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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 | $ 12.0 us-gaap_LossContingencyEstimateOfPossibleLoss |  |
SynQor, Inc [Member] |  |  |
Loss Contingencies [Line Items] |  |  |
Number of defendants | 11 us-gaap_LossContingencyNumberOfDefendants / us-gaap_LitigationCaseAxis = belfb_SynqorIncMember |  |
Damages awarded | 10.9 us-gaap_LossContingencyDamagesAwardedValue / us-gaap_LitigationCaseAxis = belfb_SynqorIncMember | 0.7 us-gaap_LossContingencyDamagesAwardedValue / us-gaap_LitigationCaseAxis = belfb_SynqorIncMember |
Damages covered through indemnification agreement |  | $ 2.1 belfb_DamagesCoveredThroughIndemnificationAgreement / us-gaap_LitigationCaseAxis = belfb_SynqorIncMember |
Molex Inc [Member] |  |  |
Loss Contingencies [Line Items] |  |  |
Number of patents | 2 belfb_NumberOfPatents / us-gaap_LitigationCaseAxis = belfb_MolexIncMember |  | 3 belfb_NumberOfPatents / us-gaap_LitigationCaseAxis = belfb_MolexIncMember

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- Definition Amount of damages awarded to the plaintiff in the legal matter. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349 + Details
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