<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.19.3.a.u2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>573</ContextCount>
  <ElementCount>681</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>143</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>11</UnitCount>
  <MyReports>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001000 - 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="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1002000 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1002501 - 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>3</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1003000 - 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>4</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1004000 - 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>5</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1004501 - 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>6</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1005000 - 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>7</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>1005501 - 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>8</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>1006000 - Statement - Consolidated Statements of Changes in Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ConsolidatedStatementsOfChangesInStockholdersEquity</Role>
      <ShortName>Consolidated Statements of Changes in Stockholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>1006001 - Statement - Consolidated Statements of Changes in Stockholders' Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ConsolidatedStatementsOfChangesInStockholdersEquityParenthetical</Role>
      <ShortName>Consolidated Statements of Changes in Stockholders' Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - Organization and Operation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/OrganizationAndOperation</Role>
      <ShortName>Organization and Operation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/SignificantAccountingPolicies</Role>
      <ShortName>Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2102100 - Disclosure - Revenue Recognition</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RevenueRecognition</Role>
      <ShortName>Revenue Recognition</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2104100 - Disclosure - Acquisitions and Divestitures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/AcquisitionsAndDivestitures</Role>
      <ShortName>Acquisitions and Divestitures</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2108100 - 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>15</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2110100 - 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>16</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2112100 - Disclosure - Regulatory Assets and Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RegulatoryAssetsAndLiabilities</Role>
      <ShortName>Regulatory Assets and Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2115100 - Disclosure - Goodwill and Other Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/GoodwillAndOtherIntangibleAssets</Role>
      <ShortName>Goodwill and Other Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2117100 - Disclosure - Shareholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShareholdersEquity</Role>
      <ShortName>Shareholders' Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2118100 - Disclosure - Stock Based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/StockBasedCompensation</Role>
      <ShortName>Stock Based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2119100 - Disclosure - Long-Term Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LongTermDebt</Role>
      <ShortName>Long-Term Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2120100 - Disclosure - Short-Term Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShortTermDebt</Role>
      <ShortName>Short-Term Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2121100 - Disclosure - General Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/GeneralTaxes</Role>
      <ShortName>General Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2123100 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2130100 - Disclosure - Employee Benefits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EmployeeBenefits</Role>
      <ShortName>Employee Benefits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2135100 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/CommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2136100 - 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>27</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2139100 - Disclosure - Fair Values of Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/FairValuesOfFinancialInstruments</Role>
      <ShortName>Fair Values of Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2145100 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/Leases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2158100 - Disclosure - Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/SegmentInformation</Role>
      <ShortName>Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2159100 - Disclosure - Unaudited Quarterly Data</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/UnauditedQuarterlyData</Role>
      <ShortName>Unaudited Quarterly Data</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2201201 - 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>32</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2301302 - 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>33</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2302301 - 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>34</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2304301 - Disclosure - Acquisitions and Divestitures Acquisitions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/AcquisitionsAndDivestituresAcquisitionsTables</Role>
      <ShortName>Acquisitions and Divestitures Acquisitions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2308301 - 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>36</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2310301 - 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>37</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2312301 - Disclosure - Regulatory Assets and Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RegulatoryAssetsAndLiabilitiesTables</Role>
      <ShortName>Regulatory Assets and Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/RegulatoryAssetsAndLiabilities</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2315301 - Disclosure - Goodwill and Other Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/GoodwillAndOtherIntangibleAssetsTables</Role>
      <ShortName>Goodwill and Other Intangible Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/GoodwillAndOtherIntangibleAssets</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2317301 - 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>40</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2318301 - 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>41</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2319301 - 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>42</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2320301 - 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>43</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2321301 - 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>44</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2323301 - 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>45</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2330301 - 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>46</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2335301 - 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>47</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2336301 - 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>48</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2339301 - Disclosure - Fair Values of Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/FairValuesOfFinancialInstrumentsTables</Role>
      <ShortName>Fair Values of Financial Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/FairValuesOfFinancialInstruments</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2345301 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/Leases</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2358301 - 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>51</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2359301 - Disclosure - Unaudited Quarterly Data (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/UnauditedQuarterlyDataTables</Role>
      <ShortName>Unaudited Quarterly Data (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://amwater.com/role/UnauditedQuarterlyData</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2401401 - Disclosure - Organization and Operation - Additional Details (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/OrganizationAndOperationAdditionalDetailsDetails</Role>
      <ShortName>Organization and Operation - Additional Details (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2401403 - 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="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>2401404 - Disclosure - Significant Accounting Policies - Reconciliation Of Cash And Cash Equivalents, And Restricted Cash Reported (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/SignificantAccountingPoliciesReconciliationOfCashAndCashEquivalentsAndRestrictedCashReportedDetails</Role>
      <ShortName>Significant Accounting Policies - Reconciliation Of Cash And Cash Equivalents, And Restricted Cash Reported (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>2401405 - 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="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>2402402 - 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>57</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>2402403 - 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>58</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>2402404 - Disclosure - Revenue Recognition - (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RevenueRecognitionDetails</Role>
      <ShortName>Revenue Recognition - (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://amwater.com/role/RevenueRecognitionTables</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>2404402 - Disclosure - Acquisitions - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/AcquisitionsAdditionalInformationDetails</Role>
      <ShortName>Acquisitions - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>2404403 - Disclosure - Acquisitions - Components of Assets Held-for-sale (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/AcquisitionsComponentsOfAssetsHeldForSaleDetails</Role>
      <ShortName>Acquisitions - Components of Assets Held-for-sale (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>2408402 - 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>62</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>2408403 - 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>63</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>2410402 - Disclosure - Allowance for Uncollectible Accounts - Schedule of Allowances for Uncollectible Accounts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/AllowanceForUncollectibleAccountsScheduleOfAllowancesForUncollectibleAccountsDetails</Role>
      <ShortName>Allowance for Uncollectible Accounts - Schedule of Allowances for Uncollectible Accounts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>2412402 - Disclosure - Regulatory Assets and Liabilities - Summary of Composition of Regulatory Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RegulatoryAssetsAndLiabilitiesSummaryOfCompositionOfRegulatoryAssetsDetails</Role>
      <ShortName>Regulatory Assets and Liabilities - Summary of Composition of Regulatory Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>2412403 - Disclosure - Regulatory Assets and Liabilities - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RegulatoryAssetsAndLiabilitiesAdditionalInformationDetails</Role>
      <ShortName>Regulatory Assets and Liabilities - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>2412404 - Disclosure - Regulatory Assets and Liabilities - Summary of Composition of Regulatory Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/RegulatoryAssetsAndLiabilitiesSummaryOfCompositionOfRegulatoryLiabilitiesDetails</Role>
