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Note 13 - Retirement Fund and Profit Sharing Plan (Details Textual) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 USD ($) shares | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
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 | $ 1,507 | $ 1,839 |
Defined Benefit Plan, Accumulated Benefit Obligation | 18,500 | 16,500 |
Investments, Fair Value Disclosure, Total | 14,700 | 13,000 |
Defined Benefit Plan, Expected Amortization, Next Fiscal Year, Total | 300 |  |
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year | 400 |  |
Defined Benefit Plan, Plan Assets, Amount, Ending Balance | $ 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 | 1,300 |
Bel Fuse Inc. Employees' Savings Plan [Member] | Common Class A [Member] |  |  |
Shares Held in Plan | shares | 166,004 |  |
Bel Fuse Inc. Employees' Savings Plan [Member] | Common Class B [Member] |  |  |
Shares Held in Plan | shares | 108,868 |  |
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 | shares | 3,323 |  |
Retirement Fund Covering Hong Kong-based Employees [Member] | Common Class B [Member] |  |  |
Shares Held in Plan | shares | 17,342 |  |

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