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RELATED PARTY TRANSACTIONS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Gain from divestiture of business	$ 0.5
Power Solutions [Member]
Related Party Transaction [Line Items]
Minority interest ownership percentage	49.00%
People's Republic of China Joint Venture [Member]
Related Party Transaction [Line Items]
Inventory purchase payment from joint venture	$ 0.0	$ 0.0	$ 1.5
Extinguishment of debt	$ 0.5

X

- Definition

Inventory purchased from the joint venture during the period.

+ References

No definition available.

+ Details

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Namespace Prefix:	belfb_
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Balance Type:	debit
Period Type:	duration

X

- Definition

Gross amount of debt extinguished.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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X

- Definition

Gain (loss) from sale or disposal of an organization or integrated set of activities (for example, but not limited to, a partnership or corporation) engaged in providing a product or service in a commercial, industrial, or professional environment.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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X

- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

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Balance Type:	na
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X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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Period Type: