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RELATED PARTY TRANSACTIONS (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2015 | Sep. 30, 2015
RELATED PARTY TRANSACTIONS [Abstract] |  |
Inventory purchase payment from joint venture | $ 0.0 | $ 1.5
Joint venture liability |  | $ 0.5

X
- Definition Inventory purchased from the joint venture. + References No definition available. + Details
Name: | belfb_InventoryPurchasePaymentFromJointVenture
Namespace Prefix: | belfb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount owed to the joint venture. + References No definition available. + Details
Name: | belfb_JointVentureLiability
Namespace Prefix: | belfb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_RelatedPartyTransactionsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration