v2.4.0.6
Document and Entity Information (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Jun. 30, 2012
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   $ 193.7
Document Fiscal Year Focus 2012  
Document Fiscal Period Focus FY  
Document Type 10-K  
Amendment Flag false  
Document Period End Date Dec. 31, 2012  
Class A Common Stock [Member]
   
Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding 2,174,912  
Class B Common Stock [Member]
   
Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding 9,192,777