      <ShortName>Regulatory Assets and Liabilities - Summary of Composition of Regulatory Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>2415402 - Disclosure - Goodwill and Other Intangible Assets - Summary of Changes in Goodwill Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/GoodwillAndOtherIntangibleAssetsSummaryOfChangesInGoodwillAssetsDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets - Summary of Changes in Goodwill Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>2415403 - Disclosure - Goodwill and Other Intangible Assets - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/GoodwillAndOtherIntangibleAssetsAdditionalInformationDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>2415404 - Disclosure - Goodwill and Other Intangible Assets - Intangible Assets - Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/GoodwillAndOtherIntangibleAssetsIntangibleAssetsCostDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets - Intangible Assets - Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>2415405 - Disclosure - Goodwill and Other Intangible Assets - Intangible Assets - Amortization (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/GoodwillAndOtherIntangibleAssetsIntangibleAssetsAmortizationDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets - Intangible Assets - Amortization (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>2415406 - Disclosure - Goodwill and Other Intangible Assets - Amortization of Intangibles (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/GoodwillAndOtherIntangibleAssetsAmortizationOfIntangiblesDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets - Amortization of Intangibles (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>2417402 - 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>73</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>2417403 - Disclosure - Shareholders' Equity - Changes in Accumulated Other Comprehensive Loss by Component, Net of Tax (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShareholdersEquityChangesInAccumulatedOtherComprehensiveLossByComponentNetOfTaxDetails</Role>
      <ShortName>Shareholders' Equity - Changes in Accumulated Other Comprehensive Loss by Component, Net of Tax (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>2417404 - Disclosure - Shareholders' Equity - Dividends (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShareholdersEquityDividendsDetails</Role>
      <ShortName>Shareholders' Equity - Dividends (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>2418402 - 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>76</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>2418403 - Disclosure - Stock-Based Compensation - Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/StockBasedCompensationExpenseDetails</Role>
      <ShortName>Stock-Based Compensation - Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>2418404 - Disclosure - Stock Based Compensation - Summary of Stock Option Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/StockBasedCompensationSummaryOfStockOptionActivityDetails</Role>
      <ShortName>Stock Based Compensation - Summary of Stock Option Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>2418405 - Disclosure - Stock Based Compensation - Additional Information to Stock Options Exercised (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/StockBasedCompensationAdditionalInformationToStockOptionsExercisedDetails</Role>
      <ShortName>Stock Based Compensation - Additional Information to Stock Options Exercised (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>2418406 - Disclosure - Stock Based Compensation - Summary of Weighted-Average Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/StockBasedCompensationSummaryOfWeightedAverageAssumptionsDetails</Role>
      <ShortName>Stock Based Compensation - Summary of Weighted-Average Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>2418407 - Disclosure - Stock Based Compensation - Summary of Restricted Stock Unit Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/StockBasedCompensationSummaryOfRestrictedStockUnitActivityDetails</Role>
      <ShortName>Stock Based Compensation - Summary of Restricted Stock Unit Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>2419402 - Disclosure - Long-Term Debt - Components of Long-Term Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LongTermDebtComponentsOfLongTermDebtDetails</Role>
      <ShortName>Long-Term Debt - Components of Long-Term Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>2419403 - Disclosure - Long-Term Debt Long-Term Debt - Components of Long-Term Debt - Footnotes (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://amwater.com/role/LongTermDebtLongTermDebtComponentsOfLongTermDebtFootnotesDetails</Role>
      <ShortName>Long-Term Debt Long-Term Debt - Components of Long-Term Debt - Footnotes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>2419404 - 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>84</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>2419405 - Disclosure - Long-Term Debt - Future Sinking Fund Payments and Debt Maturities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LongTermDebtFutureSinkingFundPaymentsAndDebtMaturitiesDetails</Role>
      <ShortName>Long-Term Debt - Future Sinking Fund Payments and Debt Maturities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>2419406 - Disclosure - Long-Term Debt - Issued (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LongTermDebtIssuedDetails</Role>
      <ShortName>Long-Term Debt - Issued (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>2419407 - Disclosure - Long-Term Debt - Retired Through Optional Redemptions or Payments at Maturities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LongTermDebtRetiredThroughOptionalRedemptionsOrPaymentsAtMaturitiesDetails</Role>
      <ShortName>Long-Term Debt - Retired Through Optional Redemptions or Payments at Maturities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>2419408 - Disclosure - Long-Term Debt - Summary of Gross Fair Value Derivative Asset and Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LongTermDebtSummaryOfGrossFairValueDerivativeAssetAndLiabilitiesDetails</Role>
