Document And Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Nov. 01, 2019
Document Information [Line Items]
Entity Registrant Name	BEL FUSE INC /NJ
Entity Central Index Key	0000729580
Trading Symbol	belfa,belfb
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Current Reporting Status	Yes
Entity Emerging Growth Company	false
Entity Small Business	true
Entity Shell Company	false
Document Type	10-Q
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Amendment Flag	false
Title of 12(b) Security	Class A Common Stock ($0.10 par value), Class B Common Stock ($0.10 par value)
Common Class A [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding (in shares)		2,144,912
Common Class B [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding (in shares)		10,128,602