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Note 13 - Retirement Fund and Profit Sharing Plan (Details Textual) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Supplemental Employee Retirement Plan [Member] |  |  |
Defined Benefit Plan, Percentage of Average Base Compensation Five Consecutive Years, Normal Retirement | 40.00% |  |
Defined Benefit Plan, Number of Monthly Payments | 120 |  |
Percentage of Participant's Annual Base Salary Received by Beneficiary, One Year From Date of Death | 100.00% |  |
Percentage of Participant's Annual Base Salary Received by Beneficiary, Following Four Years From Date of Death | 50.00% |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total | $ 1,579 | $ 1,507 |
Defined Benefit Plan, Accumulated Benefit Obligation | 22,400 | 18,500 |
Investments, Fair Value Disclosure, Total | 15,400 | 14,700 |
Defined Benefit Plan, Expected Amortization, Next Fiscal Year, Total | 500 |  |
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year | 500 |  |
Defined Benefit Plan, Plan Assets, Amount, Ending Balance | $ 0 | 0 | $ 0
Bel Fuse Inc. Employees' Savings Plan [Member] |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent Of Match, First 1% Employee Contribution | 100.00% |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Match, Next 5% Employee Contribution | 50.00% |  |
Defined Contribution Plan, Cost | $ 1,100 | 1,100 |
Bel Fuse Inc. Employees' Savings Plan [Member] | Common Class A [Member] |  |  |
Shares Held in Plan (in shares) | 257,030 |  |
Bel Fuse Inc. Employees' Savings Plan [Member] | Common Class B [Member] |  |  |
Shares Held in Plan (in shares) | 109,435 |  |
Retirement Fund Covering Hong Kong-based Employees [Member] |  |  |
Defined Contribution Plan, Cost | $ 300 | $ 300 |
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent | 5.00% |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay | 7.00% |  |
Retirement Fund Covering Hong Kong-based Employees [Member] | Minimum [Member] |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay | 5.00% |  |
Retirement Fund Covering Hong Kong-based Employees [Member] | Common Class A [Member] |  |  |
Shares Held in Plan (in shares) | 3,323 |  |
Retirement Fund Covering Hong Kong-based Employees [Member] | Common Class B [Member] |  |  |
Shares Held in Plan (in shares) | 17,342 |  |

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- Definition The percentage of average base compensation of the highest five consecutive years for normal retirement under defined benefit plan. + References No definition available. + Details
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- Definition The percentage of participant's annual base salary received by beneficiary for years two through five following date of death. + References No definition available. + Details
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- Definition Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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