| Document and Entity Information | Mar. 31, 2024 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Mar. 31, 2024 |
| Entity File Number | 000-30707 |
| Entity Registrant Name | FIRST NORTHERN COMMUNITY BANCORP |
| Entity Central Index Key | 0001114927 |
| Entity Incorporation, State or Country Code | CA |
| Entity Tax Identification Number | 68-0450397 |
| Entity Address, Address Line One | 195 N FIRST STREET |
| Entity Address, City or Town | DIXON |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 95620 |
| City Area Code | 707 |
| Local Phone Number | 678-3041 |
| Title of 12(b) Security | FNRN |
| Trading Symbol | FNRN |
| Entity Emerging Growth Company | false |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
