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Fair Value Measurements (Details) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Gross unrealized gains associated with the investment held in the rabbi trust | $ 147,000 us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax | $ 87,000 us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax | $ (251,000)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax
Fair value of investments | 100,000 us-gaap_AvailableForSaleSecurities | 100,000 us-gaap_AvailableForSaleSecurities |
Gross unrealized gains | 100,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax | 100,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax |
Transfers in out between levels | 0 belfb_TransfersInOutBetweenLevels | 0 belfb_TransfersInOutBetweenLevels |
Carrying amount of long-term debt | 232,625,000 us-gaap_LongTermDebt |  |
Investments held in Rabbi Trust [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair value of investments | 5,800,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember |  |
Available-for-sale securities measured at fair value | 6,500,000 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember | 3,300,000 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember |
Investments held in Rabbi Trust [Member] | SERP [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Gross unrealized gains associated with the investment held in the rabbi trust | 700,000 us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax / us-gaap_AwardTypeAxis = us-gaap_StockCompensationPlanMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember | 400,000 us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax / us-gaap_AwardTypeAxis = us-gaap_StockCompensationPlanMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember |
Fair value of long-term debt | 233,300,000 us-gaap_LongTermDebtFairValue / us-gaap_AwardTypeAxis = us-gaap_StockCompensationPlanMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember |  |
Carrying amount of long-term debt | 232,625,000 us-gaap_LongTermDebt / us-gaap_AwardTypeAxis = us-gaap_StockCompensationPlanMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember |  |
Available-for-sale securities measured at fair value | $ 6,500,000 us-gaap_InvestmentsFairValueDisclosure / us-gaap_AwardTypeAxis = us-gaap_StockCompensationPlanMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember | $ 3,300,000 us-gaap_InvestmentsFairValueDisclosure / us-gaap_AwardTypeAxis = us-gaap_StockCompensationPlanMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember |

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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph a, f, g -Article 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560 + Details
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