<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>156</ContextCount>
  <ElementCount>294</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>53</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>9</UnitCount>
  <MyReports>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>75000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>75010 - Statement - CONSOLIDATED BALANCE SHEETS (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_StatementCONSOLIDATEDBALANCESHEETSUNAUDITED</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS (UNAUDITED)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>75020 - Statement - CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_StatementCONSOLIDATEDBALANCESHEETSUNAUDITEDParenthetical</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>75030 - Statement - STATEMENTS OF CONSOLIDATED INCOME (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/StatementStatementsOfConsolidatedIncomeUnaudited</Role>
      <ShortName>STATEMENTS OF CONSOLIDATED INCOME (UNAUDITED)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>75040 - Statement - STATEMENTS OF CONSOLIDATED COMPREHENSIVE INCOME (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/StatementStatementsOfConsolidatedComprehensiveIncomeUnaudited</Role>
      <ShortName>STATEMENTS OF CONSOLIDATED COMPREHENSIVE INCOME (UNAUDITED)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>75050 - Statement - STATEMENTS OF CONSOLIDATED STOCKHOLDERS' EQUITY (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_StatementSTATEMENTSOFCONSOLIDATEDSTOCKHOLDERSEQUITYUNAUDITED</Role>
      <ShortName>STATEMENTS OF CONSOLIDATED STOCKHOLDERS' EQUITY (UNAUDITED)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>75060 - Statement - STATEMENTS OF CONSOLIDATED STOCKHOLDERS' EQUITY (UNAUDITED) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_StatementSTATEMENTSOFCONSOLIDATEDSTOCKHOLDERSEQUITYUNAUDITEDParenthetical</Role>
      <ShortName>STATEMENTS OF CONSOLIDATED STOCKHOLDERS' EQUITY (UNAUDITED) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>75070 - Statement - STATEMENTS OF CONSOLIDATED CASH FLOWS (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_StatementSTATEMENTSOFCONSOLIDATEDCASHFLOWSUNAUDITED</Role>
      <ShortName>STATEMENTS OF CONSOLIDATED CASH FLOWS (UNAUDITED)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>995410 - Disclosure - Pay vs Performance Disclosure</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/PvpDisclosure</Role>
      <ShortName>Pay vs Performance Disclosure</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>995445 - Disclosure - Insider Trading Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingArrangements</Role>
      <ShortName>Insider Trading Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>995455 - Disclosure - Basis of Presentation and Recent Accounting Pronouncements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureBasisOfPresentationAndRecentAccountingPronouncements</Role>
      <ShortName>Basis of Presentation and Recent Accounting Pronouncements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>995465 - Disclosure - Supplemental Balance Sheet Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureSupplementalBalanceSheetInformation</Role>
      <ShortName>Supplemental Balance Sheet Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>995475 - Disclosure - Employee Benefits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureEmployeeBenefits</Role>
      <ShortName>Employee Benefits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>995485 - Disclosure - Derivatives, Hedging Programs and Other Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDerivativesHedgingProgramsAndOtherFinancialInstruments</Role>
      <ShortName>Derivatives, Hedging Programs and Other Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>995495 - Disclosure - Debt and Credit Facility</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDebtAndCreditFacility</Role>
      <ShortName>Debt and Credit Facility</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>995505 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>995515 - Disclosure - Accumulated Other Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureAccumulatedOtherComprehensiveIncome1</Role>
      <ShortName>Accumulated Other Comprehensive Income</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>995525 - Disclosure - Other Expense, Net</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureOtherExpenseNet</Role>
      <ShortName>Other Expense, Net</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>995535 - Disclosure - Income Tax Matters</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureIncomeTaxMatters</Role>
      <ShortName>Income Tax Matters</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>995545 - Disclosure - Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureEarningsPerShare1</Role>
      <ShortName>Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>995555 - Disclosure - Supplemental Cash Flow Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureSupplementalCashFlowInformation</Role>
      <ShortName>Supplemental Cash Flow Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>995565 - Disclosure - Business, Product and Geographical Area Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureBusinessProductAndGeographicalAreaInformation</Role>
      <ShortName>Business, Product and Geographical Area Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>995575 - Disclosure - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureSubsequentEvents</Role>
      <ShortName>Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>995585 - Disclosure - Basis of Presentation and Recent Accounting Pronouncements (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureBasisOfPresentationAndRecentAccountingPronouncementsPolicies</Role>
      <ShortName>Basis of Presentation and Recent Accounting Pronouncements (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureBasisOfPresentationAndRecentAccountingPronouncements</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>995595 - Disclosure - Supplemental Balance Sheet Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureSupplementalBalanceSheetInformationTables</Role>
      <ShortName>Supplemental Balance Sheet Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureSupplementalBalanceSheetInformation</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>995605 - Disclosure - Employee Benefits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureEmployeeBenefitsTables</Role>
      <ShortName>Employee Benefits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureEmployeeBenefits</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>995615 - Disclosure - Derivatives, Hedging Programs and Other Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDerivativesHedgingProgramsAndOtherFinancialInstrumentsTables</Role>
      <ShortName>Derivatives, Hedging Programs and Other Financial Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDerivativesHedgingProgramsAndOtherFinancialInstruments</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>995625 - Disclosure - Debt and Credit Facility (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDebtAndCreditFacilityTables</Role>
      <ShortName>Debt and Credit Facility (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDebtAndCreditFacility</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>995635 - Disclosure - Accumulated Other Comprehensive Income (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureAccumulatedOtherComprehensiveIncomeTables</Role>
      <ShortName>Accumulated Other Comprehensive Income (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureAccumulatedOtherComprehensiveIncome1</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>995645 - Disclosure - Other Expense, Net (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureOtherExpenseNetTables</Role>
      <ShortName>Other Expense, Net (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureOtherExpenseNet</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>995655 - Disclosure - Income Tax Matters (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureIncomeTaxMattersTables</Role>
      <ShortName>Income Tax Matters (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureIncomeTaxMatters</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>995665 - Disclosure - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureEarningsPerShareTables</Role>
      <ShortName>Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureEarningsPerShare1</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995675 - Disclosure - Supplemental Cash Flow Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureSupplementalCashFlowInformationTables</Role>
      <ShortName>Supplemental Cash Flow Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureSupplementalCashFlowInformation</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>995685 - Disclosure - Business, Product and Geographical Area Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureBusinessProductAndGeographicalAreaInformationTables</Role>
      <ShortName>Business, Product and Geographical Area Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureBusinessProductAndGeographicalAreaInformation</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>995695 - Disclosure - Basis of Presentation and Recent Accounting Pronouncements - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureBasisOfPresentationAndRecentAccountingPronouncementsAdditionalInformationDetails</Role>
      <ShortName>Basis of Presentation and Recent Accounting Pronouncements - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>995705 - Disclosure - Supplemental Balance Sheet Information - Schedule of Supplemental Balance Sheet Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureSupplementalBalanceSheetInformationScheduleOfSupplementalBalanceSheetInformationDetails</Role>
      <ShortName>Supplemental Balance Sheet Information - Schedule of Supplemental Balance Sheet Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>995715 - Disclosure - Employee Benefits - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureEmployeeBenefitsAdditionalInformationDetails</Role>
      <ShortName>Employee Benefits - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>995725 - Disclosure - Employee Benefits - Summary of Total Expense Related to All Postretirement Benefit Plans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureEmployeeBenefitsSummaryOfTotalExpenseRelatedToAllPostretirementBenefitPlansDetails</Role>
      <ShortName>Employee Benefits - Summary of Total Expense Related to All Postretirement Benefit Plans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>995735 - Disclosure - Employee Benefits - Summary of Components of Net Periodic Postretirement Benefit Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureEmployeeBenefitsSummaryOfComponentsOfNetPeriodicPostretirementBenefitCostDetails</Role>
      <ShortName>Employee Benefits - Summary of Components of Net Periodic Postretirement Benefit Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>995745 - Disclosure - Derivatives, Hedging Programs and Other Financial Instruments - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDerivativesHedgingProgramsAndOtherFinancialInstrumentsAdditionalInformationDetails</Role>
      <ShortName>Derivatives, Hedging Programs and Other Financial Instruments - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>995755 - Disclosure - Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDerivativesHedgingProgramsAndOtherFinancialInstrumentsSummaryOfDerivativePositionsDetails</Role>
      <ShortName>Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>995765 - Disclosure - Derivatives, Hedging Programs and Other Financial Instruments - Summary of (Gain) Loss Associated with Derivative Contracts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureDerivativesHedgingProgramsAndOtherFinancialInstrumentsSummaryOfGainLossAssociatedWithDerivativeContractsDetails</Role>
      <ShortName>Derivatives, Hedging Programs and Other Financial Instruments - Summary of (Gain) Loss Associated with Derivative Contracts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDerivativesHedgingProgramsAndOtherFinancialInstrumentsTables</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>995775 - Disclosure - Derivatives, Hedging Programs and Other Financial Instruments - Schedule of Fair Value of Derivative Financial Assets and Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureDerivativesHedgingProgramsAndOtherFinancialInstrumentsScheduleOfFairValueOfDerivativeFinancialAssetsAndLiabilitiesDetails</Role>
      <ShortName>Derivatives, Hedging Programs and Other Financial Instruments - Schedule of Fair Value of Derivative Financial Assets and Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>995785 - Disclosure - Derivatives, Hedging Programs and Other Financial Instruments - Schedule of Total Amounts of Derivative Assets and Liabilities on Balance Sheets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDerivativesHedgingProgramsAndOtherFinancialInstrumentsScheduleOfTotalAmountsOfDerivativeAssetsAndLiabilitiesOnBalanceSheetsDetails</Role>
      <ShortName>Derivatives, Hedging Programs and Other Financial Instruments - Schedule of Total Amounts of Derivative Assets and Liabilities on Balance Sheets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>995795 - Disclosure - Debt and Credit Facility (Senior Notes) - Additional Information (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDebtAndCreditFacilitySeniorNotesAdditionalInformationDetails</Role>
      <ShortName>Debt and Credit Facility (Senior Notes) - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDebtAndCreditFacilityTables</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>995805 - Disclosure - Debt and Credit Facility - Summary of Senior Notes (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDebtAndCreditFacilitySummaryOfSeniorNotesDetails</Role>
      <ShortName>Debt and Credit Facility - Summary of Senior Notes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>995815 - Disclosure - Debt and Credit Facility - Summary of Senior Notes (Parenthetical) (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDebtAndCreditFacilitySummaryOfSeniorNotesParentheticalDetails</Role>
      <ShortName>Debt and Credit Facility - Summary of Senior Notes (Parenthetical) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>995825 - Disclosure - Debt and Credit Facility - Summary of Fair Value of Outstanding Senior Notes (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDebtAndCreditFacilitySummaryOfFairValueOfOutstandingSeniorNotesDetails</Role>
      <ShortName>Debt and Credit Facility - Summary of Fair Value of Outstanding Senior Notes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>995835 - Disclosure - Debt and Credit Facility (Revolving Credit Facility) - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureDebtAndCreditFacilityRevolvingCreditFacilityAdditionalInformationDetails</Role>
      <ShortName>Debt and Credit Facility (Revolving Credit Facility) - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDebtAndCreditFacilityTables</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>995845 - Disclosure - Debt and Credit Facility - Schedule of Availability and Usage of Revolving Credit Facility (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureDebtAndCreditFacilityScheduleOfAvailabilityAndUsageOfRevolvingCreditFacilityDetails</Role>
      <ShortName>Debt and Credit Facility - Schedule of Availability and Usage of Revolving Credit Facility (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>995855 - Disclosure - Debt and Credit Facility - Summary of Interest Expense Relating to Senior Notes and Revolving Credit Facility (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureDebtAndCreditFacilitySummaryOfInterestExpenseRelatingToSeniorNotesAndRevolvingCreditFacilityDetails</Role>
      <ShortName>Debt and Credit Facility - Summary of Interest Expense Relating to Senior Notes and Revolving Credit Facility (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>995865 - Disclosure - Commitments and Contingencies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureCommitmentsAndContingenciesAdditionalInformationDetails</Role>
      <ShortName>Commitments and Contingencies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>995875 - Disclosure - Accumulated Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureAccumulatedOtherComprehensiveIncomeScheduleOfAccumulatedOtherComprehensiveIncomeDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>995885 - Disclosure - Accumulated Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income (Parenthetical) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureAccumulatedOtherComprehensiveIncomeScheduleOfAccumulatedOtherComprehensiveIncomeParentheticalDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income (Parenthetical) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>995895 - Disclosure - Other Expense, Net - Components of Other Expense, Net (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureOtherExpenseNetComponentsOfOtherExpenseNetDetails</Role>
      <ShortName>Other Expense, Net - Components of Other Expense, Net (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>995905 - Disclosure - Other Expense, Net - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureOtherExpenseNetAdditionalInformationDetails</Role>
      <ShortName>Other Expense, Net - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>995915 - Disclosure - Income Tax Matters - Income Tax Provision by Region (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureIncomeTaxMattersIncomeTaxProvisionByRegionDetails</Role>
      <ShortName>Income Tax Matters - Income Tax Provision by Region (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>995925 - Disclosure - Income Tax Matters - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureIncomeTaxMattersAdditionalInformationDetails</Role>
      <ShortName>Income Tax Matters - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>995935 - Disclosure - Earnings Per Share - Schedule of Computation of Basic and Diluted Net Income Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureEarningsPerShareScheduleOfComputationOfBasicAndDilutedNetIncomePerShareDetails</Role>
      <ShortName>Earnings Per Share - Schedule of Computation of Basic and Diluted Net Income Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>995945 - Disclosure - Earnings Per Share - Schedule of Computation of Basic and Diluted Net Income Per Share (Parenthetical) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureEarningsPerShareScheduleOfComputationOfBasicAndDilutedNetIncomePerShareParentheticalDetails</Role>
      <ShortName>Earnings Per Share - Schedule of Computation of Basic and Diluted Net Income Per Share (Parenthetical) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>995955 - Disclosure - Supplemental Cash Flow Information - Schedule of Supplemental Cash Flow Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureSupplementalCashFlowInformationScheduleOfSupplementalCashFlowInformationDetails</Role>
      <ShortName>Supplemental Cash Flow Information - Schedule of Supplemental Cash Flow Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>995965 - Disclosure - Business, Product, and Geographical Area Information - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureBusinessProductAndGeographicalAreaInformationAdditionalInformationDetails</Role>
      <ShortName>Business, Product, and Geographical Area Information - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>995975 - Disclosure - Business, Product, and Geographical Area Information - Schedule of Significant Segment Expenses Provided to CODM (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureBusinessProductAndGeographicalAreaInformationScheduleOfSignificantSegmentExpensesProvidedToCodmDetails</Role>
      <ShortName>Business, Product, and Geographical Area Information - Schedule of Significant Segment Expenses Provided to CODM (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>995985 - Disclosure - Business, Product, and Geographical Area Information - Schedule of Significant Segment Expenses Provided to CODM (Parenthetical) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureBusinessProductAndGeographicalAreaInformationScheduleOfSignificantSegmentExpensesProvidedToCodmParentheticalDetails</Role>
      <ShortName>Business, Product, and Geographical Area Information - Schedule of Significant Segment Expenses Provided to CODM (Parenthetical) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>995995 - Disclosure - Business, Product and Geographical Area Information - Schedule of Net Sales by End Market Segment Applications (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/DisclosureBusinessProductAndGeographicalAreaInformationScheduleOfNetSalesByEndMarketSegmentApplicationsDetails</Role>
      <ShortName>Business, Product and Geographical Area Information - Schedule of Net Sales by End Market Segment Applications (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>996005 - Disclosure - Business, Product and Geographical Area Information - Schedule of Income Taxes Paid by Geographical Area (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureBusinessProductAndGeographicalAreaInformationScheduleOfIncomeTaxesPaidByGeographicalAreaDetails</Role>
      <ShortName>Business, Product and Geographical Area Information - Schedule of Income Taxes Paid by Geographical Area (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="kalu-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>996015 - Disclosure - Subsequent Events - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.kaiseraluminum.com/20260331/taxonomy/role/Role_DisclosureSubsequentEventsAdditionalInformationDetails</Role>
      <ShortName>Subsequent Events - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-Q" isDefinitelyFs="true" isUsgaap="true" original="kalu-20260331.htm">kalu-20260331.htm</File>
    <File>kalu-20260331.xsd</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="1">http://fasb.org/srt/2025</BaseTaxonomy>
    <BaseTaxonomy items="656">http://fasb.org/us-gaap/2025</BaseTaxonomy>
    <BaseTaxonomy items="30">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2025</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>false</HasPresentationLinkbase>
  <HasCalculationLinkbase>false</HasCalculationLinkbase>
</FilingSummary>