| Document and Entity Information | Jan. 28, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Jan. 28, 2021 |
| Entity Registrant Name | First Financial Northwest, Inc. |
| Entity Incorporation, State or Country Code | WA |
| Entity File Number | 001-33652 |
| Entity Tax Identification Number | 26-0610707 |
| Entity Address, Address Line One | 201 Wells Avenue South |
| Entity Address, City or Town | Renton |
| Entity Address, State or Province | WA |
| Entity Address, Postal Zip Code | 98057 |
| City Area Code | 425 |
| Local Phone Number | 255-4400 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001401564 |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | FFNW |
| Security Exchange Name | NASDAQ |
