Minimum prime interest rate for debt instrument.

No definition available.

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Namespace Prefix:	bfin_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

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+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

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Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the aggregate of total long-term debt, including current maturities and short-term debt.

+ References

No definition available.

+ Details

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

- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

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Balance Type:	debit
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- Definition

The carrying value as of the balance sheet date of the current portion of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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- Details

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- Details

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- Details

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Note 7 - Borrowings and Subordinated Debt (Details Textual) - USD ($)		12 Months Ended
	Apr. 14, 2021	Dec. 31, 2020	Jun. 30, 2021
Federal Home Loan Bank Advances [Member]
Debt, Long-term and Short-term, Combined Amount, Total			$ 0
Line of Credit [Member] \| Unsecured Debt [Member]
Line of Credit, Current		$ 5,000,000.0
Debt Instrument, Prime Rate Floor		2.40%
Prime Rate [Member] \| Line of Credit [Member] \| Unsecured Debt [Member]
Debt Instrument, Basis Spread on Variable Rate		0.75%
The 3.75 Percent Fixed-to-Floating Rate Subordinated Notes [Member]
Debt Instrument, Face Amount	$ 20,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	3.75%
Debt Issuance Costs, Net, Total	$ 441,000		$ 432,000
Debt Instrument, Term (Year)	10 years
The 3.75 Percent Fixed-to-Floating Rate Subordinated Notes [Member] \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]
Debt Instrument, Basis Spread on Variable Rate	2.99%