Significant Accounting Policies (Policies)	9 Months Ended
Sep. 30, 2022
Accounting Policies [Abstract]
Basis of Accounting, Policy [Policy Text Block]	Basis of Presentation: BankFinancial Corporation, a Maryland corporation headquartered in Burr Ridge, Illinois, is the owner of all of the issued and outstanding capital stock of BankFinancial, National Association (the “Bank”). The interim unaudited consolidated financial statements include the accounts and transactions of BankFinancial Corporation, the Bank, and the Bank’s wholly-owned subsidiaries, Financial Assurance Services, Inc. and BFIN Asset Recovery Company, LLC (collectively, “the Company”), and reflect all normal and recurring adjustments that are, in the opinion of management, considered necessary for a fair presentation of the financial condition and results of operations for the periods presented. Such adjustments are the only adjustments reflected in the accompanying financial statements. All significant intercompany accounts and transactions have been eliminated. The results of operations for the nine-month period ended September 30, 2022 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2022 or for any other period. Certain information and note disclosures normally included in financial statements prepared in conformity with accounting principles generally accepted in the United States of America (“US GAAP”) have been condensed or omitted pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”).
Use of Estimates, Policy [Policy Text Block]	Use of Estimates: The preparation of the consolidated financial statements in conformity with US GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Although these estimates and assumptions are based on the best available information, actual information and actual results could differ from those estimates.
Loans and Loan Income [Policy Text Block]	Factored Receivables: The Company purchases invoices from its factoring customers in schedules or batches. The face value of the invoices purchased or amount advanced is recorded by the Company as factored receivables, and the unadvanced portions of the invoices purchased, less fees, are considered customer reserves. The customer reserves are held to settle any payment disputes or collection shortfalls, may be used to pay customers’ obligations to various third parties as directed by the customer, are periodically released to or withdrawn by customers, and are reported as noninterest-bearing deposits in the Consolidated Statements of Financial Condition. The unpaid principal balances of these receivables were $7.2 million and $187,000 at September 30, 2022 and December 31, 2021, respectively, and are included in commercial loans and leases. The customer reserves associated with the factored receivables were $1.3 million and $122,000 at September 30, 2022 and December 31, 2021, respectively. Factoring fees are recognized in interest income as incurred by the customer and deducted from the customer's reserve balances. Other factoring-related fees, which include wire transfer fees, broker fees, and other similar fees, are reported by the Company as loan servicing fees in noninterest income.
Reclassification, Comparability Adjustment [Policy Text Block]	Reclassifications: Certain reclassifications have been made in the prior period’s financial statements to conform them to the current period’s presentation. These unaudited consolidated financial statements should be read in conjunction with the Company’s Annual Report on Form 10-K for the year ended December 31, 2021, as filed with the Securities and Exchange Commission.
New Accounting Pronouncements, Policy [Policy Text Block]	*Newly Issued Not* Yet Effective Accounting Standards** In June 2016, the FASB issued ASU No. 2016-13, “Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments” (“ASU 2016-13”). These amendments require the measurement of all expected credit losses for financial assets held at the reporting date based on historical experience, current conditions, and reasonable and supportable forecasts. Financial institutions and other organizations will now use forward-looking information to better inform their credit loss estimates. Many of the loss estimation techniques applied today will still be permitted, although the inputs to those techniques will change to reflect the full amount of expected credit losses. In addition, the ASU amends the accounting for credit losses on available-for-sale debt securities and purchased financial assets with credit deterioration. For SEC filers who are smaller reporting companies, such as the Company, ASU 2016-13 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2022.

Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2022

Basis of Accounting, Policy [Policy Text Block]

Basis of Presentation: BankFinancial Corporation, a Maryland corporation headquartered in Burr Ridge, Illinois, is the owner of all of the issued and outstanding capital stock of BankFinancial, National Association (the “Bank”). The interim unaudited consolidated financial statements include the accounts and transactions of BankFinancial Corporation, the Bank, and the Bank’s wholly-owned subsidiaries, Financial Assurance Services, Inc. and BFIN Asset Recovery Company, LLC (collectively, “the Company”), and reflect all normal and recurring adjustments that are, in the opinion of management, considered necessary for a fair presentation of the financial condition and results of operations for the periods presented. Such adjustments are the only adjustments reflected in the accompanying financial statements. All significant intercompany accounts and transactions have been eliminated. The results of operations for the nine-month period ended September 30, 2022 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2022 or for any other period.

Certain information and note disclosures normally included in financial statements prepared in conformity with accounting principles generally accepted in the United States of America (“US GAAP”) have been condensed or omitted pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”).

Use of Estimates, Policy [Policy Text Block]

Use of Estimates: The preparation of the consolidated financial statements in conformity with US GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Although these estimates and assumptions are based on the best available information, actual information and actual results could differ from those estimates.

Loans and Loan Income [Policy Text Block]

Factored Receivables: The Company purchases invoices from its factoring customers in schedules or batches. The face value of the invoices purchased or amount advanced is recorded by the Company as factored receivables, and the unadvanced portions of the invoices purchased, less fees, are considered customer reserves. The customer reserves are held to settle any payment disputes or collection shortfalls, may be used to pay customers’ obligations to various third parties as directed by the customer, are periodically released to or withdrawn by customers, and are reported as noninterest-bearing deposits in the Consolidated Statements of Financial Condition. The unpaid principal balances of these receivables were $7.2 million and $187,000 at September 30, 2022 and December 31, 2021, respectively, and are included in commercial loans and leases. The customer reserves associated with the factored receivables were $1.3 million and $122,000 at September 30, 2022 and December 31, 2021, respectively.

Factoring fees are recognized in interest income as incurred by the customer and deducted from the customer's reserve balances. Other factoring-related fees, which include wire transfer fees, broker fees, and other similar fees, are reported by the Company as loan servicing fees in noninterest income.

Reclassification, Comparability Adjustment [Policy Text Block]

Reclassifications: Certain reclassifications have been made in the prior period’s financial statements to conform them to the current period’s presentation.

These unaudited consolidated financial statements should be read in conjunction with the Company’s Annual Report on Form 10-K for the year ended December 31, 2021, as filed with the Securities and Exchange Commission.

New Accounting Pronouncements, Policy [Policy Text Block]

Newly Issued Not Yet Effective Accounting Standards

In June 2016, the FASB issued ASU No. 2016-13, “Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments” (“ASU 2016-13”). These amendments require the measurement of all expected credit losses for financial assets held at the reporting date based on historical experience, current conditions, and reasonable and supportable forecasts. Financial institutions and other organizations will now use forward-looking information to better inform their credit loss estimates. Many of the loss estimation techniques applied today will still be permitted, although the inputs to those techniques will change to reflect the full amount of expected credit losses. In addition, the ASU amends the accounting for credit losses on available-for-sale debt securities and purchased financial assets with credit deterioration. For SEC filers who are smaller reporting companies, such as the Company, ASU 2016-13 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2022.