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BASIS OF PRESENTATION AND ACCOUNTING POLICIES (Details)
0 Months Ended
Jun. 19, 2014
Jul. 25, 2014
Power Solutions [Member]    
Business Acquisition and Disposition [Line Items]    
Acquisition date Jun. 19, 2014  
Acquisition of issued and outstanding capital stock (in hundredths) 100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
 
Emerson [Member]    
Business Acquisition and Disposition [Line Items]    
Acquisition date   Jul. 25, 2014
Acquisition of issued and outstanding capital stock (in hundredths)   100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonMember