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Goodwill and Other Intangible Assets (Details) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Goodwill [Roll Forward] |  |  |
Goodwill, gross beginning of period | $ 45,431,000 us-gaap_GoodwillGross | $ 40,500,000 us-gaap_GoodwillGross |
Accumulated impairment charges beginning of period | (26,941,000)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss | (26,941,000)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss |
Goodwill, net beginning of period | 18,490,000 us-gaap_Goodwill | 13,559,000 us-gaap_Goodwill |
Goodwill allocation related to acquisitions | 100,016,000 us-gaap_GoodwillAcquiredDuringPeriod | 4,812,000 us-gaap_GoodwillAcquiredDuringPeriod | 9,065,000 us-gaap_GoodwillAcquiredDuringPeriod
Measurement period adjustments | (496,000)us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments |  |
Foreign currency translation | (437,000)us-gaap_GoodwillTranslationAdjustments | 119,000 us-gaap_GoodwillTranslationAdjustments |
Goodwill, gross end of period | 144,514,000 us-gaap_GoodwillGross | 45,431,000 us-gaap_GoodwillGross | 40,500,000 us-gaap_GoodwillGross
Accumulated impairment charges end of period | (26,941,000)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss | (26,941,000)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss | (26,941,000)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss
Goodwill, net end of period | 117,573,000 us-gaap_Goodwill | 18,490,000 us-gaap_Goodwill | 13,559,000 us-gaap_Goodwill
Components of intangible assets other than goodwill [Abstract] |  |  |
Gross Carrying Amount | 104,113,000 belfb_GrossCarryingAmountOfFiniteAndInfiniteIntangibleAssetsOtherThanGoodwill | 33,010,000 belfb_GrossCarryingAmountOfFiniteAndInfiniteIntangibleAssetsOtherThanGoodwill |
Accumulated Amortization | 8,611,000 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization | 3,538,000 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization |
Net Carrying Amount | 95,502,000 us-gaap_IntangibleAssetsNetExcludingGoodwill | 29,472,000 us-gaap_IntangibleAssetsNetExcludingGoodwill |
Intangible assets acquired | 73,200,000 belfb_IntangibleAssetsAcquiredFiniteAndInfinite | 8,900,000 belfb_IntangibleAssetsAcquiredFiniteAndInfinite |
Amortization expense | 5,400,000 us-gaap_AmortizationOfIntangibleAssets | 1,900,000 us-gaap_AmortizationOfIntangibleAssets | 800,000 us-gaap_AmortizationOfIntangibleAssets
Estimated amortization expense for intangible assets [Abstract] |  |  |
2015 | 7,481,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths |  |
2016 | 7,181,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo |  |
2017 | 6,848,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree |  |
2018 | 6,556,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour |  |
2019 | 6,515,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive |  |
Minimum [Member] |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Other intangible assets amortization period | 2 years |  |
Maximum [Member] |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Other intangible assets amortization period | 24 years |  |
Patents, Licenses and Technology [Member] |  |  |
Components of intangible assets other than goodwill [Abstract] |  |  |
Gross Carrying Amount | 38,872,000 belfb_GrossCarryingAmountOfFiniteAndInfiniteIntangibleAssetsOtherThanGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = belfb_PatentsLicensesAndTechnologyMember | 11,919,000 belfb_GrossCarryingAmountOfFiniteAndInfiniteIntangibleAssetsOtherThanGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = belfb_PatentsLicensesAndTechnologyMember |
Accumulated Amortization | 4,297,000 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = belfb_PatentsLicensesAndTechnologyMember | 1,864,000 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = belfb_PatentsLicensesAndTechnologyMember |
Net Carrying Amount | 34,575,000 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = belfb_PatentsLicensesAndTechnologyMember | 10,055,000 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = belfb_PatentsLicensesAndTechnologyMember |
Customer Relationships [Member] |  |  |
Components of intangible assets other than goodwill [Abstract] |  |  |
Gross Carrying Amount | 45,836,000 belfb_GrossCarryingAmountOfFiniteAndInfiniteIntangibleAssetsOtherThanGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 11,923,000 belfb_GrossCarryingAmountOfFiniteAndInfiniteIntangibleAssetsOtherThanGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Accumulated Amortization | 3,062,000 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 1,191,000 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Net Carrying Amount | 42,774,000 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 10,732,000 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Non-compete Agreements [Member] |  |  |
Components of intangible assets other than goodwill [Abstract] |  |  |
Gross Carrying Amount | 2,781,000 belfb_GrossCarryingAmountOfFiniteAndInfiniteIntangibleAssetsOtherThanGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_NoncompeteAgreementsMember | 787,000 belfb_GrossCarryingAmountOfFiniteAndInfiniteIntangibleAssetsOtherThanGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_NoncompeteAgreementsMember |
Accumulated Amortization | 1,050,000 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_NoncompeteAgreementsMember | 483,000 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_NoncompeteAgreementsMember |
Net Carrying Amount | 1,731,000 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_NoncompeteAgreementsMember | 304,000 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_NoncompeteAgreementsMember |
Trademarks [Member] |  |  |
Components of intangible assets other than goodwill [Abstract] |  |  |
Gross Carrying Amount | 16,624,000 belfb_GrossCarryingAmountOfFiniteAndInfiniteIntangibleAssetsOtherThanGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember | 8,381,000 belfb_GrossCarryingAmountOfFiniteAndInfiniteIntangibleAssetsOtherThanGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember |
Accumulated Amortization | 202,000 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember | 0 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember |
Net Carrying Amount | 16,422,000 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember | 8,381,000 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember |
North America [Member] |  |  |
Goodwill [Roll Forward] |  |  |
Goodwill, gross beginning of period | 18,865,000 us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember | 15,293,000 us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember |
Accumulated impairment charges beginning of period | (14,066,000)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember | (14,066,000)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember |
Goodwill, net beginning of period | 4,799,000 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember | 1,227,000 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember |
Goodwill allocation related to acquisitions | 50,118,000 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember | 3,572,000 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember |
Measurement period adjustments | (496,000)us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember |  |
Foreign currency translation | 0 us-gaap_GoodwillTranslationAdjustments / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember | 0 us-gaap_GoodwillTranslationAdjustments / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember |
Goodwill, gross end of period | 68,487,000 us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember | 18,865,000 us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember |
Accumulated impairment charges end of period | (14,066,000)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember | (14,066,000)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember |
Goodwill, net end of period | 54,421,000 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember | 4,799,000 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember |
Reporting unit, Percentage of fair value in excess of carrying amount (in hundredths) | 10.00%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_StatementBusinessSegmentsAxis = us-gaap_NorthAmericaMember |  |
Asia [Member] |  |  |
Goodwill [Roll Forward] |  |  |
Goodwill, gross beginning of period | 14,107,000 us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember | 12,875,000 us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember |
Accumulated impairment charges beginning of period | (12,875,000)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember | (12,875,000)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember |
Goodwill, net beginning of period | 1,232,000 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember | 0 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember |
Goodwill allocation related to acquisitions | 35,486,000 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember | 1,240,000 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember |
Measurement period adjustments | 0 us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember |  |
Foreign currency translation | (210,000)us-gaap_GoodwillTranslationAdjustments / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember | (8,000)us-gaap_GoodwillTranslationAdjustments / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember |
Goodwill, gross end of period | 49,383,000 us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember | 14,107,000 us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember |
Accumulated impairment charges end of period | (12,875,000)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember | (12,875,000)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember |
Goodwill, net end of period | 36,508,000 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember | 1,232,000 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember |
Reporting unit, Percentage of fair value in excess of carrying amount (in hundredths) | 63.00%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AsiaMember |  |
Europe [Member] |  |  |
Goodwill [Roll Forward] |  |  |
Goodwill, gross beginning of period | 12,459,000 us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember | 12,332,000 us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember |
Accumulated impairment charges beginning of period | 0 us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember | 0 us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember |
Goodwill, net beginning of period | 12,459,000 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember | 12,332,000 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember |
Goodwill allocation related to acquisitions | 14,412,000 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember | 0 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember |
Measurement period adjustments | 0 us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember |  |
Foreign currency translation | (227,000)us-gaap_GoodwillTranslationAdjustments / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember | 127,000 us-gaap_GoodwillTranslationAdjustments / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember |
Goodwill, gross end of period | 26,644,000 us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember | 12,459,000 us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember |
Accumulated impairment charges end of period | 0 us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember | 0 us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember |
Goodwill, net end of period | $ 26,644,000 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember | $ 12,459,000 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember |
Reporting unit, Percentage of fair value in excess of carrying amount (in hundredths) | 19.00%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_StatementBusinessSegmentsAxis = us-gaap_EuropeMember |  |

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- Definition The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275 + Details
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