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Note 11 - Retirement Fund and Profit Sharing Plan (Details Textual) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Bel Fuse Inc. Employees' Savings Plan [Member]    
Defined Contribution Plan, Cost $ 0.3 $ 0.3
Bel Fuse Inc. Employees' Savings Plan [Member] | Common Class A [Member]    
Shares Held in Plan (in shares) 194,235  
Bel Fuse Inc. Employees' Savings Plan [Member] | Common Class B [Member]    
Shares Held in Plan (in shares) 111,879  
Retirement Fund Covering Hong Kong-based Employees [Member]    
Defined Contribution Plan, Cost $ 0.1 $ 0.1
Retirement Fund Covering Hong Kong-based Employees [Member] | Common Class A [Member]    
Shares Held in Plan (in shares) 3,323  
Retirement Fund Covering Hong Kong-based Employees [Member] | Common Class B [Member]    
Shares Held in Plan (in shares) 17,342