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SUBSEQUENT EVENT (Details) (USD $) In Millions, unless otherwise specified | 0 Months Ended
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Apr. 25, 2014
SUBSEQUENT EVENT [Abstract] |
Expected payment for pending acquisition | $ 117

X
- Definition The anticipated cash outflow associated with the pending acquisition of a business, net of the cash acquired from the purchase. + References No definition available. + Details
Name: | belfb_ExpectedPaymentsToAcquirePendingAcquisitionNetOfCashAcquired
Namespace Prefix: | belfb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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