Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2020	Aug. 01, 2020
Cover [Abstract]
Entity Registrant Name	First Northern Community Bancorp
Entity Central Index Key	0001114927
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		12,968,567
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2
Document Quarterly Report	true
Document Transition Report	false
Entity File Number	000-30707
Entity Tax Identification Number	68-0450397
Entity Incorporation, State or Country Code	CA
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