<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>1011</ContextCount>
  <ElementCount>495</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>161</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>6</UnitCount>
  <MyReports>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0000001 - Document - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/CoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>9952151 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>9952152 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>9952153 - Statement - Consolidated Statements of Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ConsolidatedStatementsofIncome</Role>
      <ShortName>Consolidated Statements of Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>9952154 - Statement - Consolidated Statements of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ConsolidatedStatementsofComprehensiveIncome</Role>
      <ShortName>Consolidated Statements of Comprehensive Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>9952155 - Statement - Consolidated Statements of Comprehensive Income (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ConsolidatedStatementsofComprehensiveIncomeParenthetical</Role>
      <ShortName>Consolidated Statements of Comprehensive Income (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>9952156 - Statement - Consolidated Statements of Changes in Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ConsolidatedStatementsofChangesinEquity</Role>
      <ShortName>Consolidated Statements of Changes in Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>9952157 - Statement - Consolidated Statements of Changes in Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ConsolidatedStatementsofChangesinEquityParenthetical</Role>
      <ShortName>Consolidated Statements of Changes in Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>9952158 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ConsolidatedStatementsofCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>9952159 - Disclosure - Basis of Presentation and Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/BasisofPresentationandAccountingPolicies</Role>
      <ShortName>Basis of Presentation and Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>9952160 - Disclosure - Earnings Per Common Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/EarningsPerCommonShare</Role>
      <ShortName>Earnings Per Common Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>9952161 - Disclosure - Loan Portfolio</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/LoanPortfolio</Role>
      <ShortName>Loan Portfolio</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>9952162 - Disclosure - Asset Quality</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AssetQuality</Role>
      <ShortName>Asset Quality</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>9952163 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/FairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>9952164 - Disclosure - Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/Securities</Role>
      <ShortName>Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>9952165 - Disclosure - Derivatives and Hedging Activities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/DerivativesandHedgingActivities</Role>
      <ShortName>Derivatives and Hedging Activities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>9952166 - Disclosure - Mortgage Servicing Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/MortgageServicingAssets</Role>
      <ShortName>Mortgage Servicing Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>9952167 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/Leases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>9952168 - Disclosure - Variable Interest Entities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/VariableInterestEntities</Role>
      <ShortName>Variable Interest Entities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>9952169 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>9952170 - Disclosure - Discontinued Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/DiscontinuedOperations</Role>
      <ShortName>Discontinued Operations</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>9952171 - Disclosure - Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/Borrowings</Role>
      <ShortName>Borrowings</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>9952172 - Disclosure - Contingent Liabilities and Guarantees</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ContingentLiabilitiesandGuarantees</Role>
      <ShortName>Contingent Liabilities and Guarantees</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>9952173 - Disclosure - Accumulated Other Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AccumulatedOtherComprehensiveIncome</Role>
      <ShortName>Accumulated Other Comprehensive Income</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>9952174 - Disclosure - Shareholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ShareholdersEquity</Role>
      <ShortName>Shareholders' Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>9952175 - Disclosure - Business Segment Reporting</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/BusinessSegmentReporting</Role>
      <ShortName>Business Segment Reporting</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>9952176 - Disclosure - Revenue from Contracts with Customers</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/RevenuefromContractswithCustomers</Role>
      <ShortName>Revenue from Contracts with Customers</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.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>28</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>9955511 - Disclosure - Basis of Presentation and Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/BasisofPresentationandAccountingPoliciesPolicies</Role>
      <ShortName>Basis of Presentation and Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.key.com/role/BasisofPresentationandAccountingPolicies</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>9955512 - Disclosure - Earnings Per Common Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/EarningsPerCommonShareTables</Role>
      <ShortName>Earnings Per Common Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.key.com/role/EarningsPerCommonShare</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>9955513 - Disclosure - Loan Portfolio (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/LoanPortfolioTables</Role>
      <ShortName>Loan Portfolio (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.key.com/role/LoanPortfolio</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>9955514 - Disclosure - Asset Quality (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AssetQualityTables</Role>
      <ShortName>Asset Quality (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.key.com/role/AssetQuality</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>9955515 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/FairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.key.com/role/FairValueMeasurements</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>9955516 - Disclosure - Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/SecuritiesTables</Role>
      <ShortName>Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.key.com/role/Securities</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9955517 - Disclosure - Derivatives and Hedging Activities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/DerivativesandHedgingActivitiesTables</Role>
      <ShortName>Derivatives and Hedging Activities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.key.com/role/DerivativesandHedgingActivities</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9955518 - Disclosure - Mortgage Servicing Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/MortgageServicingAssetsTables</Role>
      <ShortName>Mortgage Servicing Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.key.com/role/MortgageServicingAssets</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9955519 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/LeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.key.com/role/Leases</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9955520 - Disclosure - Variable Interest Entities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/VariableInterestEntitiesTables</Role>
      <ShortName>Variable Interest Entities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.key.com/role/VariableInterestEntities</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9955521 - Disclosure - Borrowings (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/BorrowingsTables</Role>
      <ShortName>Borrowings (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.key.com/role/Borrowings</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9955522 - Disclosure - Contingent Liabilities and Guarantees (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ContingentLiabilitiesandGuaranteesTables</Role>
      <ShortName>Contingent Liabilities and Guarantees (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.key.com/role/ContingentLiabilitiesandGuarantees</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9955523 - Disclosure - Accumulated Other Comprehensive Income (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AccumulatedOtherComprehensiveIncomeTables</Role>
      <ShortName>Accumulated Other Comprehensive Income (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.key.com/role/AccumulatedOtherComprehensiveIncome</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9955524 - Disclosure - Shareholders' Equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ShareholdersEquityTables</Role>
      <ShortName>Shareholders' Equity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.key.com/role/ShareholdersEquity</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9955525 - Disclosure - Business Segment Reporting (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/BusinessSegmentReportingTables</Role>
      <ShortName>Business Segment Reporting (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.key.com/role/BusinessSegmentReporting</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9955526 - Disclosure - Revenue from Contracts with Customers (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/RevenuefromContractswithCustomersTables</Role>
      <ShortName>Revenue from Contracts with Customers (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.key.com/role/RevenuefromContractswithCustomers</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9955527 - Disclosure - Earnings Per Common Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/EarningsPerCommonShareDetails</Role>
      <ShortName>Earnings Per Common Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.key.com/role/EarningsPerCommonShareTables</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9955528 - Disclosure - Loan Portfolio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/LoanPortfolioDetails</Role>
      <ShortName>Loan Portfolio (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.key.com/role/LoanPortfolioTables</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9955529 - Disclosure - Loan Portfolio - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/LoanPortfolioNarrativeDetails</Role>
      <ShortName>Loan Portfolio - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9955530 - Disclosure - Asset Quality - Schedule of Changes in Allowance for Loan and Lease Losses by Loan Category (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AssetQualityScheduleofChangesinAllowanceforLoanandLeaseLossesbyLoanCategoryDetails</Role>
      <ShortName>Asset Quality - Schedule of Changes in Allowance for Loan and Lease Losses by Loan Category (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>9955531 - Disclosure - Asset Quality - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AssetQualityNarrativeDetails</Role>
      <ShortName>Asset Quality - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>9955532 - Disclosure - Asset Quality - Schedule of Commercial Credit Exposure (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AssetQualityScheduleofCommercialCreditExposureDetails</Role>
      <ShortName>Asset Quality - Schedule of Commercial Credit Exposure (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>9955533 - Disclosure - Asset Quality - Schedule of Consumer Credit Exposure (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AssetQualityScheduleofConsumerCreditExposureDetails</Role>
      <ShortName>Asset Quality - Schedule of Consumer Credit Exposure (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>9955534 - Disclosure - Asset Quality - Schedule of Aging Analysis of Past Due and Current Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AssetQualityScheduleofAgingAnalysisofPastDueandCurrentLoansDetails</Role>
      <ShortName>Asset Quality - Schedule of Aging Analysis of Past Due and Current Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>9955535 - Disclosure - Asset Quality - Schedule of Modifications for Borrowers Experiencing Financial Difficulty (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AssetQualityScheduleofModificationsforBorrowersExperiencingFinancialDifficultyDetails</Role>
      <ShortName>Asset Quality - Schedule of Modifications for Borrowers Experiencing Financial Difficulty (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>9955536 - Disclosure - Asset Quality - Schedule of Financial Effects of Modifications to Borrowers Experiencing Financial Difficulty (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AssetQualityScheduleofFinancialEffectsofModificationstoBorrowersExperiencingFinancialDifficultyDetails</Role>
      <ShortName>Asset Quality - Schedule of Financial Effects of Modifications to Borrowers Experiencing Financial Difficulty (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>9955537 - Disclosure - Asset Quality - Schedule of Amortized Cost Basis of Modified Loans That Subsequently Defaulted (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AssetQualityScheduleofAmortizedCostBasisofModifiedLoansThatSubsequentlyDefaultedDetails</Role>
      <ShortName>Asset Quality - Schedule of Amortized Cost Basis of Modified Loans That Subsequently Defaulted (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>9955538 - Disclosure - Asset Quality - Schedule of Performance of Loans That Have Been Modified in the Last 12 Months (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AssetQualityScheduleofPerformanceofLoansThatHaveBeenModifiedintheLast12MonthsDetails</Role>
      <ShortName>Asset Quality - Schedule of Performance of Loans That Have Been Modified in the Last 12 Months (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>9955539 - Disclosure - Asset Quality - Schedule of Changes in Liability for Credit Losses on Lending Related Commitments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AssetQualityScheduleofChangesinLiabilityforCreditLossesonLendingRelatedCommitmentsDetails</Role>
      <ShortName>Asset Quality - Schedule of Changes in Liability for Credit Losses on Lending Related Commitments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>9955540 - Disclosure - Fair Value Measurements - Schedule of Fair Value of Assets and Liabilities Measured on Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/FairValueMeasurementsScheduleofFairValueofAssetsandLiabilitiesMeasuredonRecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Fair Value of Assets and Liabilities Measured on Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>9955541 - Disclosure - Fair Value Measurements - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/FairValueMeasurementsNarrativeDetails</Role>
      <ShortName>Fair Value Measurements - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>9955542 - Disclosure - Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on Nonrecurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/FairValueMeasurementsScheduleofAssetsandLiabilitiesMeasuredatFairValueonNonrecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on Nonrecurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>9955543 - Disclosure - Fair Value Measurements - Schedule of Quantitative Information about Level 3 Fair Value Measurements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/FairValueMeasurementsScheduleofQuantitativeInformationaboutLevel3FairValueMeasurementsDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Quantitative Information about Level 3 Fair Value Measurements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>9955544 - Disclosure - Fair Value Measurements - Schedule of Fair Value Disclosures of Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/FairValueMeasurementsScheduleofFairValueDisclosuresofFinancialInstrumentsDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Fair Value Disclosures of Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>9955545 - Disclosure - Securities - Schedule of Details of Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/SecuritiesScheduleofDetailsofSecuritiesDetails</Role>
      <ShortName>Securities - Schedule of Details of Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>9955546 - Disclosure - Securities - Schedule of Available for Sale Securities in an Unrealized Loss Position (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/SecuritiesScheduleofAvailableforSaleSecuritiesinanUnrealizedLossPositionDetails</Role>
      <ShortName>Securities - Schedule of Available for Sale Securities in an Unrealized Loss Position (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>9955547 - Disclosure - Securities - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/SecuritiesNarrativeDetails</Role>
      <ShortName>Securities - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>9955548 - Disclosure - Securities - Schedule of Securities by Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/SecuritiesScheduleofSecuritiesbyMaturityDetails</Role>
      <ShortName>Securities - Schedule of Securities by Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>9955549 - Disclosure - Derivatives and Hedging Activities - Schedule of Fair Values, Volume of Activity and Gain (Loss) Information Related to Derivative Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/DerivativesandHedgingActivitiesScheduleofFairValuesVolumeofActivityandGainLossInformationRelatedtoDerivativeInstrumentsDetails</Role>
      <ShortName>Derivatives and Hedging Activities - Schedule of Fair Values, Volume of Activity and Gain (Loss) Information Related to Derivative Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>9955550 - Disclosure - Derivatives and Hedging Activities - Schedule of Cumulative Basis Adjustments on Fair Value Hedges (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/DerivativesandHedgingActivitiesScheduleofCumulativeBasisAdjustmentsonFairValueHedgesDetails</Role>
      <ShortName>Derivatives and Hedging Activities - Schedule of Cumulative Basis Adjustments on Fair Value Hedges (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>9955551 - Disclosure - Derivatives and Hedging Activities - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/DerivativesandHedgingActivitiesNarrativeDetails</Role>
      <ShortName>Derivatives and Hedging Activities - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>9955552 - Disclosure - Derivatives and Hedging Activities - Schedule of Effect of Fair Value and Cash Flow Hedges on Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/DerivativesandHedgingActivitiesScheduleofEffectofFairValueandCashFlowHedgesonIncomeDetails</Role>
      <ShortName>Derivatives and Hedging Activities - Schedule of Effect of Fair Value and Cash Flow Hedges on Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>9955553 - Disclosure - Derivatives and Hedging Activities - Schedule of Derivative Instrument Cash Flow Hedge Earning Recognized by Income Statement Location (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/DerivativesandHedgingActivitiesScheduleofDerivativeInstrumentCashFlowHedgeEarningRecognizedbyIncomeStatementLocationDetails</Role>
      <ShortName>Derivatives and Hedging Activities - Schedule of Derivative Instrument Cash Flow Hedge Earning Recognized by Income Statement Location (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>9955554 - Disclosure - Derivatives and Hedging Activities - Schedule of Pre-Tax Net Gains (Losses) on Derivatives Not Designated as Hedging Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/DerivativesandHedgingActivitiesScheduleofPreTaxNetGainsLossesonDerivativesNotDesignatedasHedgingInstrumentsDetails</Role>
      <ShortName>Derivatives and Hedging Activities - Schedule of Pre-Tax Net Gains (Losses) on Derivatives Not Designated as Hedging Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>9955555 - Disclosure - Derivatives and Hedging Activities - Schedule of Fair Value of Derivative Assets by Type (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/DerivativesandHedgingActivitiesScheduleofFairValueofDerivativeAssetsbyTypeDetails</Role>
      <ShortName>Derivatives and Hedging Activities - Schedule of Fair Value of Derivative Assets by Type (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>9955556 - Disclosure - Derivatives and Hedging Activities - Schedule of Credit Derivatives Sold and Held (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/DerivativesandHedgingActivitiesScheduleofCreditDerivativesSoldandHeldDetails</Role>
      <ShortName>Derivatives and Hedging Activities - Schedule of Credit Derivatives Sold and Held (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>9955557 - Disclosure - Derivatives and Hedging Activities - Schedule of Credit Risk Contingent Feature (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/DerivativesandHedgingActivitiesScheduleofCreditRiskContingentFeatureDetails</Role>
      <ShortName>Derivatives and Hedging Activities - Schedule of Credit Risk Contingent Feature (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>9955558 - Disclosure - Mortgage Servicing Assets - Schedule of Changes in Carrying Amount of Mortgage Servicing Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/MortgageServicingAssetsScheduleofChangesinCarryingAmountofMortgageServicingAssetsDetails</Role>
      <ShortName>Mortgage Servicing Assets - Schedule of Changes in Carrying Amount of Mortgage Servicing Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>9955559 - Disclosure - Mortgage Servicing Assets - Assumptions and Sensitivity of MSR???s (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/MortgageServicingAssetsAssumptionsandSensitivityofMSRsDetails</Role>
      <ShortName>Mortgage Servicing Assets - Assumptions and Sensitivity of MSR???s (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>9955560 - Disclosure - Mortgage Servicing Assets - OMSR (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/MortgageServicingAssetsOMSRDetails</Role>
      <ShortName>Mortgage Servicing Assets - OMSR (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>9955561 - Disclosure - Mortgage Servicing Assets - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/MortgageServicingAssetsNarrativeDetails</Role>
      <ShortName>Mortgage Servicing Assets - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>9955562 - Disclosure - Mortgage Servicing Assets - MSR's (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/MortgageServicingAssetsMSRsDetails</Role>
      <ShortName>Mortgage Servicing Assets - MSR's (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>9955563 - Disclosure - Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/LeasesDetails</Role>
      <ShortName>Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.key.com/role/LeasesTables</ParentRole>
      <Position>81</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>9955564 - Disclosure - Variable Interest Entities - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/VariableInterestEntitiesNarrativeDetails</Role>
      <ShortName>Variable Interest Entities - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>9955565 - Disclosure - Variable Interest Entities - Schedule of Variable Interest Entities Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/VariableInterestEntitiesScheduleofVariableInterestEntitiesInformationDetails</Role>
      <ShortName>Variable Interest Entities - Schedule of Variable Interest Entities Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>9955566 - Disclosure - Income Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/IncomeTaxesDetails</Role>
      <ShortName>Income Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.key.com/role/IncomeTaxes</ParentRole>
      <Position>84</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>9955567 - Disclosure - Discontinued Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/DiscontinuedOperationsDetails</Role>
      <ShortName>Discontinued Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.key.com/role/DiscontinuedOperations</ParentRole>
      <Position>85</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>9955568 - Disclosure - Borrowings - Components of Short-Term Borrowings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/BorrowingsComponentsofShortTermBorrowingsDetails</Role>
      <ShortName>Borrowings - Components of Short-Term Borrowings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>9955569 - Disclosure - Borrowings - Schedule of Components of Long-Term Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/BorrowingsScheduleofComponentsofLongTermDebtDetails</Role>
      <ShortName>Borrowings - Schedule of Components of Long-Term Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>9955570 - Disclosure - Contingent Liabilities and Guarantees - Schedule of Guarantees (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ContingentLiabilitiesandGuaranteesScheduleofGuaranteesDetails</Role>
      <ShortName>Contingent Liabilities and Guarantees - Schedule of Guarantees (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>9955571 - Disclosure - Contingent Liabilities and Guarantees - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ContingentLiabilitiesandGuaranteesNarrativeDetails</Role>
      <ShortName>Contingent Liabilities and Guarantees - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>9955572 - Disclosure - Accumulated Other Comprehensive Income - Schedule of Changes in AOCI (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AccumulatedOtherComprehensiveIncomeScheduleofChangesinAOCIDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income - Schedule of Changes in AOCI (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>9955573 - Disclosure - Accumulated Other Comprehensive Income - Schedule of Reclassifications Out of AOCI (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/AccumulatedOtherComprehensiveIncomeScheduleofReclassificationsOutofAOCIDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income - Schedule of Reclassifications Out of AOCI (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>9955574 - Disclosure - Shareholders' Equity - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ShareholdersEquityNarrativeDetails</Role>
      <ShortName>Shareholders' Equity - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>9955575 - Disclosure - Shareholders' Equity - Schedule of Stockholders Equity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/ShareholdersEquityScheduleofStockholdersEquityDetails</Role>
      <ShortName>Shareholders' Equity - Schedule of Stockholders Equity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>9955576 - Disclosure - Business Segment Reporting - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/BusinessSegmentReportingNarrativeDetails</Role>
      <ShortName>Business Segment Reporting - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>9955577 - Disclosure - Business Segment Reporting - Schedule of Selected Financial Data for Our Business Segments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/BusinessSegmentReportingScheduleofSelectedFinancialDataforOurBusinessSegmentsDetails</Role>
      <ShortName>Business Segment Reporting - Schedule of Selected Financial Data for Our Business Segments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="key-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>9955578 - Disclosure - Revenue from Contracts with Customers (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.key.com/role/RevenuefromContractswithCustomersDetails</Role>
      <ShortName>Revenue from Contracts with Customers (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.key.com/role/RevenuefromContractswithCustomersTables</ParentRole>
      <Position>96</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="key-20260331.htm">key-20260331.htm</File>
    <File>key-20260331.xsd</File>
    <File>key-20260331_cal.xml</File>
    <File>key-20260331_def.xml</File>
    <File>key-20260331_lab.xml</File>
    <File>key-20260331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>key-20260331_g1.jpg</File>
    <File>key-20260331_g10.jpg</File>
    <File>key-20260331_g11.jpg</File>
    <File>key-20260331_g12.jpg</File>
    <File>key-20260331_g13.jpg</File>
    <File>key-20260331_g2.jpg</File>
    <File>key-20260331_g3.jpg</File>
    <File>key-20260331_g4.jpg</File>
    <File>key-20260331_g5.jpg</File>
    <File>key-20260331_g6.jpg</File>
    <File>key-20260331_g7.jpg</File>
    <File>key-20260331_g8.jpg</File>
    <File>key-20260331_g9.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy items="1">http://fasb.org/srt/2025</BaseTaxonomy>
    <BaseTaxonomy items="3191">http://fasb.org/us-gaap/2025</BaseTaxonomy>
    <BaseTaxonomy items="46">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2025</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
