Summary of Significant Accounting Policies (Notes)	6 Months Ended
Jun. 30, 2019
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 six month periods ended June 30, 2019 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2019 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. Lease Accounting: The Company adopted FASB issued ASU No. 2016-02, “Leases (Topic 842)” (“ASU 2016-02”), including the adoption of the practical expedients, effective January 1, 2019. Lessees are required to recognize assets and liabilities on the balance sheet for leases with lease terms greater than 12 months. The Company recorded assets and liabilities of $6.7 million as a result of recording additional lease contracts where the Company is lessee. The Company did not restate comparative periods. The right of use assets are included in other assets and the lease obligations are included in other liabilities in the accompanying consolidated statements of financial condition. Other Intangible Assets: Intangible assets acquired in a purchase business combination with definite useful lives are amortized over their estimated useful lives to their estimated residual values. Core deposit intangible assets (“CDI”), are recognized at the time of acquisition based on valuations prepared by independent third parties or other estimates of fair value. In preparing such valuations, variables such as deposit servicing costs, attrition rates, and market discount rates are considered. CDI assets are amortized to expense over their useful lives. CDI were $68,000 and $102,000 at June 30, 2019 and December 31, 2018, respectively, and are included in other assets in the accompanying consolidated statements of financial condition. 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, 2018, 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. ASU 2016-13 is effective for SEC filers or fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019 (i.e., January 1, 2020, for calendar year entities). In July 2019, the FASB proposed changes to the effective date of ASU No. 2016-13 for smaller reporting companies, as defined by the SEC, and other non-SEC reporting entities. The proposal would delay the effective date to fiscal years beginning after December 31, 2022, including interim periods within those fiscal periods. As the Company is a smaller reporting company, the proposed delay would be applicable to the Company, if it is approved by the FASB.

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 six month periods ended June 30, 2019 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2019 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.

Lease Accounting: The Company adopted FASB issued ASU No. 2016-02, “Leases (Topic 842)” (“ASU 2016-02”), including the adoption of the practical expedients, effective January 1, 2019. Lessees are required to recognize assets and liabilities on the balance sheet for leases with lease terms greater than 12 months. The Company recorded assets and liabilities of $6.7 million as a result of recording additional lease contracts where the Company is lessee. The Company did not restate comparative periods. The right of use assets are included in other assets and the lease obligations are included in other liabilities in the accompanying consolidated statements of financial condition.

Other Intangible Assets: Intangible assets acquired in a purchase business combination with definite useful lives are amortized over their estimated useful lives to their estimated residual values. Core deposit intangible assets (“CDI”), are recognized at the time of acquisition based on valuations prepared by independent third parties or other estimates of fair value. In preparing such valuations, variables such as deposit servicing costs, attrition rates, and market discount rates are considered. CDI assets are amortized to expense over their useful lives. CDI were $68,000 and $102,000 at June 30, 2019 and December 31, 2018, respectively, and are included in other assets in the accompanying consolidated statements of financial condition.

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, 2018, 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. ASU 2016-13 is effective for SEC filers or fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019 (i.e., January 1, 2020, for calendar year entities). In July 2019, the FASB proposed changes to the effective date of ASU No. 2016-13 for smaller reporting companies, as defined by the SEC, and other non-SEC reporting entities. The proposal would delay the effective date to fiscal years beginning after December 31, 2022, including interim periods within those fiscal periods. As the Company is a smaller reporting company, the proposed delay would be applicable to the Company, if it is approved by the FASB.