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Segment Information, Supply Information (Details) (Supplier Concentration Risk, Aluminum)
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Top five major suppliers
    
Concentration Risk    
Concentration Risk, Percentage 75.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= kalu_AluminumMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_SupplierConcentrationRiskMember
/ kalu_SuppliersAxis
= kalu_TopFiveMajorSuppliersMember		 74.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= kalu_AluminumMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_SupplierConcentrationRiskMember
/ kalu_SuppliersAxis
= kalu_TopFiveMajorSuppliersMember
Largest supplier
    
Concentration Risk    
Concentration Risk, Percentage 29.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= kalu_AluminumMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_SupplierConcentrationRiskMember
/ kalu_SuppliersAxis
= kalu_LargestSupplierMember		 30.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= kalu_AluminumMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_SupplierConcentrationRiskMember
/ kalu_SuppliersAxis
= kalu_LargestSupplierMember
Second and third largest suppliers
    
Concentration Risk    
Concentration Risk, Percentage 29.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= kalu_AluminumMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_SupplierConcentrationRiskMember
/ kalu_SuppliersAxis
= kalu_SecondAndThirdLargestSuppliersMember		 25.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= kalu_AluminumMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_SupplierConcentrationRiskMember
/ kalu_SuppliersAxis
= kalu_SecondAndThirdLargestSuppliersMember