Note 11 - Retirement Fund and Profit Sharing Plan (Details Textual) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Bel Fuse Inc. Employees' Savings Plan [Member]
Defined Contribution Plan, Cost	$ 0.3	$ 0.3	$ 0.6	$ 0.6
Bel Fuse Inc. Employees' Savings Plan [Member] | Common Class A [Member]
Shares Held in Plan (in shares)	280,812		280,812
Bel Fuse Inc. Employees' Savings Plan [Member] | Common Class B [Member]
Shares Held in Plan (in shares)	106,144		106,144
Retirement Fund Covering Hong Kong-based Employees [Member]
Defined Contribution Plan, Cost	$ 0.2	$ 0.1	$ 0.3	$ 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