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Post-Employment and Other Long-Term Employees Benefits - Weighted Average Assumptions Used in Determination of Benefit Obligation and Plan Assets (Detail)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Compensation and Retirement Disclosure [Abstract]      
Discount rate 3.03%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate 3.83%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate  
Salary increase rate 2.65%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease 2.82%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease  
Expected long-term rate of return on funds for the pension expense of the year 4.76%stm_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationExpectedLongTermReturnOnAsset 4.88%stm_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationExpectedLongTermReturnOnAsset  
Net periodic benefit cost, Discount rate 3.83%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 3.43%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 4.14%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
Net periodic benefit cost, Salary increase rate 2.82%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease 2.92%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease 2.99%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
Net periodic benefit cost, Expected long-term rate of return on funds for the pension expense of the year 4.88%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets 4.43%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets 4.57%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets