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Loans Receivable - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2017	Dec. 31, 2018
Receivables [Abstract]
Reserve For Uncollected Loan Interest	$ 40,000		$ 38,000
Financing receivable modifications	17,000
Outstanding Commitments To Borrowers Loans Classified As Troubled Debt Restructurings	$ 0	$ 0

X

- Definition

Outstanding commitments to borrowers loans classified as troubled debt restructurings.

+ References

No definition available.

+ Details

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Namespace Prefix:	bfin_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Reserve for uncollected loan interest.

+ References

No definition available.

+ Details

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

X

- 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

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

X

- References

No definition available.

+ Details

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