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v2.4.1.9

Related Party Transactions (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014
Related Party Transactions [Abstract] |
Inventory purchase payment from joint venture | $ 4.3 belfb_InventoryPurchasePaymentFromJointVenture
Joint venture liability | $ 0.9 belfb_JointVentureLiability

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
- Details
Name: | us-gaap_RelatedPartyTransactionsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration