Note 12 - Accrued Expenses - Activity and Liability Balances Related to Restructuring Costs (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Accrued restructuring costs	$ 5,823	$ 5,498	$ 6,796
Restructuring charges	3,459	10,114	7,322
Cash payment and other settlements	(3,134)	(11,412)
Employee Severance [Member]
Accrued restructuring costs	1,276	1,551	3,390
Restructuring charges	2,888	7,590
Cash payment and other settlements	(3,163)	(9,429)
Disposal Of Equipment [Member]
Accrued restructuring costs	0	0	0
Restructuring charges	0	1,320
Cash payment and other settlements	0	(1,320)
Other Restructuring [Member]
Accrued restructuring costs	4,547	3,947	$ 3,406
Restructuring charges	571	1,204
Cash payment and other settlements	$ 29	$ (663)

The amount of cash payments made as the result of exit or disposal activities and any other settlements which is not reported separately during the period.

No definition available.

+ Details

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Namespace Prefix:	belfa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, after cash payment, of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

+ Details

Name:	us-gaap_RestructuringCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Carrying amount (including both current and noncurrent portions of the accrual) as of the balance sheet date pertaining to a specified type of cost associated with exit from or disposal of business activities or restructuring pursuant to a duly authorized plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 420
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 5.P.4.b.2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479823/420-10-S99-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 420
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482017/420-10-50-1

+ Details

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Balance Type:	credit
Period Type:	instant

- Details

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

Name:	us-gaap_RestructuringCostAndReserveAxis=belfa_DisposalOfEquipmentMember
Namespace Prefix:
Data Type:	na
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Period Type:

- Details

Name:	us-gaap_RestructuringCostAndReserveAxis=us-gaap_OtherRestructuringMember
Namespace Prefix:
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Period Type: