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Disclosure - Note 9 - Borrowings and Subordinated Notes - Advances From the FHLB (Details) Notes http://www.bankfinancial.com/20231231/role/statement-note-9-borrowings-and-subordinated-notes-advances-from-the-fhlb-details Note 9 - Borrowings and Subordinated Notes - Advances From the FHLB (Details) Details 71 false false R72.htm 995516 - Disclosure - Note 10 - Income Taxes (Details Textual) Sheet http://www.bankfinancial.com/20231231/role/statement-note-10-income-taxes-details-textual Note 10 - Income Taxes (Details Textual) Details http://www.bankfinancial.com/20231231/role/statement-note-10-income-taxes-tables 72 false false R73.htm 995517 - Disclosure - Note 10 - Income Taxes - Schedule of Income Tax Expense (Benefit) (Details) Sheet http://www.bankfinancial.com/20231231/role/statement-note-10-income-taxes-schedule-of-income-tax-expense-benefit-details Note 10 - Income Taxes - Schedule of Income Tax Expense (Benefit) (Details) Details 73 false false R74.htm 995518 - Disclosure - Note 10 - 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