      <ShortName>Long-Term Debt - Summary of Gross Fair Value Derivative Asset and Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>2420402 - 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>89</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>2420403 - Disclosure - Short-Term Debt - Schedule of Company's Aggregate Credit Facility Commitments, Letter of Credit Sub-Limit Under Revolving Credit Facility and Commercial Paper Limit and Available Capacity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShortTermDebtScheduleOfCompanysAggregateCreditFacilityCommitmentsLetterOfCreditSubLimitUnderRevolvingCreditFacilityAndCommercialPaperLimitAndAvailableCapacityDetails</Role>
      <ShortName>Short-Term Debt - Schedule of Company's Aggregate Credit Facility Commitments, Letter of Credit Sub-Limit Under Revolving Credit Facility and Commercial Paper Limit and Available Capacity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>2420404 - 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>91</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>2420405 - Disclosure - Short-Term Debt - Credit Facility - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/ShortTermDebtCreditFacilityAdditionalInformationDetails</Role>
      <ShortName>Short-Term Debt - Credit Facility - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>2421402 - Disclosure - General Taxes - Components of General Tax Expense from Continuing Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/GeneralTaxesComponentsOfGeneralTaxExpenseFromContinuingOperationsDetails</Role>
      <ShortName>General Taxes - Components of General Tax Expense from Continuing Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>2423402 - Disclosure - Income Taxes - Components of Income Tax Expense from Continuing Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/IncomeTaxesComponentsOfIncomeTaxExpenseFromContinuingOperationsDetails</Role>
      <ShortName>Income Taxes - Components of Income Tax Expense from Continuing Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>2423403 - Disclosure - Income Taxes - Reconciliation of Income Tax Expense from Continuing Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/IncomeTaxesReconciliationOfIncomeTaxExpenseFromContinuingOperationsDetails</Role>
      <ShortName>Income Taxes - Reconciliation of Income Tax Expense from Continuing Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>2423404 - 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>96</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>2423405 - Disclosure - Income Taxes - Components of Net Deferred Tax Liability from Continuing Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/IncomeTaxesComponentsOfNetDeferredTaxLiabilityFromContinuingOperationsDetails</Role>
      <ShortName>Income Taxes - Components of Net Deferred Tax Liability from Continuing Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>97</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>2423406 - Disclosure - Income Taxes - Changes in Gross Liability Excluding Interest and Penalties for Unrecognized Tax Benefits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/IncomeTaxesChangesInGrossLiabilityExcludingInterestAndPenaltiesForUnrecognizedTaxBenefitsDetails</Role>
      <ShortName>Income Taxes - Changes in Gross Liability Excluding Interest and Penalties for Unrecognized Tax Benefits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>98</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>2423407 - Disclosure - Income Taxes - Changes in Valuation Allowance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/IncomeTaxesChangesInValuationAllowanceDetails</Role>
      <ShortName>Income Taxes - Changes in Valuation Allowance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>99</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>2430402 - 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>100</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>2430403 - 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>101</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>2430404 - Disclosure - Employee Benefits - Schedule of Significant Unobservable Inputs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EmployeeBenefitsScheduleOfSignificantUnobservableInputsDetails</Role>
      <ShortName>Employee Benefits - Schedule of Significant Unobservable Inputs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>102</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>2430405 - 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>103</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>2430406 - Disclosure - Employee Benefits - Summary of Accumulated Other Comprehensive Income and Regulatory Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EmployeeBenefitsSummaryOfAccumulatedOtherComprehensiveIncomeAndRegulatoryAssetsDetails</Role>
      <ShortName>Employee Benefits - Summary of Accumulated Other Comprehensive Income and Regulatory Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>104</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>2430407 - 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>105</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>2430408 - 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>106</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>2430409 - 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>107</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R108.htm</HtmlFileName>
      <LongName>2430410 - 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>108</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R109.htm</HtmlFileName>
      <LongName>2430411 - 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>109</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R110.htm</HtmlFileName>
      <LongName>2435402 - 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>110</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R111.htm</HtmlFileName>
      <LongName>2435403 - Disclosure - Commitments and Contingencies - Summary 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/CommitmentsAndContingenciesSummaryOfFutureAnnualCommitmentsRelatedToMinimumQuantitiesOfPurchasedWaterHavingNonCancelableTermsDetails</Role>
      <ShortName>Commitments and Contingencies - Summary of Future Annual Commitments Related to Minimum Quantities of Purchased Water Having Non-Cancelable Terms (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>111</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R112.htm</HtmlFileName>
      <LongName>2436402 - Disclosure - Earnings Per Common Share - Reconciliation of Numerator and Denominator for Basic and Diluted Earnings Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/EarningsPerCommonShareReconciliationOfNumeratorAndDenominatorForBasicAndDilutedEarningsPerShareDetails</Role>
      <ShortName>Earnings Per Common Share - Reconciliation of Numerator and Denominator for Basic and Diluted Earnings Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>112</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R113.htm</HtmlFileName>
      <LongName>2436403 - 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>113</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R114.htm</HtmlFileName>
      <LongName>2439402 - Disclosure - Fair Values of Financial Instruments Fair Values of Financial Instruments - Carrying Amounts and Fair Values of Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/FairValuesOfFinancialInstrumentsFairValuesOfFinancialInstrumentsCarryingAmountsAndFairValuesOfFinancialInstrumentsDetails</Role>
      <ShortName>Fair Values of Financial Instruments Fair Values of Financial Instruments - Carrying Amounts and Fair Values of Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>114</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R115.htm</HtmlFileName>
      <LongName>2439403 - Disclosure - Fair Values of Financial Instruments - Fair Value Measurements of Assets and Liabilities on Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/FairValuesOfFinancialInstrumentsFairValueMeasurementsOfAssetsAndLiabilitiesOnRecurringBasisDetails</Role>
      <ShortName>Fair Values of Financial Instruments - Fair Value Measurements of Assets and Liabilities on Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>115</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R116.htm</HtmlFileName>
      <LongName>2439404 - Disclosure - Fair Values of Financial Instruments - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/FairValuesOfFinancialInstrumentsAdditionalInformationDetails</Role>
      <ShortName>Fair Values of Financial Instruments - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>116</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R117.htm</HtmlFileName>
      <LongName>2445402 - 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/LeasesTables</ParentRole>
      <Position>117</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R118.htm</HtmlFileName>
      <LongName>2445403 - Disclosure - Leases - Supplemental Cash Flow (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LeasesSupplementalCashFlowDetails</Role>
      <ShortName>Leases - Supplemental Cash Flow (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>118</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R119.htm</HtmlFileName>
      <LongName>2445404 - Disclosure - Leases - Remaining Lease Term and Discount Rate (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LeasesRemainingLeaseTermAndDiscountRateDetails</Role>
      <ShortName>Leases - Remaining Lease Term and Discount Rate (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>119</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R120.htm</HtmlFileName>
      <LongName>2445405 - Disclosure - Leases - Future Maturities of Lease Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LeasesFutureMaturitiesOfLeaseLiabilitiesDetails</Role>
      <ShortName>Leases - Future Maturities of Lease Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>120</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R121.htm</HtmlFileName>
      <LongName>2445406 - Disclosure - Leases - Future Maturities of Lease Liabilities Initial or Non Cancelable Terms (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/LeasesFutureMaturitiesOfLeaseLiabilitiesInitialOrNonCancelableTermsDetails</Role>
      <ShortName>Leases - Future Maturities of Lease Liabilities Initial or Non Cancelable Terms (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>121</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R122.htm</HtmlFileName>
      <LongName>2458402 - 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>122</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R123.htm</HtmlFileName>
      <LongName>2458403 - Disclosure - Segment Information - Summarized Segment Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/SegmentInformationSummarizedSegmentInformationDetails</Role>
      <ShortName>Segment Information - Summarized Segment Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>123</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R124.htm</HtmlFileName>
      <LongName>2459402 - Disclosure - Unaudited Quarterly Data - Schedule Of Unaudited Quarterly Data (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://amwater.com/role/UnauditedQuarterlyDataScheduleOfUnauditedQuarterlyDataDetails</Role>
      <ShortName>Unaudited Quarterly Data - Schedule Of Unaudited Quarterly Data (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>124</Position>
    </Report>
    <Report instance="a12312019-awkx10kdocum.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9999.htm</HtmlFileName>
      <LongName>Uncategorized Items - a12312019-awkx10kdocum.htm</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/role/uncategorizedFacts</Role>
      <ShortName>Uncategorized Items - a12312019-awkx10kdocum.htm</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>125</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" original="a12312019-awkx10kdocum.htm">a12312019-awkx10kdocum.htm</File>
    <File>a1014332019psubepsfor.htm</File>
    <File>a1014342019psubepsfor.htm</File>
    <File>a1014352019warnockpsu.htm</File>
    <File>a1014362019hardwickpsu.htm</File>
    <File>awk-20191231.xsd</File>
    <File>awk-20191231_cal.xml</File>
    <File>awk-20191231_def.xml</File>
    <File>awk-20191231_lab.xml</File>
    <File>awk-20191231_pre.xml</File>
    <File>ex-211x02182020xsubsid.htm</File>
    <File>ex-231x12312019xconsen.htm</File>
    <File>ex-311x12312019.htm</File>
    <File>ex-312x12312019.htm</File>
    <File>ex-321x12312019.htm</File>
    <File>ex-322x12312019.htm</File>
    <File>exh10161-pensionplanfo.htm</File>
    <File>exh10162-pensionplanam.htm</File>
    <File>exh420-securities.htm</File>
  </InputFiles>
  <SupplementalFiles>
    <File>fiveyrfuturecapexbycategory.jpg</File>
    <File>regrevenuebytype.jpg</File>
    <File>sourceswatersupply.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/us-gaap/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2019-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
