SUBSEQUENT EVENTS (Details) (USD $) In Millions, unless otherwise specified	Oct. 30, 2012
Subsequent Event [Line Items]
Maximum insurance deductible amount	$ 0.5

Maximum amount of insurance deductible related to any potential damage from a catastrophic event.

No definition available.

Name:	belfb_SubsequentEventsMaximumInsuranceDeductibleAmount
Namespace Prefix:	belfb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant