Document and Entity Information	3 Months Ended
	Mar. 31, 2013	May 01, 2013 Class A Common Stock [Member]	May 01, 2013 Class B Common Stock [Member]
Entity Information [Line Items]
Entity Registrant Name	BEL FUSE INC /NJ
Entity Central Index Key	0000729580
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		2,174,912	9,191,177
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q1
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2013