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DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) (USD $) | 3 Months Ended | 12 Months Ended | 0 Months Ended
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Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 Segment | Dec. 31, 2013 | Dec. 31, 2012 | Mar. 09, 2012 | Sep. 12, 2012 | Jul. 31, 2012 | Aug. 20, 2013 | Mar. 29, 2013 | Jun. 19, 2014 | Jul. 25, 2014
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of reportable segments |  |  |  |  |  |  |  |  | 3 us-gaap_NumberOfReportableSegments |  |  |  |  |  |  |  |  |
Foreign Currency Translation [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Change in Chinese Renminbi foreign currency exchange rate |  |  |  |  |  |  |  |  | 0.80%belfb_ChangeInChineseRenminbiForeignCurrencyExchangeRate |  |  |  |  |  |  |  |  |
Net currency exchange (losses) gains |  |  |  |  |  |  |  |  | $ 4,300,000 us-gaap_ForeignCurrencyTransactionLossBeforeTax | $ (600,000)us-gaap_ForeignCurrencyTransactionLossBeforeTax | $ 600,000 us-gaap_ForeignCurrencyTransactionLossBeforeTax |  |  |  |  |  |  |
Foreign currency translation gains (losses) |  |  |  |  |  |  |  |  | (11,269,000)us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax | 977,000 us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax | 281,000 us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax |  |  |  |  |  |  |
Numerator [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net earnings | 2,020,000 us-gaap_NetIncomeLoss | 1,507,000 us-gaap_NetIncomeLoss | 3,065,000 us-gaap_NetIncomeLoss | 2,503,000 us-gaap_NetIncomeLoss | 7,397,000 us-gaap_NetIncomeLoss | 7,380,000 us-gaap_NetIncomeLoss | 1,689,000 us-gaap_NetIncomeLoss | (558,000)us-gaap_NetIncomeLoss | 9,095,000 us-gaap_NetIncomeLoss | 15,908,000 us-gaap_NetIncomeLoss | 2,373,000 us-gaap_NetIncomeLoss |  |  |  |  |  |  |
Undistributed earnings (loss) |  |  |  |  |  |  |  |  | 5,908,000 us-gaap_UndistributedEarnings | 12,810,000 us-gaap_UndistributedEarnings | (846,000)us-gaap_UndistributedEarnings |  |  |  |  |  |  |
Undistributed earnings (loss) allocation - basic and diluted [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Undistributed earnings (loss) |  |  |  |  |  |  |  |  | 5,908,000 us-gaap_UndistributedEarnings | 12,810,000 us-gaap_UndistributedEarnings | (846,000)us-gaap_UndistributedEarnings |  |  |  |  |  |  |
Net earnings allocation - basic and diluted [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net earnings | 2,020,000 us-gaap_NetIncomeLoss | 1,507,000 us-gaap_NetIncomeLoss | 3,065,000 us-gaap_NetIncomeLoss | 2,503,000 us-gaap_NetIncomeLoss | 7,397,000 us-gaap_NetIncomeLoss | 7,380,000 us-gaap_NetIncomeLoss | 1,689,000 us-gaap_NetIncomeLoss | (558,000)us-gaap_NetIncomeLoss | 9,095,000 us-gaap_NetIncomeLoss | 15,908,000 us-gaap_NetIncomeLoss | 2,373,000 us-gaap_NetIncomeLoss |  |  |  |  |  |  |
Net earnings per share: [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Research and development costs |  |  |  |  |  |  |  |  | 21,500,000 us-gaap_ResearchAndDevelopmentExpense | 14,100,000 us-gaap_ResearchAndDevelopmentExpense | 12,400,000 us-gaap_ResearchAndDevelopmentExpense |  |  |  |  |  |  |
Class A [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Numerator [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net earnings |  |  |  |  |  |  |  |  | 1,580,000 us-gaap_NetIncomeLoss / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | 2,868,000 us-gaap_NetIncomeLoss / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | 372,000 us-gaap_NetIncomeLoss / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember |  |  |  |  |  |  |
Less dividends declared |  |  |  |  |  |  |  |  | 522,000 us-gaap_DividendsCommonStockCash / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | 522,000 us-gaap_DividendsCommonStockCash / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | 522,000 us-gaap_DividendsCommonStockCash / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember |  |  |  |  |  |  |
Undistributed earnings (loss) |  |  |  |  |  |  |  |  | 1,058,000 us-gaap_UndistributedEarnings / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | 2,346,000 us-gaap_UndistributedEarnings / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | (150,000)us-gaap_UndistributedEarnings / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember |  |  |  |  |  |  |
Undistributed earnings (loss) allocation - basic and diluted [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Undistributed earnings (loss) |  |  |  |  |  |  |  |  | 1,058,000 us-gaap_UndistributedEarnings / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | 2,346,000 us-gaap_UndistributedEarnings / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | (150,000)us-gaap_UndistributedEarnings / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember |  |  |  |  |  |  |
Net earnings allocation - basic and diluted [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net earnings |  |  |  |  |  |  |  |  | 1,580,000 us-gaap_NetIncomeLoss / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | 2,868,000 us-gaap_NetIncomeLoss / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | 372,000 us-gaap_NetIncomeLoss / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember |  |  |  |  |  |  |
Weighted average shares outstanding [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average shares outstanding (in shares) |  |  |  |  |  |  |  |  | 2,175 us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | 2,175 us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | 2,175 us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember |  |  |  |  |  |  |
Net earnings per share: [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common share - basic and diluted (in dollars per share) | $ 0.16 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | $ 0.12 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | $ 0.25 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | $ 0.20 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | $ 0.61 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | $ 0.62 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | $ 0.14 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | $ (0.05)us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | $ 0.73 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | $ 1.32 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember | $ 0.17 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember |  |  |  |  |  |  |
Class B [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividend rate Class B common stock in excess of dividend rate of Class A common stock (in hundredths) |  |  |  |  |  |  |  |  | 5.00%belfb_ExcessiveDividendRateOfOneClassOverOtherClass / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | 5.00%belfb_ExcessiveDividendRateOfOneClassOverOtherClass / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | 5.00%belfb_ExcessiveDividendRateOfOneClassOverOtherClass / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember |  |  |  |  |  |  |
