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Note 3 - Divestiture of Subsidiary (Details Textual) - USD ($) $ in Thousands | Mar. 31, 2024 | Jun. 01, 2023
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Bel Stewart s.r.o. [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] |  |
Disposal Group, Including Discontinued Operation, Consideration | $ 5,063 | $ 5,100

X
- Definition Amount of consideration received or receivable for the disposal of assets and liabilities, including discontinued operation. + References No definition available. + Details
Name: | us-gaap_DisposalGroupIncludingDiscontinuedOperationConsideration
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=belfa_BelStewartSroMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DisposalGroupClassificationAxis=us-gaap_DisposalGroupDisposedOfBySaleNotDiscontinuedOperationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |