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Pension and Other Postretirement Benefits - Fair Values of Pension Plan Assets by Asset Category (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	$ 216,754us-gaap_DefinedBenefitPlanFairValueOfPlanAssets	$ 198,367us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Large-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	82,816us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = bdc_LargeCapMember	75,306us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = bdc_LargeCapMember
Mid-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	15,276us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = bdc_MidCapMember	13,511us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = bdc_MidCapMember
Small-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	19,952us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = bdc_SmallCapMember	19,473us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = bdc_SmallCapMember
Government Bond Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	29,121us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember	25,520us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Corporate Bond Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	27,485us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CorporateDebtSecuritiesMember	21,679us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CorporateDebtSecuritiesMember
Fixed Income Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	41,975us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeFundsMember	42,847us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeFundsMember
Cash & Equivalents [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	129us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember	31us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	10,362us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member	31us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Large-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	3,414us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = bdc_LargeCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Mid-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	1,448us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = bdc_MidCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Small-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	312us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = bdc_SmallCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Government Bond Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	1,244us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Corporate Bond Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	3,815us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fixed Income Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Cash & Equivalents [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	129us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member	31us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Significant Observable Inputs (Level 2) [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	206,392us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	198,336us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Significant Observable Inputs (Level 2) [Member] | Large-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	79,402us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = bdc_LargeCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	75,306us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = bdc_LargeCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Significant Observable Inputs (Level 2) [Member] | Mid-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	13,828us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = bdc_MidCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	13,511us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = bdc_MidCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Significant Observable Inputs (Level 2) [Member] | Small-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	19,640us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = bdc_SmallCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	19,473us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = bdc_SmallCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Significant Observable Inputs (Level 2) [Member] | Government Bond Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	27,877us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	25,520us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Significant Observable Inputs (Level 2) [Member] | Corporate Bond Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	23,670us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	21,679us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Significant Observable Inputs (Level 2) [Member] | Fixed Income Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	41,975us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	42,847us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Significant Observable Inputs (Level 2) [Member] | Cash & Equivalents [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Significant Unobservable Inputs (Level 3) [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Significant Unobservable Inputs (Level 3) [Member] | Large-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Significant Unobservable Inputs (Level 3) [Member] | Mid-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Significant Unobservable Inputs (Level 3) [Member] | Small-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Significant Unobservable Inputs (Level 3) [Member] | Government Bond Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Significant Unobservable Inputs (Level 3) [Member] | Corporate Bond Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Significant Unobservable Inputs (Level 3) [Member] | Fixed Income Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Significant Unobservable Inputs (Level 3) [Member] | Cash & Equivalents [Member]
Pension And Other Employee Benefit Plans [Line Items]
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- Definition

Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 30

-Section 35

-Paragraph 50

-URI http://asc.fasb.org/extlink&oid=29635418&loc=d3e12355-114930

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