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Note 16 - Subsequent Event (Details Textual) - USD ($) $ in Millions |  | 12 Months Ended
--- | --- | ---
Nov. 11, 2019 | Sep. 30, 2019
CUI Power [Member] |  |
Revenue from Contract with Customer, Including Assessed Tax |  | $ 37
Majority Products Business of CUI Power [Member] | Subsequent Event [Member] |  |
Aggregate Purchase Price, Asset Purchase Agreement | $ 32 |

X
- Definition Represents aggregate purchase price for asset purchase agreement. + References No definition available. + Details
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Namespace Prefix: | belfa_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value-added and excise. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045 + Details
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Period Type: | duration

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- Details
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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: |

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