Note 11 - Retirement Fund and Profit Sharing Plan (Details Textual) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Bel Fuse Inc. Employees' Savings Plan [Member]
Defined Contribution Plan, Cost	$ 0.3	$ 0.3	$ 0.9	$ 0.8
Bel Fuse Inc. Employees' Savings Plan [Member] | Common Class A [Member]
Shares Held in Plan (in shares)	292,569		292,569
Bel Fuse Inc. Employees' Savings Plan [Member] | Common Class B [Member]
Shares Held in Plan (in shares)	103,511		103,511
Retirement Fund Covering Hong Kong-based Employees [Member]
Defined Contribution Plan, Cost	$ 0.6	$ 0.1	$ 1.9	$ 0.2
Retirement Fund Covering Hong Kong-based Employees [Member] | Common Class A [Member]
Shares Held in Plan (in shares)	3,323		3,323
Retirement Fund Covering Hong Kong-based Employees [Member] | Common Class B [Member]
Shares Held in Plan (in shares)	17,342		17,342