Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2021	May 03, 2021
Cover [Abstract]
Entity Registrant Name	FIRST NORTHERN COMMUNITY BANCORP
Entity Central Index Key	0001114927
Current Fiscal Year End Date	--12-31
Document Type	10-Q
Amendment Flag	false
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
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(g) Security	Common Stock, no par value
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		13,683,085