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Segment and Geographical Area Information Sales and Supply Information (Details)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Export sales | Net sales      
Concentration Risk      
Concentration risk, percentage 19.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_GeographicConcentrationRiskMember		 17.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_GeographicConcentrationRiskMember		 18.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_GeographicConcentrationRiskMember
Supplier concentration risk | Aluminum | Top five major suppliers      
Concentration Risk      
Concentration risk, percentage 71.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= kalu_AluminumMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_SupplierConcentrationRiskMember
/ kalu_SuppliersAxis
= kalu_TopFiveMajorSuppliersMember		 86.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= kalu_AluminumMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_SupplierConcentrationRiskMember
/ kalu_SuppliersAxis
= kalu_TopFiveMajorSuppliersMember		 78.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= kalu_AluminumMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_SupplierConcentrationRiskMember
/ kalu_SuppliersAxis
= kalu_TopFiveMajorSuppliersMember
Supplier concentration risk | Aluminum | Largest supplier      
Concentration Risk      
Concentration risk, percentage 30.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= kalu_AluminumMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_SupplierConcentrationRiskMember
/ kalu_SuppliersAxis
= kalu_LargestSupplierMember		 25.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= kalu_AluminumMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_SupplierConcentrationRiskMember
/ kalu_SuppliersAxis
= kalu_LargestSupplierMember		 29.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= kalu_AluminumMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_SupplierConcentrationRiskMember
/ kalu_SuppliersAxis
= kalu_LargestSupplierMember
Supplier concentration risk | Aluminum | Second and third largest suppliers      
Concentration Risk      
Concentration risk, percentage 25.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= kalu_AluminumMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_SupplierConcentrationRiskMember
/ kalu_SuppliersAxis
= kalu_SecondAndThirdLargestSuppliersMember		 35.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= kalu_AluminumMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_SupplierConcentrationRiskMember
/ kalu_SuppliersAxis
= kalu_SecondAndThirdLargestSuppliersMember		 31.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= kalu_AluminumMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_SupplierConcentrationRiskMember
/ kalu_SuppliersAxis
= kalu_SecondAndThirdLargestSuppliersMember