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v3.20.1

Note 16 - Leases (Details Textual) | 12 Months Ended
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Dec. 31, 2019
Minimum [Member] |
Operating and Finance Leases, Remaining Lease Term | 1 year
Maximum [Member] |
Operating and Finance Leases, Remaining Lease Term | 8 years

X
- Definition 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. + References No definition available. + Details
Name: | belfa_OperatingAndFinanceLeasesRemainingLeaseTerm
Namespace Prefix: | belfa_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |