
Subordinated notes	0	0
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member]
Cash and cash equivalents and time deposits in other financial institutions	39,646	30,828
Securities	146,840	40,695
Loans receivable, net of allowance for credit losses	0	0
FHLB and FRB stock	0	0
Accrued interest receivable	1,187	500
Certificates of deposit	222,221	220,222
Borrowings	24,830	24,960
Subordinated notes	16,934	17,698
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 3 [Member]
Cash and cash equivalents and time deposits in other financial institutions	0	0
Securities	0	0
Loans receivable, net of allowance for credit losses	958,233	997,897
FHLB and FRB stock	0	0
Accrued interest receivable	6,180	6,567
Certificates of deposit	0	0
Borrowings	0	$ 0
Subordinated notes	$ 0

Fair value portion of currency on hand as well as demand and time deposits with banks or financial institutions.

No definition available.

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

- Definition

The fair value disclosure of certificates of deposit liabilities.

+ References

No definition available.

+ Details

Name:	bfin_CertificatesOfDepositFairValueDisclosure
Namespace Prefix:	bfin_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fair value portion of FHLB and FRB stock.

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of amount due from customers or clients for goods or services, including trade receivables, that have been delivered or sold in the normal course of business, and amounts due from others, including related parties.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-10

+ Details

Name:	us-gaap_AccountsReceivableFairValueDisclosure
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/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 103
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-103

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 100
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-100

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)(i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (aa)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481830/320-10-45-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479130/326-30-45-1

+ Details

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

- Definition

Fair value of subordinated debt obligations including, but not limited to, subordinated loan, subordinated bond, subordinated debenture or junior debt.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2E
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2E

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

+ Details

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

- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-10

+ Details

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

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents the amount of short-term debt existing as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

+ Details

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

- 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_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: