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v2.4.1.9

Segments, Entity Wide Information (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 12 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 Customer | Dec. 31, 2013 Customer | Dec. 31, 2012 Customer
Net Sales To External Customers By Geographic Area And Major Product Line [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consolidated net sales | $ 148,650 us-gaap_SalesRevenueGoodsNet | $ 156,341 us-gaap_SalesRevenueGoodsNet | $ 99,439 us-gaap_SalesRevenueGoodsNet | $ 82,646 us-gaap_SalesRevenueGoodsNet | $ 91,016 us-gaap_SalesRevenueGoodsNet | $ 101,164 us-gaap_SalesRevenueGoodsNet | $ 93,981 us-gaap_SalesRevenueGoodsNet | $ 63,028 us-gaap_SalesRevenueGoodsNet | $ 487,076 us-gaap_SalesRevenueGoodsNet | $ 349,189 us-gaap_SalesRevenueGoodsNet | $ 286,594 us-gaap_SalesRevenueGoodsNet
Long-lived Assets by Geographic Location [Abstract] |  |  |  |  |  |  |  |  |  |  |
Long-Lived Assets | 104,361 us-gaap_NoncurrentAssets |  |  |  | 54,344 us-gaap_NoncurrentAssets |  |  |  | 104,361 us-gaap_NoncurrentAssets | 54,344 us-gaap_NoncurrentAssets |
Net sales to major customer [Abstract] |  |  |  |  |  |  |  |  |  |  |
Number of customers representing sales in excess of ten percent |  |  |  |  |  |  |  |  | 1 belfb_NumberOfCustomersRepresentingSalesInExcessOfTenPercent | 2 belfb_NumberOfCustomersRepresentingSalesInExcessOfTenPercent | 2 belfb_NumberOfCustomersRepresentingSalesInExcessOfTenPercent
Revenue from major customers |  |  |  |  |  |  |  |  | 76,400 us-gaap_Revenues | 103,300 us-gaap_Revenues | 70,600 us-gaap_Revenues
Revenue from major customers, percentage (in hundredths) |  |  |  |  |  |  |  |  | 15.70%us-gaap_ConcentrationRiskPercentage1 | 29.60%us-gaap_ConcentrationRiskPercentage1 | 24.60%us-gaap_ConcentrationRiskPercentage1
Reportable Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Net Sales To External Customers By Geographic Area And Major Product Line [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consolidated net sales |  |  |  |  |  |  |  |  | 487,076 us-gaap_SalesRevenueGoodsNet / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember | 349,189 us-gaap_SalesRevenueGoodsNet / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember | 286,594 us-gaap_SalesRevenueGoodsNet / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember
Net sales to major customer [Abstract] |  |  |  |  |  |  |  |  |  |  |
Revenue from major customers |  |  |  |  |  |  |  |  | 487,076 us-gaap_Revenues / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember | 349,189 us-gaap_Revenues / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember | 286,594 us-gaap_Revenues / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember
Connectivity Solutions [Member] |  |  |  |  |  |  |  |  |  |  |
Net Sales To External Customers By Geographic Area And Major Product Line [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consolidated net sales |  |  |  |  |  |  |  |  | 152,954 us-gaap_SalesRevenueGoodsNet / us-gaap_ProductOrServiceAxis = belfb_ConnectivitySolutionMember | 111,653 us-gaap_SalesRevenueGoodsNet / us-gaap_ProductOrServiceAxis = belfb_ConnectivitySolutionMember | 109,245 us-gaap_SalesRevenueGoodsNet / us-gaap_ProductOrServiceAxis = belfb_ConnectivitySolutionMember
Magnetic Solutions [Member] |  |  |  |  |  |  |  |  |  |  |
Net Sales To External Customers By Geographic Area And Major Product Line [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consolidated net sales |  |  |  |  |  |  |  |  | 174,255 us-gaap_SalesRevenueGoodsNet / us-gaap_ProductOrServiceAxis = belfb_MagneticSolutionsMember | 170,166 us-gaap_SalesRevenueGoodsNet / us-gaap_ProductOrServiceAxis = belfb_MagneticSolutionsMember | 100,529 us-gaap_SalesRevenueGoodsNet / us-gaap_ProductOrServiceAxis = belfb_MagneticSolutionsMember
Power Solutions and Protection [Member] |  |  |  |  |  |  |  |  |  |  |
Net Sales To External Customers By Geographic Area And Major Product Line [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consolidated net sales |  |  |  |  |  |  |  |  | 159,867 us-gaap_SalesRevenueGoodsNet / us-gaap_ProductOrServiceAxis = belfb_PowerSolutionsAndProtectionMember | 67,370 us-gaap_SalesRevenueGoodsNet / us-gaap_ProductOrServiceAxis = belfb_PowerSolutionsAndProtectionMember | 76,820 us-gaap_SalesRevenueGoodsNet / us-gaap_ProductOrServiceAxis = belfb_PowerSolutionsAndProtectionMember
United States [Member] |  |  |  |  |  |  |  |  |  |  |
Net Sales To External Customers By Geographic Area And Major Product Line [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consolidated net sales |  |  |  |  |  |  |  |  | 217,258 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = country_US | 116,548 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = country_US | 126,469 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = country_US
Long-lived Assets by Geographic Location [Abstract] |  |  |  |  |  |  |  |  |  |  |
Long-Lived Assets | 43,932 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_US |  |  |  | 30,102 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_US |  |  |  | 43,932 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_US | 30,102 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_US |
Macao [Member] |  |  |  |  |  |  |  |  |  |  |
Net Sales To External Customers By Geographic Area And Major Product Line [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consolidated net sales |  |  |  |  |  |  |  |  | 195,469 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = country_MO | 193,647 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = country_MO | 128,319 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = country_MO
China [Member] |  |  |  |  |  |  |  |  |  |  |
Long-lived Assets by Geographic Location [Abstract] |  |  |  |  |  |  |  |  |  |  |
Long-Lived Assets | 43,174 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_CN |  |  |  | 20,985 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_CN |  |  |  | 43,174 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_CN | 20,985 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_CN |
United Kingdom [Member] |  |  |  |  |  |  |  |  |  |  |
Net Sales To External Customers By Geographic Area And Major Product Line [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consolidated net sales |  |  |  |  |  |  |  |  | 22,852 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = country_GB | 16,538 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = country_GB | 13,203 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = country_GB
Long-lived Assets by Geographic Location [Abstract] |  |  |  |  |  |  |  |  |  |  |
Long-Lived Assets | 2,536 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_GB |  |  |  | 1,802 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_GB |  |  |  | 2,536 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_GB | 1,802 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_GB |
Germany [Member] |  |  |  |  |  |  |  |  |  |  |
Net Sales To External Customers By Geographic Area And Major Product Line [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consolidated net sales |  |  |  |  |  |  |  |  | 18,663 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = country_DE | 16,585 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = country_DE | 14,165 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = country_DE
Switzerland [Member] |  |  |  |  |  |  |  |  |  |  |
Net Sales To External Customers By Geographic Area And Major Product Line [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consolidated net sales |  |  |  |  |  |  |  |  | 15,236 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = country_CH | 0 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = country_CH | 0 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = country_CH
Long-lived Assets by Geographic Location [Abstract] |  |  |  |  |  |  |  |  |  |  |
Long-Lived Assets | 3,321 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_CH |  |  |  | 0 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_CH |  |  |  | 3,321 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_CH | 0 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_CH |
Slovakia [Member] |  |  |  |  |  |  |  |  |  |  |
Long-lived Assets by Geographic Location [Abstract] |  |  |  |  |  |  |  |  |  |  |
Long-Lived Assets | 9,675 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_SK |  |  |  | 0 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_SK |  |  |  | 9,675 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_SK | 0 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = country_SK |
North America [Member] | Reportable Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Net Sales To External Customers By Geographic Area And Major Product Line [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consolidated net sales |  |  |  |  |  |  |  |  | 248,007 us-gaap_SalesRevenueGoodsNet / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember | 128,472 us-gaap_SalesRevenueGoodsNet / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember | 138,966 us-gaap_SalesRevenueGoodsNet / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember
Net sales to major customer [Abstract] |  |  |  |  |  |  |  |  |  |  |
Revenue from major customers |  |  |  |  |  |  |  |  | 217,258 us-gaap_Revenues / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember | 116,548 us-gaap_Revenues / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember | 126,469 us-gaap_Revenues / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember
North America [Member] | Array [Member] | Reportable Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Net sales to major customer [Abstract] |  |  |  |  |  |  |  |  |  |  |
Revenue from major customers |  |  |  |  |  |  |  |  | 6,842 us-gaap_Revenues / us-gaap_BusinessAcquisitionAxis = belfb_ArrayMember / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember | 2,074 us-gaap_Revenues / us-gaap_BusinessAcquisitionAxis = belfb_ArrayMember / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember | 0 us-gaap_Revenues / us-gaap_BusinessAcquisitionAxis = belfb_ArrayMember / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember
Asia [Member] |  |  |  |  |  |  |  |  |  |  |
Long-lived Assets by Geographic Location [Abstract] |  |  |  |  |  |  |  |  |  |  |
Percentage of identifiable assets (in hundredths) | 35.10%belfb_PercentageOfIdentifiableAssets / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember |  |  |  |  |  |  |  | 35.10%belfb_PercentageOfIdentifiableAssets / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember |  |
Asia [Member] | Reportable Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Net Sales To External Customers By Geographic Area And Major Product Line [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consolidated net sales |  |  |  |  |  |  |  |  | 275,765 us-gaap_SalesRevenueGoodsNet / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember | 225,151 us-gaap_SalesRevenueGoodsNet / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember | 167,756 us-gaap_SalesRevenueGoodsNet / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember
Net sales to major customer [Abstract] |  |  |  |  |  |  |  |  |  |  |
Revenue from major customers |  |  |  |  |  |  |  |  | 201,338 us-gaap_Revenues / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember | 193,647 us-gaap_Revenues / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember | 128,319 us-gaap_Revenues / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember
Asia [Member] | TRP [Member] | Reportable Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Net sales to major customer [Abstract] |  |  |  |  |  |  |  |  |  |  |
Revenue from major customers |  |  |  |  |  |  |  |  | 66,082 us-gaap_Revenues / us-gaap_BusinessAcquisitionAxis = belfb_TrpMember / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember | 65,141 us-gaap_Revenues / us-gaap_BusinessAcquisitionAxis = belfb_TrpMember / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember | 0 us-gaap_Revenues / us-gaap_BusinessAcquisitionAxis = belfb_TrpMember / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember
Europe [Member] | Reportable Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Net Sales To External Customers By Geographic Area And Major Product Line [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consolidated net sales |  |  |  |  |  |  |  |  | 114,748 us-gaap_SalesRevenueGoodsNet / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember | 40,742 us-gaap_SalesRevenueGoodsNet / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember | 33,329 us-gaap_SalesRevenueGoodsNet / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember
Net sales to major customer [Abstract] |  |  |  |  |  |  |  |  |  |  |
Revenue from major customers |  |  |  |  |  |  |  |  | 68,480 us-gaap_Revenues / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember | 38,994 us-gaap_Revenues / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember | 31,806 us-gaap_Revenues / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember
Europe [Member] | TRP [Member] | Reportable Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Net sales to major customer [Abstract] |  |  |  |  |  |  |  |  |  |  |
Revenue from major customers |  |  |  |  |  |  |  |  | 2,498 us-gaap_Revenues / us-gaap_BusinessAcquisitionAxis = belfb_TrpMember / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember | 1,407 us-gaap_Revenues / us-gaap_BusinessAcquisitionAxis = belfb_TrpMember / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember | 0 us-gaap_Revenues / us-gaap_BusinessAcquisitionAxis = belfb_TrpMember / us-gaap_ConsolidationItemsAxis = us-gaap_ReportableGeographicalComponentsMember / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember
Intergeographic Revenues [Member] | Reportable Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Net Sales To External Customers By Geographic Area And Major Product Line [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consolidated 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
All Other Foreign Countries [Member] |  |  |  |  |  |  |  |  |  |  |
Net Sales To External Customers By Geographic Area And Major Product Line [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consolidated net sales |  |  |  |  |  |  |  |  | 17,598 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = belfb_AllOtherForeignCountriesMember | 5,871 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = belfb_AllOtherForeignCountriesMember | 4,438 us-gaap_SalesRevenueGoodsNet / us-gaap_StatementGeographicalAxis = belfb_AllOtherForeignCountriesMember
Long-lived Assets by Geographic Location [Abstract] |  |  |  |  |  |  |  |  |  |  |
Long-Lived Assets | $ 1,723 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = belfb_AllOtherForeignCountriesMember |  |  |  | $ 1,455 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = belfb_AllOtherForeignCountriesMember |  |  |  | $ 1,723 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = belfb_AllOtherForeignCountriesMember | $ 1,455 us-gaap_NoncurrentAssets / us-gaap_StatementGeographicalAxis = belfb_AllOtherForeignCountriesMember |

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