<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.25.4</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>830</ContextCount>
  <ElementCount>627</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>170</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>20</UnitCount>
  <MyReports>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0000001 - Document - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/CoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>0000002 - Document - Audit Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/AuditInformation</Role>
      <ShortName>Audit Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>9952151 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>9952152 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>9952153 - Statement - Consolidated Statements of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ConsolidatedStatementsofOperations</Role>
      <ShortName>Consolidated Statements of Operations</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>9952154 - Statement - Consolidated Statements of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ConsolidatedStatementsofComprehensiveIncome</Role>
      <ShortName>Consolidated Statements of Comprehensive Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>9952155 - Statement - Consolidated Statements of Comprehensive Income (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ConsolidatedStatementsofComprehensiveIncomeParenthetical</Role>
      <ShortName>Consolidated Statements of Comprehensive Income (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>9952156 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ConsolidatedStatementsofCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>9952157 - Statement - Consolidated Statements of Cash Flows (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ConsolidatedStatementsofCashFlowsParenthetical</Role>
      <ShortName>Consolidated Statements of Cash Flows (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>9952158 - Statement - Consolidated Statements of Changes in Shareholders??? Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ConsolidatedStatementsofChangesinShareholdersEquity</Role>
      <ShortName>Consolidated Statements of Changes in Shareholders??? Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>9952159 - Statement - Consolidated Statements of Changes in Shareholders??? Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ConsolidatedStatementsofChangesinShareholdersEquityParenthetical</Role>
      <ShortName>Consolidated Statements of Changes in Shareholders??? Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>9952160 - Disclosure - Organization and Operation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/OrganizationandOperation</Role>
      <ShortName>Organization and Operation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>9952161 - Disclosure - Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/SignificantAccountingPolicies</Role>
      <ShortName>Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>9952162 - Disclosure - Regulatory Matters</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RegulatoryMatters</Role>
      <ShortName>Regulatory Matters</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>9952163 - Disclosure - Revenue Recognition</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RevenueRecognition</Role>
      <ShortName>Revenue Recognition</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>9952164 - Disclosure - Mergers, Acquisitions and Divestitures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/MergersAcquisitionsandDivestitures</Role>
      <ShortName>Mergers, Acquisitions and Divestitures</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>9952165 - Disclosure - Property, Plant and Equipment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/PropertyPlantandEquipment</Role>
      <ShortName>Property, Plant and Equipment</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>9952166 - Disclosure - Allowance for Uncollectible Accounts</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/AllowanceforUncollectibleAccounts</Role>
      <ShortName>Allowance for Uncollectible Accounts</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>9952167 - Disclosure - Goodwill</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/Goodwill</Role>
      <ShortName>Goodwill</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>9952168 - Disclosure - Shareholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShareholdersEquity</Role>
      <ShortName>Shareholders' Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>9952169 - Disclosure - Stock Based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/StockBasedCompensation</Role>
      <ShortName>Stock Based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>9952170 - Disclosure - Long-Term Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LongTermDebt</Role>
      <ShortName>Long-Term Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>9952171 - Disclosure - Short-Term Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShortTermDebt</Role>
      <ShortName>Short-Term Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>9952172 - Disclosure - General Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/GeneralTaxes</Role>
      <ShortName>General Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>9952173 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>9952174 - Disclosure - Employee Benefits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EmployeeBenefits</Role>
      <ShortName>Employee Benefits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>9952175 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/CommitmentsandContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>9952176 - Disclosure - Earnings Per Common Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EarningsPerCommonShare</Role>
      <ShortName>Earnings Per Common Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>9952177 - Disclosure - Fair Value of Financial Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/FairValueofFinancialInformation</Role>
      <ShortName>Fair Value of Financial Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>9952178 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/Leases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>9952179 - Disclosure - Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/SegmentInformation</Role>
      <ShortName>Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.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>32</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995447 - Disclosure - Insider Trading Policies and Procedures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingPoliciesProc</Role>
      <ShortName>Insider Trading Policies and Procedures</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>995550 - Disclosure - Cybersecurity Risk Management and Strategy Disclosure</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/cyd/role/CybersecurityRiskManagementAndStrategyDisclosure</Role>
      <ShortName>Cybersecurity Risk Management and Strategy Disclosure</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9955511 - Disclosure - Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/SignificantAccountingPoliciesPolicies</Role>
      <ShortName>Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://amwater.com/role/SignificantAccountingPolicies</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9955512 - Disclosure - Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/SignificantAccountingPoliciesTables</Role>
      <ShortName>Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/SignificantAccountingPolicies</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9955513 - Disclosure - Regulatory Matters (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RegulatoryMattersTables</Role>
      <ShortName>Regulatory Matters (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/RegulatoryMatters</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9955514 - Disclosure - Revenue Recognition (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RevenueRecognitionTables</Role>
      <ShortName>Revenue Recognition (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/RevenueRecognition</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9955515 - Disclosure - Property, Plant and Equipment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/PropertyPlantandEquipmentTables</Role>
      <ShortName>Property, Plant and Equipment (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/PropertyPlantandEquipment</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9955516 - Disclosure - Allowance for Uncollectible Accounts (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/AllowanceforUncollectibleAccountsTables</Role>
      <ShortName>Allowance for Uncollectible Accounts (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/AllowanceforUncollectibleAccounts</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9955517 - Disclosure - Goodwill (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/GoodwillTables</Role>
      <ShortName>Goodwill (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/Goodwill</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9955518 - Disclosure - Shareholders' Equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShareholdersEquityTables</Role>
      <ShortName>Shareholders' Equity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/ShareholdersEquity</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9955519 - Disclosure - Stock Based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/StockBasedCompensationTables</Role>
      <ShortName>Stock Based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/StockBasedCompensation</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9955520 - Disclosure - Long-Term Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LongTermDebtTables</Role>
      <ShortName>Long-Term Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/LongTermDebt</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9955521 - Disclosure - Short-Term Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShortTermDebtTables</Role>
      <ShortName>Short-Term Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/ShortTermDebt</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9955522 - Disclosure - General Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/GeneralTaxesTables</Role>
      <ShortName>General Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/GeneralTaxes</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9955523 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/IncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/IncomeTaxes</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9955524 - Disclosure - Employee Benefits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EmployeeBenefitsTables</Role>
      <ShortName>Employee Benefits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/EmployeeBenefits</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>9955525 - Disclosure - Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/CommitmentsandContingenciesTables</Role>
      <ShortName>Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/CommitmentsandContingencies</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>9955526 - Disclosure - Earnings Per Common Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EarningsPerCommonShareTables</Role>
      <ShortName>Earnings Per Common Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/EarningsPerCommonShare</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>9955527 - Disclosure - Fair Value of Financial Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/FairValueofFinancialInformationTables</Role>
      <ShortName>Fair Value of Financial Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/FairValueofFinancialInformation</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>9955528 - Disclosure - Segment Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/SegmentInformationTables</Role>
      <ShortName>Segment Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/SegmentInformation</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>9955529 - Disclosure - Organization and Operation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/OrganizationandOperationDetails</Role>
      <ShortName>Organization and Operation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://amwater.com/role/OrganizationandOperation</ParentRole>
      <Position>53</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>9955530 - Disclosure - Significant Accounting Policies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/SignificantAccountingPoliciesAdditionalInformationDetails</Role>
      <ShortName>Significant Accounting Policies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>9955531 - Disclosure - Significant Accounting Policies - Schedule of Cash, Cash Equivalents and Restricted Funds (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/SignificantAccountingPoliciesScheduleofCashCashEquivalentsandRestrictedFundsDetails</Role>
      <ShortName>Significant Accounting Policies - Schedule of Cash, Cash Equivalents and Restricted Funds (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>9955532 - Disclosure - Significant Accounting Policies - Schedule of Allowance for Funds Used During Construction (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/SignificantAccountingPoliciesScheduleofAllowanceforFundsUsedDuringConstructionDetails</Role>
      <ShortName>Significant Accounting Policies - Schedule of Allowance for Funds Used During Construction (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>9955533 - Disclosure - Regulatory Matters - Schedule of General Rate Cases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RegulatoryMattersScheduleofGeneralRateCasesDetails</Role>
      <ShortName>Regulatory Matters - Schedule of General Rate Cases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>9955534 - Disclosure - Regulatory Matters - General Rate Cases Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RegulatoryMattersGeneralRateCasesAdditionalInformationDetails</Role>
      <ShortName>Regulatory Matters - General Rate Cases Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>9955535 - Disclosure - Regulatory Matters - Pending General Rate Case Filings Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RegulatoryMattersPendingGeneralRateCaseFilingsAdditionalInformationDetails</Role>
      <ShortName>Regulatory Matters - Pending General Rate Case Filings Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>9955536 - Disclosure - Regulatory Matters - Schedule of Infrastructure Surcharge (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RegulatoryMattersScheduleofInfrastructureSurchargeDetails</Role>
      <ShortName>Regulatory Matters - Schedule of Infrastructure Surcharge (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>9955537 - Disclosure - Regulatory Matters - Pending Infrastructure Surcharge Filings Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RegulatoryMattersPendingInfrastructureSurchargeFilingsAdditionalInformationDetails</Role>
      <ShortName>Regulatory Matters - Pending Infrastructure Surcharge Filings Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>9955538 - Disclosure - Regulatory Matters - Regulatory Assets and Liabilities Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RegulatoryMattersRegulatoryAssetsandLiabilitiesAdditionalInformationDetails</Role>
      <ShortName>Regulatory Matters - Regulatory Assets and Liabilities Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>9955539 - Disclosure - Regulatory Matters - Schedule of Composition of Regulatory Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RegulatoryMattersScheduleofCompositionofRegulatoryAssetsDetails</Role>
      <ShortName>Regulatory Matters - Schedule of Composition of Regulatory Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>9955540 - Disclosure - Regulatory Matters - Schedule of Composition of Regulatory Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RegulatoryMattersScheduleofCompositionofRegulatoryLiabilitiesDetails</Role>
      <ShortName>Regulatory Matters - Schedule of Composition of Regulatory Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>9955541 - Disclosure - Revenue Recognition - Disaggregated Revenues (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RevenueRecognitionDisaggregatedRevenuesDetails</Role>
      <ShortName>Revenue Recognition - Disaggregated Revenues (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>9955542 - Disclosure - Revenue Recognition - Contract Assets and Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RevenueRecognitionContractAssetsandLiabilitiesDetails</Role>
      <ShortName>Revenue Recognition - Contract Assets and Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>9955543 - Disclosure - Revenue Recognition - Remaining Performance Obligations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RevenueRecognitionRemainingPerformanceObligationsDetails</Role>
      <ShortName>Revenue Recognition - Remaining Performance Obligations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>9955544 - Disclosure - Mergers, Acquisitions and Divestitures (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/MergersAcquisitionsandDivestituresDetails</Role>
      <ShortName>Mergers, Acquisitions and Divestitures (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://amwater.com/role/MergersAcquisitionsandDivestitures</ParentRole>
      <Position>68</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>9955545 - Disclosure - Property, Plant and Equipment - Schedule of Major Classes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/PropertyPlantandEquipmentScheduleofMajorClassesDetails</Role>
      <ShortName>Property, Plant and Equipment - Schedule of Major Classes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>9955546 - Disclosure - Property, Plant and Equipment - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/PropertyPlantandEquipmentAdditionalInformationDetails</Role>
      <ShortName>Property, Plant and Equipment - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>9955547 - Disclosure - Allowance for Uncollectible Accounts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/AllowanceforUncollectibleAccountsDetails</Role>
      <ShortName>Allowance for Uncollectible Accounts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://amwater.com/role/AllowanceforUncollectibleAccountsTables</ParentRole>
      <Position>71</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>9955548 - Disclosure - Goodwill - Schedule of Changes in Goodwill Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/GoodwillScheduleofChangesinGoodwillAssetsDetails</Role>
      <ShortName>Goodwill - Schedule of Changes in Goodwill Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>9955549 - Disclosure - Goodwill - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/GoodwillNarrativeDetails</Role>
      <ShortName>Goodwill - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>9955550 - Disclosure - Shareholders' Equity - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShareholdersEquityAdditionalInformationDetails</Role>
      <ShortName>Shareholders' Equity - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>9955551 - Disclosure - Shareholders' Equity - Schedule of Changes in Accumulated Other Comprehensive Loss by Component, Net of Tax (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShareholdersEquityScheduleofChangesinAccumulatedOtherComprehensiveLossbyComponentNetofTaxDetails</Role>
      <ShortName>Shareholders' Equity - Schedule of Changes in Accumulated Other Comprehensive Loss by Component, Net of Tax (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>9955552 - Disclosure - Shareholders' Equity - Schedule of Dividends (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShareholdersEquityScheduleofDividendsDetails</Role>
      <ShortName>Shareholders' Equity - Schedule of Dividends (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>9955553 - Disclosure - Stock Based Compensation - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/StockBasedCompensationAdditionalInformationDetails</Role>
      <ShortName>Stock Based Compensation - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>9955554 - Disclosure - Stock-Based Compensation - Schedule of Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/StockBasedCompensationScheduleofExpenseDetails</Role>
      <ShortName>Stock-Based Compensation - Schedule of Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>9955555 - Disclosure - Stock Based Compensation - Schedule of Stock Unit Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/StockBasedCompensationScheduleofStockUnitActivityDetails</Role>
      <ShortName>Stock Based Compensation - Schedule of Stock Unit Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>9955556 - Disclosure - Stock Based Compensation -Schedule of Weighted-Average Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/StockBasedCompensationScheduleofWeightedAverageAssumptionsDetails</Role>
      <ShortName>Stock Based Compensation -Schedule of Weighted-Average Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>9955557 - Disclosure - Long-Term Debt - Schedule of Components of Long-Term Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LongTermDebtScheduleofComponentsofLongTermDebtDetails</Role>
      <ShortName>Long-Term Debt - Schedule of Components of Long-Term Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>9955558 - Disclosure - Long-Term Debt - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LongTermDebtAdditionalInformationDetails</Role>
      <ShortName>Long-Term Debt - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>9955559 - Disclosure - Long-Term Debt - Schedule of Future Sinking Fund Payments and Debt Maturities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LongTermDebtScheduleofFutureSinkingFundPaymentsandDebtMaturitiesDetails</Role>
      <ShortName>Long-Term Debt - Schedule of Future Sinking Fund Payments and Debt Maturities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>9955560 - Disclosure - Long-Term Debt - Schedule of Issued (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LongTermDebtScheduleofIssuedDetails</Role>
      <ShortName>Long-Term Debt - Schedule of Issued (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>9955561 - Disclosure - Long-Term Debt - Schedule of Retired Through Optional Redemptions or Payments at Maturities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LongTermDebtScheduleofRetiredThroughOptionalRedemptionsorPaymentsatMaturitiesDetails</Role>
      <ShortName>Long-Term Debt - Schedule of Retired Through Optional Redemptions or Payments at Maturities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>9955562 - Disclosure - Short-Term Debt - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShortTermDebtAdditionalInformationDetails</Role>
      <ShortName>Short-Term Debt - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>9955563 - Disclosure - Short-Term Debt - Schedule of Company's Aggregate Credit Facility Commitments, Commercial Paper Limit, Letter of Credit Availability and Availability Capacity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShortTermDebtScheduleofCompanysAggregateCreditFacilityCommitmentsCommercialPaperLimitLetterofCreditAvailabilityandAvailabilityCapacityDetails</Role>
      <ShortName>Short-Term Debt - Schedule of Company's Aggregate Credit Facility Commitments, Commercial Paper Limit, Letter of Credit Availability and Availability Capacity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>9955564 - Disclosure - Short-Term Debt - Schedule of Availability Liquidity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShortTermDebtScheduleofAvailabilityLiquidityDetails</Role>
      <ShortName>Short-Term Debt - Schedule of Availability Liquidity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>9955565 - Disclosure - Short-Term Debt - Schedule of Short-Term Borrowings Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShortTermDebtScheduleofShortTermBorrowingsActivityDetails</Role>
      <ShortName>Short-Term Debt - Schedule of Short-Term Borrowings Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>9955566 - Disclosure - General Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/GeneralTaxesDetails</Role>
      <ShortName>General Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://amwater.com/role/GeneralTaxesTables</ParentRole>
      <Position>90</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>9955567 - Disclosure - Income Taxes - Schedule of Components of Income Tax Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/IncomeTaxesScheduleofComponentsofIncomeTaxExpenseDetails</Role>
      <ShortName>Income Taxes - Schedule of Components of Income Tax Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>9955568 - Disclosure - Income Taxes - Schedule of Reconciliation to Effective Tax Amount (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/IncomeTaxesScheduleofReconciliationtoEffectiveTaxAmountDetails</Role>
      <ShortName>Income Taxes - Schedule of Reconciliation to Effective Tax Amount (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>9955569 - Disclosure - Income Taxes - Schedule of Income Taxes Paid, Net (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/IncomeTaxesScheduleofIncomeTaxesPaidNetDetails</Role>
      <ShortName>Income Taxes - Schedule of Income Taxes Paid, Net (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>9955570 - Disclosure - Income Taxes - Schedule of Components of Net Deferred Tax Liability (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/IncomeTaxesScheduleofComponentsofNetDeferredTaxLiabilityDetails</Role>
      <ShortName>Income Taxes - Schedule of Components of Net Deferred Tax Liability (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>9955571 - Disclosure - Income Taxes - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/IncomeTaxesAdditionalInformationDetails</Role>
      <ShortName>Income Taxes - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>9955572 - Disclosure - Income Taxes - Schedule of Changes in Gross Liability Excluding Interest and Penalties for Unrecognized Tax Benefits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/IncomeTaxesScheduleofChangesinGrossLiabilityExcludingInterestandPenaltiesforUnrecognizedTaxBenefitsDetails</Role>
      <ShortName>Income Taxes - Schedule of Changes in Gross Liability Excluding Interest and Penalties for Unrecognized Tax Benefits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>96</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>9955573 - Disclosure - Income Taxes - Schedule of Changes in Valuation Allowance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/IncomeTaxesScheduleofChangesinValuationAllowanceDetails</Role>
      <ShortName>Income Taxes - Schedule of Changes in Valuation Allowance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>97</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>9955574 - Disclosure - Employee Benefits - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EmployeeBenefitsAdditionalInformationDetails</Role>
      <ShortName>Employee Benefits - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>98</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>9955575 - Disclosure - Employee Benefits - Schedule of Changes in Fair Value of Plan Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EmployeeBenefitsScheduleofChangesinFairValueofPlanAssetsDetails</Role>
      <ShortName>Employee Benefits - Schedule of Changes in Fair Value of Plan Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>99</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>9955576 - Disclosure - Employee Benefits - Schedule of Rollforward Changes in Benefit Obligation and Plan Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EmployeeBenefitsScheduleofRollforwardChangesinBenefitObligationandPlanAssetsDetails</Role>
      <ShortName>Employee Benefits - Schedule of Rollforward Changes in Benefit Obligation and Plan Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>100</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>9955577 - Disclosure - Employee Benefits - Schedule of Accumulated Other Comprehensive Income and Regulatory Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EmployeeBenefitsScheduleofAccumulatedOtherComprehensiveIncomeandRegulatoryAssetsDetails</Role>
      <ShortName>Employee Benefits - Schedule of Accumulated Other Comprehensive Income and Regulatory Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>101</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>9955578 - Disclosure - Employee Benefits - Schedule of Projected Benefit Obligation, Accumulated Benefit Obligation and Fair Value of Plan Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EmployeeBenefitsScheduleofProjectedBenefitObligationAccumulatedBenefitObligationandFairValueofPlanAssetsDetails</Role>
      <ShortName>Employee Benefits - Schedule of Projected Benefit Obligation, Accumulated Benefit Obligation and Fair Value of Plan Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>102</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>9955579 - Disclosure - Employee Benefits - Schedule of Expected Cash Flow for Pension and Post Retirement Benefit Plans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EmployeeBenefitsScheduleofExpectedCashFlowforPensionandPostRetirementBenefitPlansDetails</Role>
      <ShortName>Employee Benefits - Schedule of Expected Cash Flow for Pension and Post Retirement Benefit Plans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>103</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>9955580 - Disclosure - Employee Benefits - Schedule of Expected Benefit Payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EmployeeBenefitsScheduleofExpectedBenefitPaymentsDetails</Role>
      <ShortName>Employee Benefits - Schedule of Expected Benefit Payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>104</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>9955581 - Disclosure - Employee Benefits - Schedule of Significant Assumptions of Pension and Other Postretirement Benefit Plans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EmployeeBenefitsScheduleofSignificantAssumptionsofPensionandOtherPostretirementBenefitPlansDetails</Role>
      <ShortName>Employee Benefits - Schedule of Significant Assumptions of Pension and Other Postretirement Benefit Plans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>105</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>9955582 - Disclosure - Employee Benefits - Schedule of Net Periodic Benefit Cost Components (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EmployeeBenefitsScheduleofNetPeriodicBenefitCostComponentsDetails</Role>
      <ShortName>Employee Benefits - Schedule of Net Periodic Benefit Cost Components (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>106</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>9955583 - Disclosure - Commitments and Contingencies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/CommitmentsandContingenciesAdditionalInformationDetails</Role>
      <ShortName>Commitments and Contingencies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>107</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R108.htm</HtmlFileName>
      <LongName>9955584 - Disclosure - Commitments and Contingencies - Schedule of Future Annual Commitments Related to Minimum Quantities of Purchased Water Having Non-Cancelable Terms (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/CommitmentsandContingenciesScheduleofFutureAnnualCommitmentsRelatedtoMinimumQuantitiesofPurchasedWaterHavingNonCancelableTermsDetails</Role>
      <ShortName>Commitments and Contingencies - Schedule of Future Annual Commitments Related to Minimum Quantities of Purchased Water Having Non-Cancelable Terms (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>108</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R109.htm</HtmlFileName>
      <LongName>9955585 - Disclosure - Earnings Per Common Share - Schedule of Reconciliation of Numerator and Denominator for Basic and Diluted Earnings Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EarningsPerCommonShareScheduleofReconciliationofNumeratorandDenominatorforBasicandDilutedEarningsPerShareDetails</Role>
      <ShortName>Earnings Per Common Share - Schedule of Reconciliation of Numerator and Denominator for Basic and Diluted Earnings Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>109</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R110.htm</HtmlFileName>
      <LongName>9955586 - Disclosure - Earnings Per Common Share - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EarningsPerCommonShareAdditionalInformationDetails</Role>
      <ShortName>Earnings Per Common Share - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>110</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R111.htm</HtmlFileName>
      <LongName>9955587 - Disclosure - Fair Value of Financial Information - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/FairValueofFinancialInformationAdditionalInformationDetails</Role>
      <ShortName>Fair Value of Financial Information - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>111</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R112.htm</HtmlFileName>
      <LongName>9955588 - Disclosure - Fair Value of Financial Information - Schedule of Carrying Amounts and Fair Values of Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/FairValueofFinancialInformationScheduleofCarryingAmountsandFairValuesofFinancialInstrumentsDetails</Role>
      <ShortName>Fair Value of Financial Information - Schedule of Carrying Amounts and Fair Values of Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>112</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R113.htm</HtmlFileName>
      <LongName>9955589 - Disclosure - Fair Value of Financial Information - Schedule of Measurements of Assets and Liabilities on Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/FairValueofFinancialInformationScheduleofMeasurementsofAssetsandLiabilitiesonRecurringBasisDetails</Role>
      <ShortName>Fair Value of Financial Information - Schedule of Measurements of Assets and Liabilities on Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>113</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R114.htm</HtmlFileName>
      <LongName>9955590 - Disclosure - Fair Value of Financial Information - Schedule of Unrealized Positions for Available-for-sale Fixed-Income Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/FairValueofFinancialInformationScheduleofUnrealizedPositionsforAvailableforsaleFixedIncomeSecuritiesDetails</Role>
      <ShortName>Fair Value of Financial Information - Schedule of Unrealized Positions for Available-for-sale Fixed-Income Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>114</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R115.htm</HtmlFileName>
      <LongName>9955591 - Disclosure - Fair Value of Financial Information - Schedule of Available-for-sale Fixed-Income Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/FairValueofFinancialInformationScheduleofAvailableforsaleFixedIncomeSecuritiesDetails</Role>
      <ShortName>Fair Value of Financial Information - Schedule of Available-for-sale Fixed-Income Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>115</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R116.htm</HtmlFileName>
      <LongName>9955592 - Disclosure - Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LeasesDetails</Role>
      <ShortName>Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://amwater.com/role/Leases</ParentRole>
      <Position>116</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R117.htm</HtmlFileName>
      <LongName>9955593 - Disclosure - Segment Information - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/SegmentInformationAdditionalInformationDetails</Role>
      <ShortName>Segment Information - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>117</Position>
    </Report>
    <Report instance="awk-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R118.htm</HtmlFileName>
      <LongName>9955594 - Disclosure - Segment Information - Schedule of Summarized Segment Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/SegmentInformationScheduleofSummarizedSegmentInformationDetails</Role>
      <ShortName>Segment Information - Schedule of Summarized Segment Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>118</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-K" isDefinitelyFs="true" isUsgaap="true" original="awk-20251231.htm">awk-20251231.htm</File>
    <File>awk-20251231.xsd</File>
    <File>awk-20251231_cal.xml</File>
    <File>awk-20251231_def.xml</File>
    <File>awk-20251231_lab.xml</File>
    <File>awk-20251231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="1">http://fasb.org/srt/2025</BaseTaxonomy>
    <BaseTaxonomy items="2008">http://fasb.org/us-gaap/2025</BaseTaxonomy>
    <BaseTaxonomy items="15">http://xbrl.sec.gov/cyd/2025</BaseTaxonomy>
    <BaseTaxonomy items="38">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
    <BaseTaxonomy items="5">http://xbrl.sec.gov/ecd/2025</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
