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FAIR VALUE MEASUREMENTS (Details) (USD $) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
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 | $ 54,000 us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax | $ 28,000 us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax |
Combined fair value of available-for-sale securities | 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
Fair value of long-term debt | 223,800,000 us-gaap_LongTermDebtFairValue |  |
Carrying amount of long-term debt | 220,900,000 us-gaap_LongTermDebt |  |
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 | 800,000 us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax / us-gaap_AwardTypeAxis = us-gaap_StockCompensationPlanMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember |  | 700,000 us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax / us-gaap_AwardTypeAxis = us-gaap_StockCompensationPlanMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember
Available-for-sale securities measured at fair value | $ 6,600,000 us-gaap_InvestmentsFairValueDisclosure / us-gaap_AwardTypeAxis = us-gaap_StockCompensationPlanMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember |  | $ 6,500,000 us-gaap_InvestmentsFairValueDisclosure / us-gaap_AwardTypeAxis = us-gaap_StockCompensationPlanMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = belfb_InvestmentsHeldInRabbiTrustMember

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- Definition The amount of the transfers of assets or liabilities measured on a recurring basis between different levels of the fair value hierarchy. + References No definition available. + Details
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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 Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
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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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- Definition Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Amount after tax, before reclassification adjustments, of unrealized holding gain (loss) on available-for-sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580 + Details
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