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Document And Entity Information - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Feb. 28, 2024
Jun. 30, 2023
Document Information [Line Items]      
Entity, Registrant Name BANKFINANCIAL CORPORATION    
Current Fiscal Year End Date --12-31    
Document, Fiscal Period Focus FY    
Document, Fiscal Year Focus 2023    
Document, Type 10-K/A    
Document, Annual Report true    
Document, Period End Date Dec. 31, 2023    
Document, Transition Report false    
Entity, File Number 0-51331    
Entity, Incorporation, State or Country Code MD    
Entity, Tax Identification Number 75-3199276    
Entity, Address, Address Line One 60 North Frontage Road    
Entity, Address, City or Town Burr Ridge    
Entity, Address, State or Province IL    
Entity, Address, Postal Zip Code 60527    
City Area Code 800    
Local Phone Number 894-6900    
Title of 12(b) Security Common Stock, par value $0.01 per share    
Trading Symbol BFIN    
Security Exchange Name NASDAQ    
Entity, Well-known Seasoned Issuer No    
Entity, Voluntary Filers No    
Entity, Current Reporting Status Yes    
Entity, Interactive Data, Current Yes    
Entity, Filer Category Non-accelerated Filer    
Entity, Small Business true    
Entity, Emerging Growth Company false    
ICFR Auditor Attestation Flag false    
Document, Financial Statement Error Correction Flag false    
Entity, Shell Company false    
Entity, Public Float     $ 94.8
Entity, Common Stock Shares, Outstanding   12,460,678  
Amendment Description BankFinancial Corporation (the “Company”) is filing this Amendment No. 1 on Form 10-K/A to its Report on Form 10-K for the year ended December 31, 2023, as filed with the Securities and Exchange Commission on March 1, 2024. In accordance with General Instruction G(3), the Company is now filing this amendment to include in the Form 10-K the information required to be filed pursuant to Part III of Form 10-K.    
Auditor Name RSM US LLP     
Auditor Location Chicago, Illinois    
Auditor Firm ID 49    
Amendment Flag true    
Entity, Central Index Key 0001303942