v3.10.0.1
Document and Entity Information - shares
9 Months Ended
Sep. 30, 2018
Nov. 01, 2018
Entity Information [Line Items]    
Entity Registrant Name BEL FUSE INC /NJ  
Entity Central Index Key 0000729580  
Current Fiscal Year End Date --12-31  
Entity Filer Category Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Ex Transition Period false  
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q3  
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   10,072,352