Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2021	Mar. 01, 2022	Jun. 30, 2021
Document Information [Line Items]
Entity Central Index Key	0000729580
Entity Registrant Name	BEL FUSE INC /NJ
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2021
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
Document Transition Report	false
Entity File Number	000-11676
Entity Address, Address Line One	206 Van Vorst Street
Entity Address, City or Town	Jersey City
Entity Address, State or Province	NJ
Entity Address, Postal Zip Code	07302
City Area Code	201
Local Phone Number	432-0463
Entity Incorporation, State or Country Code	NJ
Entity Tax Identification Number	22-1463699
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 170,100,000
Auditor Name	Grant Thornton LLP
Auditor Location	Iselin, New Jersey
Auditor Firm ID	248
Deloitte & Touche LLP [Member]
Document Information [Line Items]
Auditor Name	Deloitte & Touche LLP
Auditor Location	New York, New York
Auditor Firm ID	34
Common Class B [Member]
Document Information [Line Items]
Title of 12(b) Security	Class B Common Stock ($0.10 par value)
Trading Symbol	BELFB
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		10,373,102
Common Class A [Member]
Document Information [Line Items]
Title of 12(b) Security	Class A Common Stock ($0.10 par value)
Trading Symbol	BELFA
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		2,144,912