XML 84 R58.htm IDEA: XBRL DOCUMENT v2.4.1.9
Segments (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
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
Industry
Segment
Dec. 31, 2013
Dec. 31, 2012
Segments [Abstract]                      
Number of industry in which entity operates                 1belfb_NumberOfIndustryInWhichEntityOperates    
Number of reportable operating segments                 3us-gaap_NumberOfReportableSegments    
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 $ 76,400us-gaap_Revenues $ 103,300us-gaap_Revenues $ 70,600us-gaap_Revenues
Net Sales 148,650us-gaap_SalesRevenueGoodsNet 156,341us-gaap_SalesRevenueGoodsNet 99,439us-gaap_SalesRevenueGoodsNet 82,646us-gaap_SalesRevenueGoodsNet 91,016us-gaap_SalesRevenueGoodsNet 101,164us-gaap_SalesRevenueGoodsNet 93,981us-gaap_SalesRevenueGoodsNet 63,028us-gaap_SalesRevenueGoodsNet 487,076us-gaap_SalesRevenueGoodsNet 349,189us-gaap_SalesRevenueGoodsNet 286,594us-gaap_SalesRevenueGoodsNet
Income (loss) from operations                 14,093us-gaap_OperatingIncomeLoss 15,047us-gaap_OperatingIncomeLoss 1,663us-gaap_OperatingIncomeLoss
Total assets 636,025us-gaap_Assets       308,141us-gaap_Assets       636,025us-gaap_Assets 308,141us-gaap_Assets  
Capital Expenditures                 9,042us-gaap_PaymentsToAcquirePropertyPlantAndEquipment 6,940us-gaap_PaymentsToAcquirePropertyPlantAndEquipment 4,744us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
Depreciation and Amortization Expense                 19,746us-gaap_DepreciationDepletionAndAmortization 12,382us-gaap_DepreciationDepletionAndAmortization 9,113us-gaap_DepreciationDepletionAndAmortization
Restructuring charges                 1,832us-gaap_RestructuringCharges 1,387us-gaap_RestructuringCharges 5,245us-gaap_RestructuringCharges
Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 487,076us-gaap_Revenues
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
349,189us-gaap_Revenues
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
286,594us-gaap_Revenues
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
Net Sales                 487,076us-gaap_SalesRevenueGoodsNet
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
349,189us-gaap_SalesRevenueGoodsNet
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
286,594us-gaap_SalesRevenueGoodsNet
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
Income (loss) from operations                 14,093us-gaap_OperatingIncomeLoss
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
15,047us-gaap_OperatingIncomeLoss
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
1,663us-gaap_OperatingIncomeLoss
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
Total assets 636,025us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
      308,141us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
      636,025us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
308,141us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
275,189us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
Capital Expenditures                 9,042us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
6,940us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
4,744us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
Depreciation and Amortization Expense                 19,746us-gaap_DepreciationDepletionAndAmortization
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
12,382us-gaap_DepreciationDepletionAndAmortization
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
9,113us-gaap_DepreciationDepletionAndAmortization
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
Restructuring charges                 1,832us-gaap_RestructuringCharges
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
1,387us-gaap_RestructuringCharges
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
5,245us-gaap_RestructuringCharges
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
Power Solutions [Member]                      
Summary of key financial data [Abstract]                      
Total assets 203,700us-gaap_Assets
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
              203,700us-gaap_Assets
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
   
Connectivity Solutions [Member]                      
Summary of key financial data [Abstract]                      
Total assets 133,100us-gaap_Assets
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
              133,100us-gaap_Assets
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
   
2012-2014 Acquired Companies [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 218,864us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
79,361us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
3,202us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
Income (loss) from operations                 9,727us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
9,083us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
(85)us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
North America [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 217,258us-gaap_Revenues
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
116,548us-gaap_Revenues
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
126,469us-gaap_Revenues
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
Net Sales                 248,007us-gaap_SalesRevenueGoodsNet
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
128,472us-gaap_SalesRevenueGoodsNet
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
138,966us-gaap_SalesRevenueGoodsNet
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
Income (loss) from operations                 (4,465)us-gaap_OperatingIncomeLoss
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
(1,560)us-gaap_OperatingIncomeLoss
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
1,336us-gaap_OperatingIncomeLoss
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
Total assets 311,298us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
      117,261us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
      311,298us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
117,261us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
84,609us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
Capital Expenditures                 3,862us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
2,064us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
2,455us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
Depreciation and Amortization Expense                 7,921us-gaap_DepreciationDepletionAndAmortization
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
4,282us-gaap_DepreciationDepletionAndAmortization
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
4,081us-gaap_DepreciationDepletionAndAmortization
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
Restructuring charges                 1,539us-gaap_RestructuringCharges
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
963us-gaap_RestructuringCharges
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
4,558us-gaap_RestructuringCharges
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
North America [Member] | Power Solutions [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 73,530us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
Income (loss) from operations                 (712)us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
North America [Member] | Connectivity Solutions [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 28,242us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
Income (loss) from operations                 (2,808)us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
North America [Member] | Array [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 6,842us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_ArrayMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
2,074us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_ArrayMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_ArrayMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
Income (loss) from operations                 (801)us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_ArrayMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
(936)us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_ArrayMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_ArrayMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
North America [Member] | 2012-2014 Acquired Companies [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 108,614us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
2,074us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
Income (loss) from operations                 (4,321)us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
(936)us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
Asia [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 201,338us-gaap_Revenues
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
193,647us-gaap_Revenues
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
128,319us-gaap_Revenues
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
Net Sales                 275,765us-gaap_SalesRevenueGoodsNet
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
225,151us-gaap_SalesRevenueGoodsNet
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
167,756us-gaap_SalesRevenueGoodsNet
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
Income (loss) from operations                 13,338us-gaap_OperatingIncomeLoss
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
15,356us-gaap_OperatingIncomeLoss
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
(42)us-gaap_OperatingIncomeLoss
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
Total assets 254,822us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
      148,780us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
      254,822us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
148,780us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
148,351us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
Capital Expenditures                 4,089us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
4,551us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
2,003us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
Depreciation and Amortization Expense                 8,143us-gaap_DepreciationDepletionAndAmortization
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
6,540us-gaap_DepreciationDepletionAndAmortization
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
4,076us-gaap_DepreciationDepletionAndAmortization
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
Restructuring charges                 0us-gaap_RestructuringCharges
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
249us-gaap_RestructuringCharges
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
612us-gaap_RestructuringCharges
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
Asia [Member] | Power Solutions [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 3,401us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
Income (loss) from operations                 (3,603)us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
Asia [Member] | Connectivity Solutions [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 2,469us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
Income (loss) from operations                 493us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
Asia [Member] | TRP [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 66,082us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_TrpMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
65,141us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_TrpMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_TrpMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
Income (loss) from operations                 13,152us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_TrpMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
9,007us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_TrpMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_TrpMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
Asia [Member] | 2012-2014 Acquired Companies [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 71,952us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
65,141us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
Income (loss) from operations                 10,042us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
9,007us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaMember
Europe [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 68,480us-gaap_Revenues
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
38,994us-gaap_Revenues
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
31,806us-gaap_Revenues
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Net Sales                 114,748us-gaap_SalesRevenueGoodsNet
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
40,742us-gaap_SalesRevenueGoodsNet
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
33,329us-gaap_SalesRevenueGoodsNet
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Income (loss) from operations                 5,220us-gaap_OperatingIncomeLoss
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
1,251us-gaap_OperatingIncomeLoss
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
369us-gaap_OperatingIncomeLoss
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Total assets 69,905us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
      42,100us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
      69,905us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
42,100us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
42,229us-gaap_Assets
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Capital Expenditures                 1,091us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
325us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
286us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Depreciation and Amortization Expense                 3,682us-gaap_DepreciationDepletionAndAmortization
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
1,560us-gaap_DepreciationDepletionAndAmortization
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
956us-gaap_DepreciationDepletionAndAmortization
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Restructuring charges                 293us-gaap_RestructuringCharges
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
175us-gaap_RestructuringCharges
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
75us-gaap_RestructuringCharges
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Europe [Member] | Power Solutions [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 23,882us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Income (loss) from operations                 4,882us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerSolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Europe [Member] | Connectivity Solutions [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 2,812us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Income (loss) from operations                 (167)us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_EmersonNetworkPowerConnectivitySolutionsMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Europe [Member] | TRP [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 2,498us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_TrpMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
1,407us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_TrpMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
0us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_TrpMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Income (loss) from operations                 366us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_TrpMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
289us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_TrpMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
0us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_TrpMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Europe [Member] | GigaCom [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 8us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_GigacomMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
4us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_GigacomMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
57us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_GigacomMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Income (loss) from operations                 (23)us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_GigacomMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
(986)us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_GigacomMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
(392)us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_GigacomMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Europe [Member] | Fibreco [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 6,197us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_FibrecoMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
7,483us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_FibrecoMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
2,062us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_FibrecoMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Income (loss) from operations                 817us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_FibrecoMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
2,101us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_FibrecoMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
297us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_FibrecoMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Europe [Member] | Powerbox [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 2,901us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerboxItalyMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
3,252us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerboxItalyMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
1,083us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerboxItalyMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Income (loss) from operations                 (1,869)us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerboxItalyMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
(392)us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerboxItalyMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
10us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_PowerboxItalyMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Europe [Member] | 2012-2014 Acquired Companies [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales to External Customers                 38,298us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
12,146us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
3,202us-gaap_Revenues
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Income (loss) from operations                 4,006us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
1,012us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
(85)us-gaap_OperatingIncomeLoss
/ us-gaap_BusinessAcquisitionAxis
= belfb_AcquiredCompaniesMember
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_EuropeMember
Intergeographic Revenues [Member] | Reportable Operating Segments [Member]                      
Summary of key financial data [Abstract]                      
Net Sales                 (151,444)us-gaap_SalesRevenueGoodsNet
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_IntersegmentEliminationMember
(45,176)us-gaap_SalesRevenueGoodsNet
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_IntersegmentEliminationMember
(53,457)us-gaap_SalesRevenueGoodsNet
/ us-gaap_ConsolidationItemsAxis
= us-gaap_ReportableGeographicalComponentsMember
/ us-gaap_StatementGeographicalAxis
= us-gaap_IntersegmentEliminationMember
United States [Member]                      
Summary of key financial data [Abstract]                      
Net Sales                 217,258us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= country_US
116,548us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= country_US
126,469us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= country_US
Macao [Member]                      
Summary of key financial data [Abstract]                      
Net Sales                 195,469us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= country_MO
193,647us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= country_MO
128,319us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= country_MO
Germany [Member]                      
Summary of key financial data [Abstract]                      
Net Sales                 18,663us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= country_DE
16,585us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= country_DE
14,165us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= country_DE
United Kingdom [Member]                      
Summary of key financial data [Abstract]                      
Net Sales                 22,852us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= country_GB
16,538us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= country_GB
13,203us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= country_GB
All Other Foreign Countries [Member]                      
Summary of key financial data [Abstract]                      
Net Sales                 $ 17,598us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= belfb_AllOtherForeignCountriesMember
$ 5,871us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= belfb_AllOtherForeignCountriesMember
$ 4,438us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= belfb_AllOtherForeignCountriesMember