v3.8.0.1
Document and Entity Information - shares
9 Months Ended
Sep. 30, 2017
Nov. 01, 2017
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  
Document Fiscal Year Focus 2017  
Document Fiscal Period Focus Q3  
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2017  
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   9,858,802