Document And Entity Information - shares	9 Months Ended
	Sep. 30, 2020	Nov. 01, 2020
Document Information [Line Items]
Entity Central Index Key	0000729580
Entity Registrant Name	BEL FUSE INC /NJ
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	Q3
Document Fiscal Year Focus	2020
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2020
Document Transition Report	false
Entity File Number	0-11676
Entity Address, Address Line One	206 Van Vorst Street
Entity Address, City or Town	Jersey City
Entity Address, State or Province	NJ
Entity Address, Postal Zip Code	07302
City Area Code	201
Local Phone Number	432-0463
Entity Incorporation, State or Country Code	NJ
Entity Tax Identification Number	22-1463699
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Common Class B [Member]
Document Information [Line Items]
Title of 12(b) Security	Class B Common Stock ($0.10 par value)
Trading Symbol	BELFB
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		10,214,352
Common Class A [Member]
Document Information [Line Items]
Title of 12(b) Security	Class A Common Stock ($0.10 par value)
Trading Symbol	BELFA
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		2,144,912