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Note 5 - Goodwill and Other Intangible Assets - Estimated Fair Values Over Carrying Value, Including Goodwill (Details) - Europe [Member] | Dec. 31, 2024
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Power Solutions and Protection [Member] |
% by Which Estimated Fair Value Exceeds Carrying Value | 500.50%
Connectivity Solutions [Member] |
% by Which Estimated Fair Value Exceeds Carrying Value | 156.00%
CUI Power [Member] |
% by Which Estimated Fair Value Exceeds Carrying Value | 43.60%

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- Definition Percentage of fair value of reporting unit in excess of carrying amount. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | srt_StatementGeographicalAxis=srt_EuropeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_ReportingUnitAxis=belfa_PowerSolutionsAndProtectionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_ReportingUnitAxis=belfa_ConnectivitySolutionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |