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Employee Benefits, Defined Benefit Plans (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |
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Mar. 31, 2015 | Dec. 31, 2014
VEBA Postretirement Medical Obligations |  |
Accrued annual contribution to VEBAs | $ 0 kalu_AccruedAnnualVebaContribution | $ 13.7 kalu_AccruedAnnualVebaContribution
VEBAs |  |
VEBA Postretirement Medical Obligations |  |
Cash flow in determining VEBA obligation | 20.0 kalu_CashFlowInDetermingVebaObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = kalu_VebasMember |
VEBAs | Maximum |  |
VEBA Postretirement Medical Obligations |  |
Administrative expenses of the VEBAs | 0.3 kalu_AdministrativeExpenseObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = kalu_VebasMember / us-gaap_RangeAxis = us-gaap_MaximumMember |
Union VEBA |  |
VEBA Postretirement Medical Obligations |  |
Percent allocation of total contribution between VEBAs | 85.50%kalu_PercentAllocationOfTotalContributionBetweenVebas / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = kalu_UnionVebaMember |
Accrued Veba contingent contribution - total | 45.4 kalu_AccruedVebacontingentcontributiontotal / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = kalu_UnionVebaMember |
Defined Benefit Plan, Benefit Obligation |  | 391.5 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = kalu_UnionVebaMember
Defined Benefit Plan, Fair Value of Plan Assets |  | 731.6 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = kalu_UnionVebaMember
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments | 307.8 kalu_LossonsettlementofUnionVEBAnetoftax / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = kalu_UnionVebaMember |
Income Tax Expense (Benefit) | 184.4 kalu_IncometaxbenefitonremovalofUnionVEBAnetassets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = kalu_UnionVebaMember |
Salaried VEBA |  |
VEBA Postretirement Medical Obligations |  |
Percent allocation of total contribution between VEBAs | 14.50%kalu_PercentAllocationOfTotalContributionBetweenVebas / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = kalu_SalariedVebaMember |
Variable cash contribution | VEBAs |  |
VEBA Postretirement Medical Obligations |  |
Accrued annual contribution to VEBAs |  | 13.7 kalu_AccruedAnnualVebaContribution / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = kalu_VebasMember / kalu_TypesOfContributionsAxis = kalu_VariableCashContributionMember
Variable cash contribution | VEBAs | Maximum |  |
VEBA Postretirement Medical Obligations |  |
Variable cash contribution obligation to VEBAs | 20.0 kalu_PostretirementMedicalPlanContributionObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = kalu_VebasMember / us-gaap_RangeAxis = us-gaap_MaximumMember / kalu_TypesOfContributionsAxis = kalu_VariableCashContributionMember |
Variable cash contribution | VEBAs | Annual Cash Flows up to $20 Million |  |
VEBA Postretirement Medical Obligations |  |
Postretirement medical plan contribution obligation percentage | 10.00%kalu_PostretirementMedicalPlanContributionObligationPercentage / kalu_ContributionPercentageDeterminationAxis = kalu_AnnualCashFlowsUpToTwentyMillionDollarsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = kalu_VebasMember / kalu_TypesOfContributionsAxis = kalu_VariableCashContributionMember |
Variable cash contribution | VEBAs | Annual Cash Flows in Excess of $20 Million |  |
VEBA Postretirement Medical Obligations |  |
Postretirement medical plan contribution obligation percentage | 20.00%kalu_PostretirementMedicalPlanContributionObligationPercentage / kalu_ContributionPercentageDeterminationAxis = kalu_AnnualCashFlowsInExcessOfTwentyMillionDollarsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = kalu_VebasMember / kalu_TypesOfContributionsAxis = kalu_VariableCashContributionMember |
Variable cash contribution | Union VEBA |  |
VEBA Postretirement Medical Obligations |  |
Accrued annual contribution to VEBAs |  | 11.7 kalu_AccruedAnnualVebaContribution / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = kalu_UnionVebaMember / kalu_TypesOfContributionsAxis = kalu_VariableCashContributionMember
Variable cash contribution | Salaried VEBA |  |
VEBA Postretirement Medical Obligations |  |
Accrued annual contribution to VEBAs |  | $ 2.0 kalu_AccruedAnnualVebaContribution / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = kalu_SalariedVebaMember / kalu_TypesOfContributionsAxis = kalu_VariableCashContributionMember

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- Definition Accrued Veba contingent contribution - total + References No definition available. + Details
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- Definition Administrative Expense Obligation + References No definition available. + Details
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- Definition Cash flow in determining VEBA obligation + References No definition available. + Details
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- Definition Income tax benefit on removal of Union VEBA net assets + References No definition available. + Details
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- Definition Loss on settlement of Union VEBA - net of tax + References No definition available. + Details
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- Definition Percent allocation of total contribution between VEBAs + References No definition available. + Details
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- Definition Postretirement medical plan contribution obligation. + References No definition available. + Details
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- Definition 1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Projected Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6522206 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accumulated Postretirement Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6503904 + Details
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- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=49178875&loc=d3e12355-114930 + Details
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