| Document And Entity Information | Oct. 30, 2020 |
| Document Information [Line Items] | |
| Entity, Registrant Name | BELFUSE INC /NJ |
| Document, Type | 8-K |
| Document, Period End Date | Oct. 30, 2020 |
| Entity, Incorporation, State or Country Code | NJ |
| Entity, File Number | 0-11676 |
| Entity, Tax Identification Number | 22-1463699 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity, Emerging Growth Company | false |
| Amendment Flag | false |
| Entity, Central Index Key | 0000729580 |
| ClassACommonStock Custom [Member] | |
| Document Information [Line Items] | |
| Title of 12(b) Security | Class A Common Stock |
| Trading Symbol | BELFA |
| Security Exchange Name | NASDAQ |
| ClassBCommonStock Custom [Member] | |
| Document Information [Line Items] | |
| Title of 12(b) Security | Class B Common Stock |
| Trading Symbol | BELFB |
| Security Exchange Name | NASDAQ |
