<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.25.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>276</ContextCount>
  <ElementCount>425</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>60</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>8</UnitCount>
  <MyReports>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>75000 - Document - Document &amp; Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DocumentEntityInformation</Role>
      <ShortName>Document &amp; Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>75010 - Statement - Condensed Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/StatementCondensedConsolidatedBalanceSheets</Role>
      <ShortName>Condensed Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>75030 - Statement - Condensed Consolidated Balance Sheets (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/StatementCondensedConsolidatedBalanceSheetsParentheticals</Role>
      <ShortName>Condensed Consolidated Balance Sheets (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>75050 - Statement - Consolidated Statements of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/ConsolidatedStatementsofComprehensiveIncome</Role>
      <ShortName>Consolidated Statements of Comprehensive Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>75060 - Statement - Condensed Consolidated Statements of Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/StatementCondensedConsolidatedStatementsOfStockholdersEquity</Role>
      <ShortName>Condensed Consolidated Statements of Stockholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>75070 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/ConsolidatedStatementsofCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>995410 - Disclosure - Pay vs Performance Disclosure</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/PvpDisclosure</Role>
      <ShortName>Pay vs Performance Disclosure</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.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>8</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>995455 - Disclosure - Organization, Consolidation and Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/OrganizationConsolidationandPresentation</Role>
      <ShortName>Organization, Consolidation and Presentation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>995465 - Disclosure - Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureSignificantAccountingPolicies</Role>
      <ShortName>Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>995475 - Disclosure - Disaggregation of Relevant Expense Captions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureDisaggregationOfRelevantExpenseCaptions</Role>
      <ShortName>Disaggregation of Relevant Expense Captions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>995485 - Disclosure - Discontinued Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureDiscontinuedOperations1</Role>
      <ShortName>Discontinued Operations</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>995495 - Disclosure - Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/Investments</Role>
      <ShortName>Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>995505 - Disclosure - Earnings per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/EarningsperShare</Role>
      <ShortName>Earnings per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>995515 - Disclosure - Property and Equipment, Net</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/PropertyandEquipmentNet</Role>
      <ShortName>Property and Equipment, Net</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>995525 - Disclosure - Goodwill and Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/GoodwillandIntangibleAssets</Role>
      <ShortName>Goodwill and Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>995535 - Disclosure - Reinsurance</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/Reinsurance</Role>
      <ShortName>Reinsurance</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>995545 - Disclosure - Liability for Unpaid Losses and Loss Adjustment Expenses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/LiabilityforUnpaidLossesandLossAdjustmentExpenses</Role>
      <ShortName>Liability for Unpaid Losses and Loss Adjustment Expenses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>995555 - Disclosure - Long-Term Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/LongTermDebt</Role>
      <ShortName>Long-Term Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>995565 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/CommitmentsandContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>995575 - Disclosure - Allowance For Expected Credit Losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureAllowanceForExpectedCreditLosses</Role>
      <ShortName>Allowance For Expected Credit Losses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>995585 - Disclosure - Statutory Accounting and Regulation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/StatutoryAccountingandRegulation</Role>
      <ShortName>Statutory Accounting and Regulation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>995605 - Disclosure - Accumulated Other Comprehensive Loss</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureAccumulatedOtherComprehensiveLoss</Role>
      <ShortName>Accumulated Other Comprehensive Loss</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>995615 - Disclosure - Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/StockholdersEquity</Role>
      <ShortName>Stockholders' Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>995625 - Disclosure - Stock-Based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/StockBasedCompensation</Role>
      <ShortName>Stock-Based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>995635 - Disclosure - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/SubsequentEvents</Role>
      <ShortName>Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>995645 - Disclosure - Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/SignificantAccountingPoliciesPolicies</Role>
      <ShortName>Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureSignificantAccountingPolicies</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>995655 - Disclosure - Disaggregation of Relevant Expense Captions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureDisaggregationOfRelevantExpenseCaptionsTables</Role>
      <ShortName>Disaggregation of Relevant Expense Captions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureDisaggregationOfRelevantExpenseCaptions</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>995665 - Disclosure - Discontinued Operations (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureDiscontinuedOperationsTables</Role>
      <ShortName>Discontinued Operations (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureDiscontinuedOperations1</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>995675 - Disclosure - Investments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/InvestmentsTables</Role>
      <ShortName>Investments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/Investments</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>995685 - Disclosure - Earnings per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/EarningsperShareTables</Role>
      <ShortName>Earnings per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/EarningsperShare</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>995695 - Disclosure - Property and Equipment, Net (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/PropertyandEquipmentNetTables</Role>
      <ShortName>Property and Equipment, Net (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/PropertyandEquipmentNet</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995705 - Disclosure - Goodwill and Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/GoodwillandIntangibleAssetsTables</Role>
      <ShortName>Goodwill and Intangible Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/GoodwillandIntangibleAssets</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>995715 - Disclosure - Reinsurance (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/ReinsuranceTables</Role>
      <ShortName>Reinsurance (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/Reinsurance</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>995725 - Disclosure - Liability for Unpaid Losses and Loss Adjustment Expense (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/LiabilityforUnpaidLossesandLossAdjustmentExpenseTables</Role>
      <ShortName>Liability for Unpaid Losses and Loss Adjustment Expense (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/LiabilityforUnpaidLossesandLossAdjustmentExpenses</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>995735 - Disclosure - Long-Term Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/LongTermDebtTables</Role>
      <ShortName>Long-Term Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/LongTermDebt</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>995745 - Disclosure - Commitments and Contingencies Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/CommitmentsandContingenciesLeasesTables</Role>
      <ShortName>Commitments and Contingencies Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/CommitmentsandContingencies</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>995755 - Disclosure - Allowance For Expected Credit Losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureAllowanceForExpectedCreditLossesTables</Role>
      <ShortName>Allowance For Expected Credit Losses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureAllowanceForExpectedCreditLosses</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>995765 - Disclosure - Statutory Accounting and Regulation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureStatutoryAccountingAndRegulationTables</Role>
      <ShortName>Statutory Accounting and Regulation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/StatutoryAccountingandRegulation</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>995775 - Disclosure - Accumulated Other Comprehensive Loss (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureAccumulatedOtherComprehensiveLossTables</Role>
      <ShortName>Accumulated Other Comprehensive Loss (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureAccumulatedOtherComprehensiveLoss</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>995795 - Disclosure - Stock-Based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/StockBasedCompensationTables</Role>
      <ShortName>Stock-Based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/StockBasedCompensation</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>995805 - Disclosure - Other Income and Expenses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/OtherIncomeandExpensesTables</Role>
      <ShortName>Other Income and Expenses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>995815 - Disclosure - Organization, Consolidation and Presentation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/OrganizationConsolidationandPresentationDetails</Role>
      <ShortName>Organization, Consolidation and Presentation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/OrganizationConsolidationandPresentation</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>995835 - Disclosure - Disaggregation of Relevant Expense Captions - Schedule of Other Operating Cost and Expense, by Component (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureDisaggregationOfRelevantExpenseCaptionsScheduleOfOtherOperatingCostAndExpenseByComponentDetails</Role>
      <ShortName>Disaggregation of Relevant Expense Captions - Schedule of Other Operating Cost and Expense, by Component (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>995845 - Disclosure - Disaggregation Of Relevant Expense Captions - Schedule Of Other Operating Cost And Expense, By Component (Parenthetical) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureDisaggregationOfRelevantExpenseCaptionsScheduleOfOtherOperatingCostAndExpenseByComponentParentheticalDetails</Role>
      <ShortName>Disaggregation Of Relevant Expense Captions - Schedule Of Other Operating Cost And Expense, By Component (Parenthetical) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>995855 - Disclosure - Discontinued Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureDiscontinuedOperationsDetails</Role>
      <ShortName>Discontinued Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureDiscontinuedOperationsTables</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>995865 - Disclosure - Discontinued Operations - Schedule of Results of Ops IIC (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureDiscontinuedOperationsScheduleOfResultsOfOpsIicDetails</Role>
      <ShortName>Discontinued Operations - Schedule of Results of Ops IIC (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>995875 - Disclosure - Discontinued Operations - Schedule of Results of Ops IIC (Parenthetical) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureDiscontinuedOperationsScheduleOfResultsOfOpsIicParentheticalDetails</Role>
      <ShortName>Discontinued Operations - Schedule of Results of Ops IIC (Parenthetical) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>995885 - Disclosure - Discontinued Operations - Schedule of Assets &amp; Liabilities IIC (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureDiscontinuedOperationsScheduleOfAssetsLiabilitiesIicDetails</Role>
      <ShortName>Discontinued Operations - Schedule of Assets &amp; Liabilities IIC (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>995895 - Disclosure - Investments - Schedule of Available-for-sale Securities Reconciliation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/InvestmentsScheduleofAvailableforsaleSecuritiesReconciliationDetails</Role>
      <ShortName>Investments - Schedule of Available-for-sale Securities Reconciliation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>995905 - Disclosure - Investments - Equity Securities, FV-NI (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/InvestmentsEquitySecuritiesFVNIDetails</Role>
      <ShortName>Investments - Equity Securities, FV-NI (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>995915 - Disclosure - Investments - Schedule of Realized Gain (Loss) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/InvestmentsScheduleofRealizedGainLossDetails</Role>
      <ShortName>Investments - Schedule of Realized Gain (Loss) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>995925 - Disclosure - Investments - Investments Classified by Contractual Maturity Date (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/InvestmentsInvestmentsClassifiedbyContractualMaturityDateDetails</Role>
      <ShortName>Investments - Investments Classified by Contractual Maturity Date (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>995935 - Disclosure - Investments - Investment Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/InvestmentsInvestmentIncomeDetails</Role>
      <ShortName>Investments - Investment Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>995945 - Disclosure - Investments - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureInvestmentsAdditionalInformationDetails</Role>
      <ShortName>Investments - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>995955 - Disclosure - Investments - Schedule of Unrealized Loss on Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/InvestmentsScheduleofUnrealizedLossonInvestmentsDetails</Role>
      <ShortName>Investments - Schedule of Unrealized Loss on Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>995965 - Disclosure - Investments - Schedule of Fair Value of Financial Instruments Measured on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/InvestmentsScheduleofFairValueofFinancialInstrumentsMeasuredonaRecurringBasisDetails</Role>
      <ShortName>Investments - Schedule of Fair Value of Financial Instruments Measured on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>995985 - Disclosure - Investments - Schedule of Other Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/InvestmentsScheduleofOtherInvestmentsDetails</Role>
      <ShortName>Investments - Schedule of Other Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>995995 - Disclosure - Investments - Restricted Cash (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/InvestmentsRestrictedCashDetails</Role>
      <ShortName>Investments - Restricted Cash (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>996015 - Disclosure - Earnings per Share - Schedule of Earnings per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/EarningsperShareScheduleofEarningsperShareDetails</Role>
      <ShortName>Earnings per Share - Schedule of Earnings per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>996025 - Disclosure - Property and Equipment, Net - Schedule of Property and Equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/PropertyandEquipmentNetScheduleofPropertyandEquipmentDetails</Role>
      <ShortName>Property and Equipment, Net - Schedule of Property and Equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>996035 - Disclosure - Property and Equipment, Net (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/PropertyandEquipmentNetDetails</Role>
      <ShortName>Property and Equipment, Net (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/PropertyandEquipmentNetTables</ParentRole>
      <Position>62</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>996045 - Disclosure - Goodwill and Intangible Assets - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/GoodwillandIntangibleAssetsNarrativeDetails</Role>
      <ShortName>Goodwill and Intangible Assets - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>996055 - Disclosure - Goodwill and Intangible Assets - Summary of Intangible Assets Excluding Goodwill (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureGoodwillAndIntangibleAssetsSummaryOfIntangibleAssetsExcludingGoodwillDetails</Role>
      <ShortName>Goodwill and Intangible Assets - Summary of Intangible Assets Excluding Goodwill (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>996065 - Disclosure - Goodwill and Intangible Assets - Summary of Intangible Assets Subject to Amortization and Not Fully Amortized (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureGoodwillAndIntangibleAssetsSummaryOfIntangibleAssetsSubjectToAmortizationAndNotFullyAmortizedDetails</Role>
      <ShortName>Goodwill and Intangible Assets - Summary of Intangible Assets Subject to Amortization and Not Fully Amortized (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>996075 - Disclosure - Goodwill and Intangible Assets - Schedule of Finite-Lived Intangible Assets, Estimated Amortization Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureGoodwillAndIntangibleAssetsScheduleOfFinitelivedIntangibleAssetsEstimatedAmortizationExpenseDetails</Role>
      <ShortName>Goodwill and Intangible Assets - Schedule of Finite-Lived Intangible Assets, Estimated Amortization Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>996095 - Disclosure - Reinsurance - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureReinsuranceAdditionalInformationDetails</Role>
      <ShortName>Reinsurance - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>996105 - Disclosure - Reinsurance - Summary of Premium Received and Capital at Risk (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureReinsuranceSummaryOfPremiumReceivedAndCapitalAtRiskDetails</Role>
      <ShortName>Reinsurance - Summary of Premium Received and Capital at Risk (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>996115 - Disclosure - Reinsurance - Schedule of External Quota Share Agreements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureReinsuranceScheduleOfExternalQuotaShareAgreementsDetails</Role>
      <ShortName>Reinsurance - Schedule of External Quota Share Agreements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>996125 - Disclosure - Reinsurance - Schedule of External Quota Share Agreements (Parenthetical) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureReinsuranceScheduleOfExternalQuotaShareAgreementsParentheticalDetails</Role>
      <ShortName>Reinsurance - Schedule of External Quota Share Agreements (Parenthetical) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>996135 - Disclosure - Reinsurance - Reinsurance Recoverables (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureReinsuranceReinsuranceRecoverablesDetails</Role>
      <ShortName>Reinsurance - Reinsurance Recoverables (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>996145 - Disclosure - Liability for Unpaid Losses and Loss Adjustment Expenses - Schedule of Liability for Unpaid Claims and Claims Adjustment Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureLiabilityForUnpaidLossesAndLossAdjustmentExpensesScheduleOfLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseDetails</Role>
      <ShortName>Liability for Unpaid Losses and Loss Adjustment Expenses - Schedule of Liability for Unpaid Claims and Claims Adjustment Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>996155 - Disclosure - Long-Term Debt - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureLongtermDebtAdditionalInformationDetails</Role>
      <ShortName>Long-Term Debt - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>996175 - Disclosure - Long-Term Debt - Debt Issuance Cost Rollforward (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureLongtermDebtDebtIssuanceCostRollforwardDetails</Role>
      <ShortName>Long-Term Debt - Debt Issuance Cost Rollforward (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>996185 - Disclosure - Commitments and Contingencies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureCommitmentsAndContingenciesAdditionalInformationDetails</Role>
      <ShortName>Commitments and Contingencies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>996195 - Disclosure - Commitments and Contingencies - Summary of Operating and Lease Asset and Liability Balances (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureCommitmentsAndContingenciesSummaryOfOperatingAndLeaseAssetAndLiabilityBalancesDetails</Role>
      <ShortName>Commitments and Contingencies - Summary of Operating and Lease Asset and Liability Balances (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>996205 - Disclosure - Commitments and Contingencies - Summary of Lease Expenses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureCommitmentsAndContingenciesSummaryOfLeaseExpensesDetails</Role>
