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Employee Benefits - Summary of Total Expense Related to All Postretirement Benefit Plans (Parenthetical) (Details) $ in Millions | 6 Months Ended
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Sep. 30, 2024 USD ($)
2024 Plan |
Defined Benefit Plan Disclosure [Line Items] |
Multiemployer pension plans excludes charge to restructuring costs | $ 4.6

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- Definition Restructuring cost, pension obligation expected + References No definition available. + Details
Name: | kalu_RestructuringCostPensionObligationExpected
Namespace Prefix: | kalu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_RestructuringPlanAxis=kalu_TwoThousandAndTwentyFourRestructuringPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |