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Income Tax Matters, Effective Tax Rate Reconciliation (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Reconciliation between income tax provision and statutory income tax provision:      
Amount of federal income tax provision based on the statutory rate $ (37.5)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate $ (50.1)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate $ (48.9)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
Decrease in federal valuation allowances 0us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance 0.1us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance 0.1us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance
Non-deductible compensation expense (0.1)us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost (0.3)us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost (0.4)us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost
Non-deductible expense (0.3)us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther (0.9)us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther (0.3)us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther
State income taxes, net of federal benefit 0us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes [1] (4.4)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes [1] (3.8)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes [1]
Decrease in state tax rates 1.6kalu_Changeinstatetaxrates    
Foreign income tax (expense) benefit 0.3us-gaap_IncomeTaxReconciliationForeignIncomeTaxRateDifferential 0us-gaap_IncomeTaxReconciliationForeignIncomeTaxRateDifferential (0.5)us-gaap_IncomeTaxReconciliationForeignIncomeTaxRateDifferential
Expiration of statute of limitations 2.3kalu_Incometaxreconciliationlapseofstatuteoflimitations 4.6kalu_Incometaxreconciliationlapseofstatuteoflimitations 0kalu_Incometaxreconciliationlapseofstatuteoflimitations
Settlement with taxing authorities 0us-gaap_IncomeTaxReconciliationTaxSettlementsForeign 4.4us-gaap_IncomeTaxReconciliationTaxSettlementsForeign 0us-gaap_IncomeTaxReconciliationTaxSettlementsForeign
Advance pricing agreement 0us-gaap_IncomeTaxReconciliationOtherAdjustments 2.9us-gaap_IncomeTaxReconciliationOtherAdjustments 0us-gaap_IncomeTaxReconciliationOtherAdjustments
Competent Authority settlement 0us-gaap_IncomeTaxReconciliationTaxSettlementsOther 5.3us-gaap_IncomeTaxReconciliationTaxSettlementsOther 0us-gaap_IncomeTaxReconciliationTaxSettlementsOther
Income tax provision (35.3)us-gaap_IncomeTaxExpenseBenefit (38.4)us-gaap_IncomeTaxExpenseBenefit (53.8)us-gaap_IncomeTaxExpenseBenefit
Valuation Allowance      
Current State and Local Tax Expense (Benefit) 2.3kalu_Stateincometaxprovisionbenefitbeforeratereconciliationadjustments [1]    
Deferred Tax Assets, Net of Valuation Allowance, Current 86.4us-gaap_DeferredTaxAssetsNetCurrent 36.7us-gaap_DeferredTaxAssetsNetCurrent  
Increase (Decrease) in valuation allowance 0.7us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount (1.2)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount 0.1us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
Unused state net operating losses      
Valuation Allowance      
Increase (Decrease) in valuation allowance   $ (1.2)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
/ us-gaap_ValuationAllowanceByDeferredTaxAssetAxis
= kalu_UnusedStateNetOperatingLossMember		  
[1] State income taxes were $2.3 in 2014, but were offset by a $1.6 decrease due to lower tax rates in various states and a $0.7 decrease in the valuation allowance relating to certain state net operating losses. State income taxes of $4.4 in 2013 included a $1.2 increase in the valuation allowance relating to certain unused state net operating losses expected to expire.