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RETIREMENT FUND AND PROFIT SHARING PLAN (Details) (USD $)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Balance sheet amounts [Abstract]      
Minimum pension obligation and unfunded pension liability $ 14,469,000us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent   $ 14,205,000us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
SERP [Member]      
Components of SERP expense [Abstract]      
Service cost 138,000us-gaap_DefinedBenefitPlanServiceCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
138,000us-gaap_DefinedBenefitPlanServiceCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
 
Interest cost 142,000us-gaap_DefinedBenefitPlanInterestCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
135,000us-gaap_DefinedBenefitPlanInterestCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
 
Net amortization 92,000us-gaap_DefinedBenefitPlanOtherCosts
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
46,000us-gaap_DefinedBenefitPlanOtherCosts
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
 
Net periodic benefit cost 372,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
319,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
 
Balance sheet amounts [Abstract]      
Minimum pension obligation and unfunded pension liability 14,469,000us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
  14,205,000us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
Amounts recognized in accumulated other comprehensive loss, pretax [Abstract]      
Prior service cost 1,002,000us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
  1,048,000us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
Net loss 3,256,000us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
  3,302,000us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
Total amounts recognized in accumulated other comprehensive loss 4,258,000us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
  4,350,000us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
401(k) Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Employer match of the first 1% of compensation contributed by participants (in hundredths) 100.00%belfb_EmployerMatchOfFirstOnePercentOfCompensationContributedByParticipants
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Percentage of participant contribution under condition one (in hundredths) 1.00%belfb_PercentageOfParticipantContributionOne
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Employer match of the next 5% compensation contributed by participants (in hundredths) 50.00%belfb_EmployerMatchOfNextFivePercentCompensationContributedByParticipants
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Percentage of employee deferrals under condition two (in hundredths) 5.00%belfb_PercentageOfParticipantContributionTwo
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Compensation expense 300,000us-gaap_PensionAndOtherPostretirementBenefitExpense
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
200,000us-gaap_PensionAndOtherPostretirementBenefitExpense
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
 
401(k) Plan [Member] | Class A [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Shares owned by plan (in shares) 13,940belfb_SharesHeldInPlan
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
   
401(k) Plan [Member] | Class B [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Shares owned by plan (in shares) 173,683belfb_SharesHeldInPlan
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
   
Non-defined Retirement Fund [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Compensation expense $ 100,000us-gaap_PensionAndOtherPostretirementBenefitExpense
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= belfb_NonDefinedRetirementFundMember
$ 100,000us-gaap_PensionAndOtherPostretirementBenefitExpense
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= belfb_NonDefinedRetirementFundMember
 
Maximum annual contribution by eligible employee (in hundredths) 5.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= belfb_NonDefinedRetirementFundMember
   
Minimum employer contribution to plan (in hundredths) 5.00%belfb_DefinedContributionPlanMinimumEmployerContributionPercent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= belfb_NonDefinedRetirementFundMember
   
Employer contribution of eligible salary (in hundredths) 7.00%belfb_DefinedContributionPlanEmployerContributionPercent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= belfb_NonDefinedRetirementFundMember
   
Non-defined Retirement Fund [Member] | Class A [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Shares owned by plan (in shares) 3,323belfb_SharesHeldInPlan
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= belfb_NonDefinedRetirementFundMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
   
Non-defined Retirement Fund [Member] | Class B [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Shares owned by plan (in shares) 17,342belfb_SharesHeldInPlan
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= belfb_NonDefinedRetirementFundMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember