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Share-based Compensation (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Pretax stock based compensation cost $ 2,717,000us-gaap_ShareBasedCompensation $ 1,879,000us-gaap_ShareBasedCompensation $ 1,767,000us-gaap_ShareBasedCompensation
Percentage of increments earned for share awards (in hundredths) 25.00%belfb_PercentageOfIncrementsEarnedForShareAwards    
Amortization period for market value of common stock awarded 5 years    
Restricted Stock [Member]      
Shares [Roll Forward]      
Outstanding, beginning of period (in shares) 412,350us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Granted (in shares) 378,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Vested (in shares) (120,175)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Forfeited (in shares) (26,900)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Outstanding, end of period (in shares) 643,275us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
412,350us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Weighted Average Award Price [Roll Forward]      
Outstanding, beginning of period (in dollars per share) $ 18.85belfb_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Granted (in dollars per share) $ 23.57belfb_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Vested (in dollars per share) $ 18.69belfb_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Forfeited (in dollars per share) $ 22.03belfb_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Outstanding, end of period (in dollars per share) $ 21.52belfb_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
$ 18.85belfb_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Weighted Average Remaining Contractual Term [Abstract]      
Outstanding 3 years 9 months 18 days 3 years 4 months 24 days  
Pretax unrecognized compensation cost $ 10,000,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Period over which compensation cost is expected to be recognized 4 years 7 months 6 days    
Restricted Stock [Member] | Common Class B [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares issued (in shares) 378,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
162,200us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
130,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
2011 Equity Compensation Plan [Member] | Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares authorized under the plan (in shares) 1,400,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= belfb_EquityCompensationPlanMember
   
Common shares available for future issuance (in shares) 766,500us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= belfb_EquityCompensationPlanMember