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Employee Benefit Plans - Components of Net Periodic Benefit Cost (Credit) (Parenthetical) (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]      
Tax benefit in plan assets and benefits obligations recognition $ 7,119laz_TaxChargeBenefitOnPlanAssetsAndBenefitObligation $ 4,459laz_TaxChargeBenefitOnPlanAssetsAndBenefitObligation $ 11,805laz_TaxChargeBenefitOnPlanAssetsAndBenefitObligation
Recognition of actuarial losses     2,167laz_DefinedBenefitPlanActuarialNetGainsLossesRecognized
Settlement loss     1,135laz_RecognitionOfSettlementLossesChargedToCompensationAndBenefitsExpense
Settlement charges     $ 1,032laz_NetChargeToOtherComprehensiveIncomeDueToRemeasurement