Document And Entity Information (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Jun. 30, 2011	Mar. 01, 2012 Class A [Member]	Mar. 01, 2012 Class B [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		$ 239
Entity Common Stock, Shares Outstanding			2,174,912	9,633,643
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2011