LEGAL PROCEEDINGS (Details)	9 Months Ended
Sep. 30, 2012
LEGAL PROCEEDINGS [Abstract]
Number of patents claimed to be infringed	3

Number of patents claimed to be infringed related to integrated magnetic connector products.

No definition available.

Name:	belfb_NumberOfPatentsClaimedToBeInfringed
Namespace Prefix:	belfb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration