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Note 13 - Retirement, Savings and Deferred Compensation Plans (Details Textual) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2025 USD ($) shares	Sep. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) shares	Sep. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)
Supplemental Employee Retirement Plan [Member]
Deferred Compensation Arrangement with Individual, Compensation Expense	$ 0.1			$ 0.3	$ 0.2
Maximum [Member] | Supplemental Employee Retirement Plan [Member]
Deferred Compensation Arrangement with Individual, Compensation Expense		$ 0.1
Bel Fuse Inc. Employees' Savings Plan [Member]
Defined Contribution Plan, Cost	$ 0.3		$ 0.3	$ 1.2	1.2
Bel Fuse Inc. Employees' Savings Plan [Member] | Common Class A [Member]
Shares Held in Plan (in shares) | shares	238,062			238,062
Bel Fuse Inc. Employees' Savings Plan [Member] | Common Class B [Member]
Shares Held in Plan (in shares) | shares	47,641			47,641
Nonqualified Deferred Compensation Plan [Member]
Deferred Compensation Arrangement, Vesting Age	65			65
Deferred Compensation Plan Assets	$ 1.7			$ 1.7		$ 1.3
Nonqualified Deferred Compensation Plan [Member] | Maximum [Member]
Deferred Compensation Arrangement with Individual, Compensation Expense	$ 0.1	$ 0.1		$ 0.1	$ 0.1

X

- Definition

The age of employees for contributions to become fully vested under deferred compensation arrangement.

+ References

No definition available.

+ Details

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X

- Definition

The number of shares of the company's common stock owned by the plan.

+ References

No definition available.

+ Details

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X

- Definition

The compensation expense recognized during the period pertaining to the deferred compensation arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 710
-SubTopic 10
-Name Accounting Standards Codification
-Section 30
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483043/710-10-30-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480429/718-10-50-1

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X

- Definition

Carrying amount as of the balance sheet date of assets held under deferred compensation agreements.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(17))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

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X

- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 70
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480794/715-70-50-1

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