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

The anticipated cash outflow associated with the pending acquisition of a business, net of the cash acquired from the purchase.

No definition available.

Name:	belfb_ExpectedPaymentsToAcquirePendingAcquisitionNetOfCashAcquired
Namespace Prefix:	belfb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration