| Document And Entity Information | Jul. 02, 2021 |
| Document Information [Line Items] | |
| Entity, Registrant Name | BANKFINANCIAL CORPORATION |
| Document, Type | 8-K |
| Document, Period End Date | Jul. 2, 2021 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | BFIN |
| Security Exchange Name | NASDAQ |
| Entity, Emerging Growth Company | false |
| Amendment Flag | false |
| Entity, Central Index Key | 0001303942 |
