Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Significant Accounting Policies	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 three month period ended March 31, 2020 is not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2020 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. Use of Estimates: The preparation of the consolidation 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: The Company has evaluated subsequent events for potential recognition and/or disclosure through the date the unaudited consolidated financial statements included in this Quarterly Report on Form 10-Q were issued. On March 11, 2020, the World Health Organization declared the outbreak of a novel coronavirus (“COVID-19”) as a global pandemic, which continues to spread throughout the United States and around the world. 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 could 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 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 impact could occur though such potential impact is unknown at this time. 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, 2019, as filed with the Securities and Exchange Commission. 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, ASU 2016-13 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2022.
Basis of Presentation	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”).
Principles of Consolidation	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, 2020 is not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2020 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.
Use of Estimates	Use of Estimates: The preparation of the consolidation 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.
Uncollected Interest Policy	The Company’s reserve for uncollected loan interest was $94,000 and $81,000 at March 31, 2020 and December 31, 2019, respectively. When a loan is on nonaccrual status and the ultimate collectability of the total principal of an impaired loan is in doubt, all payments are applied to principal under the cost recovery method. Alternatively, when a loan is on nonaccrual status but there is doubt concerning only the ultimate collectability of interest, contractual interest is credited to interest income only when received, under the cash basis method pursuant to the provisions of FASB ASC 310–10, as applicable. In all cases, the average balances are calculated based on the month–end balances of the financing receivables within the period reported pursuant to the provisions of FASB ASC 310–10, as applicable.
Reclassifications	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, 2019, as filed with the Securities and Exchange Commission.
Recent Accounting Pronouncements	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, ASU 2016-13 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2022.