Note 16 - Leases (Details Textual)	12 Months Ended
Dec. 31, 2020
Minimum [Member]
Operating and Finance Leases, Remaining Lease Term (Year)	1 year
Maximum [Member]
Operating and Finance Leases, Remaining Lease Term (Year)	8 years

The remaining lease term for operating and finance lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

No definition available.

Name:	belfa_OperatingAndFinanceLeasesRemainingLeaseTerm
Namespace Prefix:	belfa_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration