Note 11 - Retirement Fund and Profit Sharing Plan (Details Textual) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Bel Fuse Inc. Employees' Savings Plan [Member]
Defined Contribution Plan, Cost	$ 0.3	$ 0.3	$ 0.8	$ 0.8
Bel Fuse Inc. Employees' Savings Plan [Member] | Common Class A [Member]
Shares Held in Plan	153,352		153,352
Bel Fuse Inc. Employees' Savings Plan [Member] | Common Class B [Member]
Shares Held in Plan	109,442		109,442
Retirement Fund Covering Hong Kong-based Employees [Member]
Defined Contribution Plan, Cost	$ 0.1	$ 0.1	$ 0.3	$ 0.3
Retirement Fund Covering Hong Kong-based Employees [Member] | Common Class A [Member]
Shares Held in Plan	3,323		3,323
Retirement Fund Covering Hong Kong-based Employees [Member] | Common Class B [Member]
Shares Held in Plan	17,342		17,342