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COMMITMENTS AND CONTINGENCIES (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2015
Defendant
Dec. 31, 2014
Dec. 31, 2013
Patent
Loss Contingencies [Line Items]      
Estimated liability $ 12.0us-gaap_LossContingencyEstimateOfPossibleLoss    
SynQor, Inc [Member]      
Loss Contingencies [Line Items]      
Number of defendants 11us-gaap_LossContingencyNumberOfDefendants
/ us-gaap_LitigationCaseAxis
= belfb_SynqorIncMember
   
Damages awarded 10.9us-gaap_LossContingencyDamagesAwardedValue
/ us-gaap_LitigationCaseAxis
= belfb_SynqorIncMember
0.7us-gaap_LossContingencyDamagesAwardedValue
/ us-gaap_LitigationCaseAxis
= belfb_SynqorIncMember
 
Damages covered through indemnification agreement   $ 2.1belfb_DamagesCoveredThroughIndemnificationAgreement
/ us-gaap_LitigationCaseAxis
= belfb_SynqorIncMember
 
Molex Inc [Member]      
Loss Contingencies [Line Items]      
Number of patents 2belfb_NumberOfPatents
/ us-gaap_LitigationCaseAxis
= belfb_MolexIncMember
  3belfb_NumberOfPatents
/ us-gaap_LitigationCaseAxis
= belfb_MolexIncMember