Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2018
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, 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 and nine month periods ended September 30, 2018 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2018 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 (“GAAP”) have been condensed or omitted pursuant to the rules and regulations of the Securities and Exchange Commission. Use of Estimates: To prepare financial statements in conformity with GAAP, management makes estimates and assumptions based on available information. These estimates and assumptions affect the amounts reported in the financial statements and the disclosures provided, and future results could differ. 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, 2017, as filed with the Securities and Exchange Commission. Recent Accounting Pronouncements In May 2014, the FASB issued an update (ASU No. 2014-09, Revenue from Contracts with Customers) creating FASB Topic 606, Revenue from Contracts with Customers. The guidance in this update affects any entity that either enters into contracts with customers to transfer goods or services or enters into contracts for the transfer of nonfinancial assets unless those contracts are within the scope of other standards (for example, insurance contracts or lease contracts). The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The guidance provides steps to follow to achieve the core principle. An entity should disclose sufficient information to enable users of financial statements to understand the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. Qualitative and quantitative information is required about contracts with customers, significant judgments and changes in judgments, and assets recognized from the costs to obtain or fulfill a contract. The amendments in this update became effective for annual periods and interim periods within those annual periods beginning after December 15, 2017. We have evaluated the impact of adopting the update and have concluded that it does not have a significant impact to our consolidated financial statements. The Company’s revenue streams that are in-scope from the update include: financed OREO sales; deposit fees, including ATM fees, overdraft fees, maintenance fees and dormancy fees; debit card fees, and trust fees. For the in-scope revenue streams, our current revenue recognition is not different than our prior revenue recognition under the update. The Company has infrequently financed an OREO sale. Our customer contracts generally do not have performance obligations and fees are assessed and collected as the transaction occurs. The Company’s fee income is not material for any individual income streams. The adoption of ASC 606 did not result in a change to the accounting for any of the in-scope revenue stream; as such, no cumulative effect adjustment was recorded. Refer to Note 8 - Revenue for Contracts with Customers for further discussion on the Company's accounting policies for revenue sources within the scope of ASC 606. In January 2016, the FASB issued an update (ASU No. 2016-01, Financial Instruments - Recognition and Measurement of Financial Assets and Liabilities). The new guidance is intended to improve the recognition and measurement of financial instruments by requiring: equity investments (other than equity method or consolidation) to be measured at fair value with changes in fair value recognized in net income; public business entities to use the exit price notion when measuring the fair value of financial instruments for disclosure purposes; separate presentation of financial assets and financial liabilities by measurement category and form of financial assets (i.e., securities or loans and receivables) on the balance sheet or the accompanying notes to the financial statements; eliminating the requirement to disclose the fair value of financial instruments measured at amortized cost for organizations that are not public business entities; eliminating the requirement for non-public business entities to disclose the method(s) and significant assumptions used to estimate the fair value that is to be required to be disclosed for financial instruments measured at amortized cost on the balance sheet; and requiring a reporting organization to present separately in other comprehensive income the portion of the total change in fair value of a liability resulting from the change in the instrument-specific credit risk (also referred to as “own credit”) when the organization has elected to measure the liability at fair value in accordance with the fair value option for financial instruments. The new guidance became effective for public business entities for fiscal years beginning after December 15, 2017. The new pronouncement does not have a significant impact on our Statement of Operations, as we had one equity security that was valued at $499,000 at December 31, 2017 and none at September 30, 2018. In February 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842)” (“ASU 2016-02”). The standard requires a lessee to recognize assets and liabilities on the balance sheet for leases with lease terms greater than 12 months. ASU 2016-02 is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018, and early adoption is permitted. We are currently evaluating the impact that the standard will have on our consolidated financial statements. Our preliminary finding is that the new pronouncement will not have a significant impact on our consolidated financial statements as the projected minimum lease payments under existing leases subject to the new pronouncement are less than one percent of our current total assets. 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. ASU 2016-13 is effective for SEC filers for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019 (i.e., January 1, 2020, for calendar year entities). Early application will be permitted for all organizations for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. We are currently evaluating the impact that the standard will have on our consolidated financial statements. Our initial review indicates that we have maintained sufficient historical loan data to support the requirements of this pronouncement. In addition, we have begun tracking the average life of the various segments of our loan portfolio. We are currently evaluating various loss methodologies to determine their correlation to our various loan categories' historical performance. In August 2018, we contracted with a third-party vendor to provide a model and assist with assessing processes, portfolio segmentation, and model development. In March 2017, the FASB issued ASU No. 2017-08, “Receivables-Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities” (“ASU 2017-08”). This guidance shortens the amortization period for premiums on certain callable debt securities to the earliest call date (with an explicit, noncontingent call feature that is callable at a fixed price and on a preset dates), rather than contractual maturity date as currently required under GAAP. The ASU does not impact instruments without preset call dates such as mortgage-backed securities. For instruments with contingent call features, once the contingency is resolved and the security is callable at a fixed price and preset date, the security is within the scope of the ASU. ASU 2017-08 is effective for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years, and early adoption is permitted. Effective January 2017, we early adopted the pronouncement. Adoption of the new pronouncement was immaterial to the consolidated financial statements.
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, NA (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 and nine month periods ended September 30, 2018 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2018 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 (“GAAP”) have been condensed or omitted pursuant to the rules and regulations of the Securities and Exchange Commission. Use of Estimate
Use of Estimates	Use of Estimates: To prepare financial statements in conformity with GAAP, management makes estimates and assumptions based on available information. These estimates and assumptions affect the amounts reported in the financial statements and the disclosures provided, and future results could differ.
Uncollected Interest Policy	The Company’s reserve for uncollected loan interest was $36,000 and $103,000 at September 30, 2018 and December 31, 2017, 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 non-accrual 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, 2017, as filed with the Securities and Exchange Commission.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the FASB issued an update (ASU No. 2014-09, Revenue from Contracts with Customers) creating FASB Topic 606, Revenue from Contracts with Customers. The guidance in this update affects any entity that either enters into contracts with customers to transfer goods or services or enters into contracts for the transfer of nonfinancial assets unless those contracts are within the scope of other standards (for example, insurance contracts or lease contracts). The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The guidance provides steps to follow to achieve the core principle. An entity should disclose sufficient information to enable users of financial statements to understand the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. Qualitative and quantitative information is required about contracts with customers, significant judgments and changes in judgments, and assets recognized from the costs to obtain or fulfill a contract. The amendments in this update became effective for annual periods and interim periods within those annual periods beginning after December 15, 2017. We have evaluated the impact of adopting the update and have concluded that it does not have a significant impact to our consolidated financial statements. The Company’s revenue streams that are in-scope from the update include: financed OREO sales; deposit fees, including ATM fees, overdraft fees, maintenance fees and dormancy fees; debit card fees, and trust fees. For the in-scope revenue streams, our current revenue recognition is not different than our prior revenue recognition under the update. The Company has infrequently financed an OREO sale. Our customer contracts generally do not have performance obligations and fees are assessed and collected as the transaction occurs. The Company’s fee income is not material for any individual income streams. The adoption of ASC 606 did not result in a change to the accounting for any of the in-scope revenue stream; as such, no cumulative effect adjustment was recorded. Refer to Note 8 - Revenue for Contracts with Customers for further discussion on the Company's accounting policies for revenue sources within the scope of ASC 606. In January 2016, the FASB issued an update (ASU No. 2016-01, Financial Instruments - Recognition and Measurement of Financial Assets and Liabilities). The new guidance is intended to improve the recognition and measurement of financial instruments by requiring: equity investments (other than equity method or consolidation) to be measured at fair value with changes in fair value recognized in net income; public business entities to use the exit price notion when measuring the fair value of financial instruments for disclosure purposes; separate presentation of financial assets and financial liabilities by measurement category and form of financial assets (i.e., securities or loans and receivables) on the balance sheet or the accompanying notes to the financial statements; eliminating the requirement to disclose the fair value of financial instruments measured at amortized cost for organizations that are not public business entities; eliminating the requirement for non-public business entities to disclose the method(s) and significant assumptions used to estimate the fair value that is to be required to be disclosed for financial instruments measured at amortized cost on the balance sheet; and requiring a reporting organization to present separately in other comprehensive income the portion of the total change in fair value of a liability resulting from the change in the instrument-specific credit risk (also referred to as “own credit”) when the organization has elected to measure the liability at fair value in accordance with the fair value option for financial instruments. The new guidance became effective for public business entities for fiscal years beginning after December 15, 2017. The new pronouncement does not have a significant impact on our Statement of Operations, as we had one equity security that was valued at $499,000 at December 31, 2017 and none at September 30, 2018. In February 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842)” (“ASU 2016-02”). The standard requires a lessee to recognize assets and liabilities on the balance sheet for leases with lease terms greater than 12 months. ASU 2016-02 is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018, and early adoption is permitted. We are currently evaluating the impact that the standard will have on our consolidated financial statements. Our preliminary finding is that the new pronouncement will not have a significant impact on our consolidated financial statements as the projected minimum lease payments under existing leases subject to the new pronouncement are less than one percent of our current total assets. 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. ASU 2016-13 is effective for SEC filers for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019 (i.e., January 1, 2020, for calendar year entities). Early application will be permitted for all organizations for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. We are currently evaluating the impact that the standard will have on our consolidated financial statements. Our initial review indicates that we have maintained sufficient historical loan data to support the requirements of this pronouncement. In addition, we have begun tracking the average life of the various segments of our loan portfolio. We are currently evaluating various loss methodologies to determine their correlation to our various loan categories' historical performance. In August 2018, we contracted with a third-party vendor to provide a model and assist with assessing processes, portfolio segmentation, and model development. In March 2017, the FASB issued ASU No. 2017-08, “Receivables-Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities” (“ASU 2017-08”). This guidance shortens the amortization period for premiums on certain callable debt securities to the earliest call date (with an explicit, noncontingent call feature that is callable at a fixed price and on a preset dates), rather than contractual maturity date as currently required under GAAP. The ASU does not impact instruments without preset call dates such as mortgage-backed securities. For instruments with contingent call features, once the contingency is resolved and the security is callable at a fixed price and preset date, the security is within the scope of the ASU. ASU 2017-08 is effective for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years, and early adoption is permitted. Effective January 2017, we early adopted the pronouncement. Adoption of the new pronouncement was immaterial to the consolidated financial statements.