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v2.4.1.9

Other Assets (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items] |  |  |
Cash surrender value of the COLI | $ 5,800,000 us-gaap_CashSurrenderValueFairValueDisclosure | $ 8,600,000 us-gaap_CashSurrenderValueFairValueDisclosure |
Entity surrender value | 5,756,000 us-gaap_ProceedsFromLifeInsurancePolicies | 0 us-gaap_ProceedsFromLifeInsurancePolicies | 0 us-gaap_ProceedsFromLifeInsurancePolicies
Increase (decrease) in cash surrender value | 200,000 belfb_IncreaseOrDecreaseInCashSurrenderValue | 700,000 belfb_IncreaseOrDecreaseInCashSurrenderValue | 300,000 belfb_IncreaseOrDecreaseInCashSurrenderValue
Payments to acquire marketable securities | 2,936,000 us-gaap_PaymentsToAcquireMarketableSecurities | 0 us-gaap_PaymentsToAcquireMarketableSecurities | 24,000 us-gaap_PaymentsToAcquireMarketableSecurities
Fair value of investments | 100,000 us-gaap_AvailableForSaleSecurities | 100,000 us-gaap_AvailableForSaleSecurities |
Investments held in Rabbi Trust [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Entity surrender value | 2,900,000 us-gaap_ProceedsFromLifeInsurancePolicies / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember |  |
Fair value of investments | 5,800,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember |  |
Fair value of investments | 6,500,000 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember | 3,300,000 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember |
Unrealized gain on investments | 700,000 us-gaap_AvailableforsaleSecuritiesGrossUnrealizedGainLoss1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember | 400,000 us-gaap_AvailableforsaleSecuritiesGrossUnrealizedGainLoss1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember |
Supplemental Employee Retirement Plans [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Benefit obligation | 14,205,000 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember | 10,830,000 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember | 11,045,000 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
Value of assets earmarked for SERP use but not restricted to that use | 12,300,000 us-gaap_OtherAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember | 11,900,000 us-gaap_OtherAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember |
Payments to acquire marketable securities | $ 2,900,000 us-gaap_PaymentsToAcquireMarketableSecurities / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember |  |

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 + Details
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- Definition Fair value portion of the amount that could be realized under a life insurance contract or contracts owned by the entity, commonly known as corporate-owned life insurance (COLI) or bank-owned life insurance (BOLI). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Surrender Value -URI http://asc.fasb.org/extlink&oid=6507153 + Details
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- Definition 1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Projected Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6522206 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accumulated Postretirement Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6503904 + Details
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