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RETIREMENT FUND AND PROFIT SHARING PLAN (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Balance sheet amounts [Abstract]          
Minimum pension obligation and unfunded pension liability $ 14,733   $ 14,733   $ 14,205 [1]
Bel Fuse Inc Employees Savings Plan [Member]          
Defined Contribution Plan Disclosure [Line Items]          
Employer match of the first 1% of compensation contributed by participants (in hundredths)     100.00%    
Percentage of participant contribution under condition one (in hundredths) 1.00%   1.00%    
Employer match of the next 5% compensation contributed by participants (in hundredths)     50.00%    
Percentage of employee deferrals under condition two (in hundredths) 5.00%   5.00%    
Compensation expense $ 300 $ 100 $ 600 $ 300  
Maximum annual contribution by eligible employee (in hundredths)     5.00%    
Minimum employer contribution to plan (in hundredths)     5.00%    
Employer contribution of eligible salary (in hundredths)     7.00%    
Bel Fuse Inc Employees Savings Plan [Member] | Common Class A [Member]          
Defined Contribution Plan Disclosure [Line Items]          
Shares owned by plan (in shares) 13,928   13,928    
Bel Fuse Inc Employees Savings Plan [Member] | Common Class B [Member]          
Defined Contribution Plan Disclosure [Line Items]          
Shares owned by plan (in shares) 172,269   172,269    
Non Defined Retirement Fund [Member]          
Defined Contribution Plan Disclosure [Line Items]          
Compensation expense $ 100 100 $ 200 100  
Non Defined Retirement Fund [Member] | Common Class A [Member]          
Defined Contribution Plan Disclosure [Line Items]          
Shares owned by plan (in shares) 3,323   3,323    
Non Defined Retirement Fund [Member] | Common Class B [Member]          
Defined Contribution Plan Disclosure [Line Items]          
Shares owned by plan (in shares) 17,342   17,342    
SERP [Member]          
Components of SERP expense [Abstract]          
Service cost $ 138 138 $ 276 276  
Interest cost 142 135 283 270  
Net amortization 92 46 183 92  
Net periodic benefit cost 372 $ 319 742 $ 638  
Balance sheet amounts [Abstract]          
Minimum pension obligation and unfunded pension liability 14,733   14,733   14,205
Amounts recognized in accumulated other comprehensive loss, pretax [Abstract]          
Prior service cost 957   957   1,048
Net loss 3,210   3,210   3,302
Total amounts recognized in accumulated other comprehensive loss $ 4,167   $ 4,167   $ 4,350
[1] Revised