      <ShortName>Commitments and Contingencies - Summary of Lease Expenses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>996215 - Disclosure - Commitments and Contingencies - Schedule of Future Minimum Gross Lease Payments Relating to the Non-Cancellable Operating Lease Agreements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureCommitmentsAndContingenciesScheduleOfFutureMinimumGrossLeasePaymentsRelatingToTheNoncancellableOperatingLeaseAgreementsDetails</Role>
      <ShortName>Commitments and Contingencies - Schedule of Future Minimum Gross Lease Payments Relating to the Non-Cancellable Operating Lease Agreements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>996225 - Disclosure - Commitments and Contingencies - Summary of Weighted Average Remaining Lease Term and Discount Rate Related to Operating Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureCommitmentsAndContingenciesSummaryOfWeightedAverageRemainingLeaseTermAndDiscountRateRelatedToOperatingLeasesDetails</Role>
      <ShortName>Commitments and Contingencies - Summary of Weighted Average Remaining Lease Term and Discount Rate Related to Operating Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>996235 - Disclosure - Allowance For Expected Credit Losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureAllowanceForExpectedCreditLossesDetails</Role>
      <ShortName>Allowance For Expected Credit Losses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureAllowanceForExpectedCreditLossesTables</ParentRole>
      <Position>80</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>996245 - Disclosure - Statutory Accounting and Regulation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureStatutoryAccountingAndRegulationDetails</Role>
      <ShortName>Statutory Accounting and Regulation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureStatutoryAccountingAndRegulationTables</ParentRole>
      <Position>81</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>996255 - Disclosure - Statutory Accounting and Regulation - Schedule of Statutory Accounting and Regulation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureStatutoryAccountingAndRegulationScheduleOfStatutoryAccountingAndRegulationDetails</Role>
      <ShortName>Statutory Accounting and Regulation - Schedule of Statutory Accounting and Regulation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>996275 - Disclosure - Accumulated Other Comprehensive Loss - Schedule of Accumulated Other Comprehensive Income (Loss) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureAccumulatedOtherComprehensiveLossScheduleOfAccumulatedOtherComprehensiveIncomeLossDetails</Role>
      <ShortName>Accumulated Other Comprehensive Loss - Schedule of Accumulated Other Comprehensive Income (Loss) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>996295 - Disclosure - Stockholders' Equity - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureStockholdersEquityAdditionalInformationDetails</Role>
      <ShortName>Stockholders' Equity - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>996305 - Disclosure - Stock-Based Compensation - Stock-based Compensation Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/StockBasedCompensationStockbasedCompensationExpenseDetails</Role>
      <ShortName>Stock-Based Compensation - Stock-based Compensation Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>996315 - Disclosure - Stock-Based Compensation - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/DisclosureStockbasedCompensationAdditionalInformationDetails</Role>
      <ShortName>Stock-Based Compensation - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>996325 - Disclosure - Stock-Based Compensation - Non-Vested Common Stock Grants (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/StockBasedCompensationNonVestedCommonStockGrantsDetails</Role>
      <ShortName>Stock-Based Compensation - Non-Vested Common Stock Grants (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>996335 - Disclosure - Stock-Based Compensation - Weighted-Average Assumptions - Stock Options (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/StockBasedCompensationWeightedAverageAssumptionsStockOptionsDetails</Role>
      <ShortName>Stock-Based Compensation - Weighted-Average Assumptions - Stock Options (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>996345 - Disclosure - Stock-Based Compensation - Non-Vested Stock Options (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/StockBasedCompensationNonVestedStockOptionsDetails</Role>
      <ShortName>Stock-Based Compensation - Non-Vested Stock Options (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="acic-20250630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>996355 - Disclosure - Subsequent Events (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.amcoastal.com/20250630/taxonomy/role/SubsequentEventsDetails</Role>
      <ShortName>Subsequent Events (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.amcoastal.com/20250630/taxonomy/role/SubsequentEvents</ParentRole>
      <Position>90</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-Q" isDefinitelyFs="true" isUsgaap="true" original="acic-20250630.htm">acic-20250630.htm</File>
    <File>acic-20250630.xsd</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="1">http://fasb.org/srt/2025</BaseTaxonomy>
    <BaseTaxonomy items="1220">http://fasb.org/us-gaap/2025</BaseTaxonomy>
    <BaseTaxonomy items="29">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2025</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>false</HasPresentationLinkbase>
  <HasCalculationLinkbase>false</HasCalculationLinkbase>
</FilingSummary>
