Note 5 - Other Real Estate Owned - Real Estate Acquired Through Foreclosure (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Valuation allowance	$ 0	$ 0	$ 0	$ (38)	$ (21)	$ (23)
Other real estate owned, net	110	$ 143	186	$ 269	$ 497	$ 1,226
One-to-Four Family Residential Real Estate [Member]
Balance	110		186
Valuation allowance	0		0
Other real estate owned, net	$ 110		$ 186

Carrying amount of other real estate not separately disclosed, including, but not limited to, real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned, before valuation allowance.

No definition available.

+ Details

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

- Definition

+ 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=120398452&loc=d3e534808-122878

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

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

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=bfin_OnetoFourFamilyResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: