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Employee Incentive Plans, Short Term Incentive Plans (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Compensation charges associated with STI Plans    
Costs recorded in connection with STI plans $ 3.3kalu_ShortTermIncentiveCompensationExpense $ 2.4kalu_ShortTermIncentiveCompensationExpense
Fabricated Products    
Compensation charges associated with STI Plans    
Costs recorded in connection with STI plans 2.4kalu_ShortTermIncentiveCompensationExpense
/ us-gaap_StatementBusinessSegmentsAxis
= kalu_FabricatedProductsMember		 1.9kalu_ShortTermIncentiveCompensationExpense
/ us-gaap_StatementBusinessSegmentsAxis
= kalu_FabricatedProductsMember
All Other    
Compensation charges associated with STI Plans    
Costs recorded in connection with STI plans 0.9kalu_ShortTermIncentiveCompensationExpense
/ us-gaap_StatementBusinessSegmentsAxis
= kalu_AllOtherMember		 0.5kalu_ShortTermIncentiveCompensationExpense
/ us-gaap_StatementBusinessSegmentsAxis
= kalu_AllOtherMember
Cost of products sold, excluding depreciation and amortization and other items    
Compensation charges associated with STI Plans    
Costs recorded in connection with STI plans 0.8kalu_ShortTermIncentiveCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember		 1.1kalu_ShortTermIncentiveCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
Selling, general, administrative, research and development    
Compensation charges associated with STI Plans    
Costs recorded in connection with STI plans $ 2.5kalu_ShortTermIncentiveCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= kalu_SellingAdministrativeResearchAndDevelopmentAndGeneralExpensesMember		 $ 1.3kalu_ShortTermIncentiveCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= kalu_SellingAdministrativeResearchAndDevelopmentAndGeneralExpensesMember