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Note 5 - Fair Value Measurements - Fair Value Estimate Calculation (Details) - North America Reporting Unit [Member] | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Valuation Technique, Discounted Cash Flow [Member] |  |
Valuation Method Allocation Of Use Percentage | 75.00% | 75.00%
Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] |  |
Goodwill Impairment, Measurement Input | 0.015 | 0.026
Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, EBITDA Margins [Member] |  |
Goodwill Impairment, Measurement Input | 0.039 | 0.076
Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Cost of Equity Capital [Member] |  |
Goodwill Impairment, Measurement Input | 0.154 | 0.142
Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Cost of Debt [Member] |  |
Goodwill Impairment, Measurement Input | 0.04 | 0.037
Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Weighted Average Cost of Capital [Member] |  |
Goodwill Impairment, Measurement Input | 0.14 | 0.13
Valuation, Market Approach [Member] |  |
Valuation Method Allocation Of Use Percentage | 25.00% | 25.00%
Valuation, Market Approach [Member] | Measurement Input, Revenue Multiple [Member] |  |
Goodwill Impairment, Measurement Input | 0.3 | 0.4
Valuation, Market Approach [Member] | Measurement Input, EBITDA Multiple [Member] | Minimum [Member] |  |
Goodwill Impairment, Measurement Input | 7 | 5.1
Valuation, Market Approach [Member] | Measurement Input, EBITDA Multiple [Member] | Maximum [Member] |  |
Goodwill Impairment, Measurement Input | 8 | 5.7
Valuation, Market Approach [Member] | Measurement Input, Invested Capital Control Premium [Member] |  |
Goodwill Impairment, Measurement Input | 0.25 | 0.25

X
- Definition Value of input used to measure goodwill impairment. + References No definition available. + Details
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Namespace Prefix: | belfa_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents valuation method allocation of use percentage. + References No definition available. + Details
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Namespace Prefix: | belfa_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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

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- Details
Name: | us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputRevenueMultipleMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputEbitdaMultipleMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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- Details
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Namespace Prefix: |
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