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