Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2021	Mar. 07, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Amendment Flag	false
Document Period End Date	Dec. 31, 2021
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
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
Entity Shell Company	false
Entity Public Float			$ 139,650,593
Entity Common Stock, Shares Outstanding		13,267,243
Auditor Firm ID	659
Auditor Name	MOSS ADAMS LLP
Auditor Location	Sacramento, California