Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2024	Mar. 03, 2025	Jun. 30, 2024
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Annual Report	true
Document Period End Date	Dec. 31, 2024
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	FY
Document Transition Report	false
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 St.
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(g) Security	Common Stock, no par value
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
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
ICFR Auditor Attestation Flag	false
Document Financial Statement Error Correction [Flag]	false
Entity Shell Company	false
Entity Public Float			$ 125,391,497
Entity Common Stock, Shares Outstanding		15,184,687
Auditor Firm ID	659
Auditor Name	MOSS ADAMS LLP
Auditor Location	Sacramento, California