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Assumptions Used to Determine Postretirement Cost (Detail)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.10%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate 4.80%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate  
Discount rate 4.80%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 4.10%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 4.80%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate