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Derivative Financial Instruments and Related Hedging Programs, Notional Quantity Table (Details)
Mar. 31, 2015
Remainder of 2015  
Summary of material derivative positions  
Percentage of natural gas purchases for which the Company's exposure to fluctuations in gas prices had been substantially reduced 79.00%kalu_PercentageOfNaturalGasPurchasesForWhichCompanysExposureToFluctuationsInGasPricesHaveBeenReduced
/ kalu_DatePeriodAxis
= kalu_RemainderOfCurrentYearMember
Percentage of Electricity Purchases For Which Company Exposure To Fluctuations In Electricity Prices Have Been Reduced 55.00%kalu_PercentageofElectricityPurchasesForWhichCompanyExposureToFluctuationsInElectricityPricesHaveBeenReduced
/ kalu_DatePeriodAxis
= kalu_RemainderOfCurrentYearMember
2016  
Summary of material derivative positions  
Percentage of natural gas purchases for which the Company's exposure to fluctuations in gas prices had been substantially reduced 77.00%kalu_PercentageOfNaturalGasPurchasesForWhichCompanysExposureToFluctuationsInGasPricesHaveBeenReduced
/ kalu_DatePeriodAxis
= kalu_YearOneMember
Percentage of Electricity Purchases For Which Company Exposure To Fluctuations In Electricity Prices Have Been Reduced 54.00%kalu_PercentageofElectricityPurchasesForWhichCompanyExposureToFluctuationsInElectricityPricesHaveBeenReduced
/ kalu_DatePeriodAxis
= kalu_YearOneMember
2017  
Summary of material derivative positions  
Percentage of natural gas purchases for which the Company's exposure to fluctuations in gas prices had been substantially reduced 29.00%kalu_PercentageOfNaturalGasPurchasesForWhichCompanysExposureToFluctuationsInGasPricesHaveBeenReduced
/ kalu_DatePeriodAxis
= kalu_YearTwoMember
Percentage of Electricity Purchases For Which Company Exposure To Fluctuations In Electricity Prices Have Been Reduced 27.00%kalu_PercentageofElectricityPurchasesForWhichCompanyExposureToFluctuationsInElectricityPricesHaveBeenReduced
/ kalu_DatePeriodAxis
= kalu_YearTwoMember
Not Designated as Hedging Instrument | Aluminum | Fixed priced contracts | Purchase  
Summary of material derivative positions  
Notional amount of contracts 82.5invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kalu_AluminumMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_PositionAxis
= us-gaap_LongMember
Not Designated as Hedging Instrument | Aluminum | Fixed priced contracts | Sales  
Summary of material derivative positions  
Notional amount of contracts 0.2invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kalu_AluminumMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_PositionAxis
= us-gaap_ShortMember
Not Designated as Hedging Instrument | Aluminum | Midwest premium swap contracts | Purchase  
Summary of material derivative positions  
Notional amount of contracts 82.1invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= kalu_MidwestPremiumSwapContractsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kalu_AluminumMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_PositionAxis
= us-gaap_LongMember
Not Designated as Hedging Instrument | Natural Gas | Fixed priced contracts | Purchase  
Summary of material derivative positions  
Notional amount of contracts 6,700,000invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kalu_NaturalGasMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_PositionAxis
= us-gaap_LongMember
Not Designated as Hedging Instrument | Electricity | Fixed priced contracts | Purchase  
Summary of material derivative positions  
Notional amount of contracts 132,020invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kalu_ElectricityMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_PositionAxis
= us-gaap_LongMember
Not Designated as Hedging Instrument | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature  
Summary of material derivative positions  
Notional amount of contracts 3,668,449invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_EmbeddedDerivativeFinancialInstrumentsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kalu_HedgesRelatingToNotesMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Not Designated as Hedging Instrument | Hedges Relating to the Convertible Notes | Call Options  
Summary of material derivative positions  
Notional amount of contracts 3,668,449invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_CallOptionMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kalu_HedgesRelatingToNotesMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember