v3.8.0.1
Document and Entity Information - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Mar. 01, 2018
Jun. 30, 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    
Entity Public Float     $ 273.5
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY    
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 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   9,859,352