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Summary of Significant Accounting Policies, Textuals (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Significant accounting policies    
Percentage of Employees Covered by Collective Bargaining Agreements 100.00%kalu_PercentageofEmployeesCoveredbyCollectiveBargainingAgreements  
Percentage of Employees Covered by Collective Bargaining Agreements Expiring Within One Year 0.00%kalu_PercentageofEmployeesCoveredbyCollectiveBargainingAgreementsExpiringWithinOneYear  
Fabricated Products    
Significant accounting policies    
Excess of current cost over the stated LIFO value of inventory $ 24.7us-gaap_InventoryLIFOReserve
/ us-gaap_StatementBusinessSegmentsAxis
= kalu_FabricatedProductsMember		 $ 37.6us-gaap_InventoryLIFOReserve
/ us-gaap_StatementBusinessSegmentsAxis
= kalu_FabricatedProductsMember