XML 56 R45.htm IDEA: XBRL DOCUMENT
v3.19.2

RETIREMENT FUND AND PROFIT SHARING PLAN, Employees' Savings Plan (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018
UNITED STATES | Class A Common Stock [Member] |  |  |  |
Compensation Expense [Abstract] |  |  |  |
Shares owned by plan (in shares) | 133,280 |  | 133,280 |
UNITED STATES | Class B Common Stock [Member] |  |  |  |
Compensation Expense [Abstract] |  |  |  |
Shares owned by plan (in shares) | 107,962 |  | 107,962 |
401K Plan [Member] | UNITED STATES |  |  |  |
Compensation Expense [Abstract] |  |  |  |
Compensation expense | $ 300 | $ 300 | $ 600 | $ 600
Retirement Fund [Member] | HONG KONG |  |  |  |
Compensation Expense [Abstract] |  |  |  |
Compensation expense | $ 100 | $ 100 | $ 200 | $ 200
Retirement Fund [Member] | HONG KONG | Class A Common Stock [Member] |  |  |  |
Compensation Expense [Abstract] |  |  |  |
Shares owned by plan (in shares) | 3,323 |  | 3,323 |
Retirement Fund [Member] | HONG KONG | Class B Common Stock [Member] |  |  |  |
Compensation Expense [Abstract] |  |  |  |
Shares owned by plan (in shares) | 17,342 |  | 17,342 |

X
- Definition Number of shares of the company's common stock owned by the plan. + References No definition available. + Details
Name: | belfb_SharesHeldInPlan
Namespace Prefix: | belfb_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_CompensationRelatedCostsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of cost for defined contribution plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
Name: | us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_RetirementPlanSponsorLocationAxis=country_US
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassBMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RetirementPlanNameAxis=belfb_FourZeroOneKPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RetirementPlanNameAxis=belfb_RetirementFundMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RetirementPlanSponsorLocationAxis=country_HK
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |