Significant Accounting Policies (Policies)	3 Months Ended
Mar. 31, 2021
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, NA (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 three month period ended March 31, 2021 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2021 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, actual results could differ from those estimates.
Covid-19 [Policy Text Block]	COVID-19: On March 11, 2020, the World Health Organization declared the outbreak of a novel coronavirus (“COVID-19”) as a global pandemic. The declaration of a global pandemic indicates that almost all public commerce and related business activities must be, to varying degrees, curtailed with the goal of decreasing the rate of new infections. The outbreak of COVID-19 has adversely impacted, and could further adversely impact, a broad range of industries in which the Company’s customers operate and impair their ability to fulfill their financial obligations to the Company. On March 3, 2020, the Federal Open Market Committee reduced the target federal funds rate range by 50 basis points to 1.00% to 1.25%. This rate was further reduced to a target range of 0% to 0.25% on March 16, 2020. These reductions in interest rates and other effects of the COVID-19 outbreak may adversely affect the Company’s financial condition and results of operations. As a result of the spread of the COVID-19 coronavirus, economic uncertainties have arisen which are likely to negatively impact net interest income and noninterest income. Other financial impacts could occur though such potential impact is unknown at this time.
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, 2020, 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)

3 Months Ended

Mar. 31, 2021

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, NA (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 three month period ended March 31, 2021 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2021 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, actual results could differ from those estimates.

Covid-19 [Policy Text Block]

COVID-19: On March 11, 2020, the World Health Organization declared the outbreak of a novel coronavirus (“COVID-19”) as a global pandemic. The declaration of a global pandemic indicates that almost all public commerce and related business activities must be, to varying degrees, curtailed with the goal of decreasing the rate of new infections. The outbreak of COVID-19 has adversely impacted, and could further adversely impact, a broad range of industries in which the Company’s customers operate and impair their ability to fulfill their financial obligations to the Company. On March 3, 2020, the Federal Open Market Committee reduced the target federal funds rate range by 50 basis points to 1.00% to 1.25%. This rate was further reduced to a target range of 0% to 0.25% on March 16, 2020. These reductions in interest rates and other effects of the COVID-19 outbreak may adversely affect the Company’s financial condition and results of operations. As a result of the spread of the COVID-19 coronavirus, economic uncertainties have arisen which are likely to negatively impact net interest income and noninterest income. Other financial impacts could occur though such potential impact is unknown at this time.

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, 2020, 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.