v3.26.1
Document And Entity Information - shares
 6 Months Ended
Jun. 30, 2026
Aug. 05, 2026
Document Information [Line Items]    
Entity Central Index Key 0001114927  
Entity Registrant Name FIRST NORTHERN COMMUNITY BANCORP  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Document Fiscal Period Focus Q2  
Document Fiscal Year Focus 2026  
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2026  
Document Transition Report false  
Entity File Number 000-30707  
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 Common stock, without par value  
Trading Symbol FNRN  
Security Exchange Name NASDAQ  
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  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   16,395,303