| Document And Entity Information | Mar. 25, 2021 |
| Document Information [Line Items] | |
| Entity, Registrant Name | FIRST NORTHWEST BANCORP |
| Document, Type | 8-K |
| Document, Period End Date | Mar. 25, 2021 |
| Entity, Incorporation, State or Country Code | WA |
| Entity, File Number | 001-36741 |
| Entity, Tax Identification Number | 46-1259100 |
| Entity, Address, Address Line One | 105 West 8th Street |
| Entity, Address, City or Town | Port Angeles |
| Entity, Address, State or Province | WA |
| Entity, Address, Postal Zip Code | 98362 |
| City Area Code | 360 |
| Local Phone Number | 457-0461 |
| 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 | FNWB |
| Security Exchange Name | NASDAQ |
| Entity, Emerging Growth Company | false |
| Amendment Flag | false |
| Entity, Central Index Key | 0001556727 |
