RETIREMENT FUND AND PROFIT SHARING PLAN (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Bel Fuse Inc Employees Savings Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Compensation expense	$ 300	$ 300	$ 800	$ 900
Bel Fuse Inc Employees Savings Plan [Member] | Class A Common Stock [Member]
Defined Contribution Plan Disclosure [Line Items]
Shares owned by plan (in shares)	13,928		13,928
Bel Fuse Inc Employees Savings Plan [Member] | Class B Common Stock [Member]
Defined Contribution Plan Disclosure [Line Items]
Shares owned by plan (in shares)	164,462		164,462
Non Defined Retirement Fund [Member]
Defined Contribution Plan Disclosure [Line Items]
Compensation expense	$ 100	100	$ 200	200
Non Defined Retirement Fund [Member] | Class A Common Stock [Member]
Defined Contribution Plan Disclosure [Line Items]
Shares owned by plan (in shares)	3,323		3,323
Non Defined Retirement Fund [Member] | Class B Common Stock [Member]
Defined Contribution Plan Disclosure [Line Items]
Shares owned by plan (in shares)	17,342		17,342
SERP [Member]
Components of SERP expense [Abstract]
Service cost	$ 148	138	$ 445	414
Interest cost	165	142	494	425
Net amortization	98	92	293	275
Net periodic benefit cost	411	$ 372	1,232	$ 1,114
Amounts recognized in accumulated other comprehensive loss, pretax [Abstract]
Prior service cost	730		730		$ 866
Net loss	2,900		2,900		3,465
Total amounts recognized in accumulated other comprehensive loss	$ 3,630		$ 3,630		$ 4,331