Undistributed earning allocation rate of Class B common stock in excess of Class A common stock (in hundredths) |  |  |  |  |  |  |  |  | 5.00%belfb_ExcessiveUndistributedEarningsAllocationRateOfOneClassOverOtherClass / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | 5.00%belfb_ExcessiveUndistributedEarningsAllocationRateOfOneClassOverOtherClass / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | 5.00%belfb_ExcessiveUndistributedEarningsAllocationRateOfOneClassOverOtherClass / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember |  |  |  |  |  |  |
Numerator [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net earnings |  |  |  |  |  |  |  |  | 7,515,000 us-gaap_NetIncomeLoss / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | 13,040,000 us-gaap_NetIncomeLoss / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | 2,001,000 us-gaap_NetIncomeLoss / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember |  |  |  |  |  |  |
Less dividends declared |  |  |  |  |  |  |  |  | 2,665,000 us-gaap_DividendsCommonStockCash / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | 2,576,000 us-gaap_DividendsCommonStockCash / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | 2,697,000 us-gaap_DividendsCommonStockCash / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember |  |  |  |  |  |  |
Undistributed earnings (loss) |  |  |  |  |  |  |  |  | 4,850,000 us-gaap_UndistributedEarnings / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | 10,464,000 us-gaap_UndistributedEarnings / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | (696,000)us-gaap_UndistributedEarnings / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember |  |  |  |  |  |  |
Undistributed earnings (loss) allocation - basic and diluted [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Undistributed earnings (loss) |  |  |  |  |  |  |  |  | 4,850,000 us-gaap_UndistributedEarnings / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | 10,464,000 us-gaap_UndistributedEarnings / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | (696,000)us-gaap_UndistributedEarnings / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember |  |  |  |  |  |  |
Net earnings allocation - basic and diluted [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net earnings |  |  |  |  |  |  |  |  | $ 7,515,000 us-gaap_NetIncomeLoss / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | $ 13,040,000 us-gaap_NetIncomeLoss / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | $ 2,001,000 us-gaap_NetIncomeLoss / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember |  |  |  |  |  |  |
Weighted average shares outstanding [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average shares outstanding (in shares) |  |  |  |  |  |  |  |  | 9,491 us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | 9,240 us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | 9,625 us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember |  |  |  |  |  |  |
Net earnings per share: [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common share - basic and diluted (in dollars per share) | $ 0.17 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | $ 0.13 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | $ 0.27 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | $ 0.22 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | $ 0.65 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | $ 0.65 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | $ 0.15 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | $ (0.05)us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | $ 0.79 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | $ 1.41 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember | $ 0.21 us-gaap_EarningsPerShareBasicAndDiluted / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember |  |  |  |  |  |  |
Machinery and Equipment [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  |  |  |  |  |  |  | 1 year |  |  |  |  |  |  |  |  |
Machinery and Equipment [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  |  |  |  |  |  |  | 15 years |  |  |  |  |  |  |  |  |
Buildings and Leasehold Improvements [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  |  |  |  |  |  |  | 1 year |  |  |  |  |  |  |  |  |
Buildings and Leasehold Improvements [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  |  |  |  |  |  |  | 39 years |  |  |  |  |  |  |  |  |
GigaCom Interconnect [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition date |  |  |  |  |  |  |  |  |  |  |  | Mar. 09, 2012 |  |  |  |  |  |
Acquisition of issued and outstanding capital stock (in hundredths) |  |  |  |  |  |  |  |  |  |  |  | 100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired / us-gaap_BusinessAcquisitionAxis = belfb_GigacomInterconnectMember |  |  |  |  |  |
Powerbox [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition date |  |  |  |  |  |  |  |  |  |  |  |  | Sep. 12, 2012 |  |  |  |  |
Acquisition of issued and outstanding capital stock (in hundredths) |  |  |  |  |  |  |  |  |  |  |  |  | 100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired / us-gaap_BusinessAcquisitionAxis = belfb_PowerboxItalyMember |  |  |  |  |
Fibreco [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition date |  |  |  |  |  |  |  |  |  |  |  |  |  | Jul. 31, 2012 |  |  |  |
Acquisition of issued and outstanding capital stock (in hundredths) |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired / us-gaap_BusinessAcquisitionAxis = belfb_FibrecoMember |  |  |  |
Array [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition date |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Aug. 20, 2013 |  |  |
Acquisition of issued and outstanding capital stock (in hundredths) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired / us-gaap_BusinessAcquisitionAxis = belfb_ArrayMember |  |  |
TRP [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition date |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Mar. 29, 2013 |  |
Acquisition of issued and outstanding capital stock (in hundredths) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired / us-gaap_BusinessAcquisitionAxis = belfb_TrpMember |  |
Power Solutions [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition date |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Jun. 19, 2014 |
Acquisition of issued and outstanding capital stock (in hundredths) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired / us-gaap_BusinessAcquisitionAxis = belfb_PowerSolutionsMember |
Emerson [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition date |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Jul. 25, 2014
Acquisition of issued and outstanding capital stock (in hundredths) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired / us-gaap_BusinessAcquisitionAxis = belfb_EmersonMember

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