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Document And Entity Information (USD $) In Millions, except Share data	9 Months Ended
	Sep. 30, 2011	Jun. 30, 2010	Nov. 01, 2011 Class A Common Stock [Member]	Nov. 01, 2011 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 Public Float		$ 177.5
Entity Common Stock, Shares Outstanding			2,174,912	9,639,443
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	Q3
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2011