<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>360</ContextCount>
  <ElementCount>359</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>80</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>8</UnitCount>
  <MyReports>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0000001 - Document - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/CoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>9952151 - Statement - Consolidated Statement of Earnings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/ConsolidatedStatementofEarnings</Role>
      <ShortName>Consolidated Statement of Earnings</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>9952152 - Statement - Consolidated Statement of Comprehensive Earnings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/ConsolidatedStatementofComprehensiveEarnings</Role>
      <ShortName>Consolidated Statement of Comprehensive Earnings</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>9952153 - Statement - Consolidated Balance Sheet</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/ConsolidatedBalanceSheet</Role>
      <ShortName>Consolidated Balance Sheet</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>9952154 - Statement - Consolidated Balance Sheet (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/ConsolidatedBalanceSheetParenthetical</Role>
      <ShortName>Consolidated Balance Sheet (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>9952155 - Statement - Consolidated Statement of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/ConsolidatedStatementofCashFlows</Role>
      <ShortName>Consolidated Statement of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>9952156 - Statement - Consolidated Statement of Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/ConsolidatedStatementofStockholdersEquity</Role>
      <ShortName>Consolidated Statement of Stockholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>9952157 - Statement - Consolidated Statement of Stockholders' Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/ConsolidatedStatementofStockholdersEquityParenthetical</Role>
      <ShortName>Consolidated Statement of Stockholders' Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>9952158 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/SummaryofSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>9952159 - Disclosure - Effect of New Accounting Pronouncements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/EffectofNewAccountingPronouncements</Role>
      <ShortName>Effect of New Accounting Pronouncements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>9952160 - Disclosure - Business Combinations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/BusinessCombinations</Role>
      <ShortName>Business Combinations</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>9952161 - Disclosure - Contracts with Customers</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/ContractswithCustomers</Role>
      <ShortName>Contracts with Customers</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>9952162 - Disclosure - Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/IntangibleAssets</Role>
      <ShortName>Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>9952163 - Disclosure - Credit and Other Debt Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/CreditandOtherDebtAgreements</Role>
      <ShortName>Credit and Other Debt Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>9952164 - Disclosure - Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/EarningsPerShare</Role>
      <ShortName>Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>9952165 - Disclosure - Stock Option Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/StockOptionPlans</Role>
      <ShortName>Stock Option Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>9952166 - Disclosure - Deferred Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/DeferredCompensation</Role>
      <ShortName>Deferred Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>9952167 - Disclosure - Restricted Stock and Performance Share Awards</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RestrictedStockandPerformanceShareAwards</Role>
      <ShortName>Restricted Stock and Performance Share Awards</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>9952168 - Disclosure - Derivatives and Hedging Activity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/DerivativesandHedgingActivity</Role>
      <ShortName>Derivatives and Hedging Activity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>9952169 - Disclosure - Commitments, Contingencies and Off-Balance Sheet Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/CommitmentsContingenciesandOffBalanceSheetArrangements</Role>
      <ShortName>Commitments, Contingencies and Off-Balance Sheet Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>9952170 - Disclosure - Supplemental Disclosures of Cash Flow Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/SupplementalDisclosuresofCashFlowInformation</Role>
      <ShortName>Supplemental Disclosures of Cash Flow Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>9952171 - Disclosure - Accumulated Other Comprehensive Loss</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/AccumulatedOtherComprehensiveLoss</Role>
      <ShortName>Accumulated Other Comprehensive Loss</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>9952172 - Disclosure - Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/SegmentInformation</Role>
      <ShortName>Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.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>24</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>9955511 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/SummaryofSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.ajg.com/role/SummaryofSignificantAccountingPolicies</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>9955512 - Disclosure - Business Combinations (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/BusinessCombinationsTables</Role>
      <ShortName>Business Combinations (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/BusinessCombinations</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>9955513 - Disclosure - Contracts with Customers (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/ContractswithCustomersTables</Role>
      <ShortName>Contracts with Customers (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/ContractswithCustomers</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>9955514 - Disclosure - Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/IntangibleAssetsTables</Role>
      <ShortName>Intangible Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/IntangibleAssets</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>9955515 - Disclosure - Credit and Other Debt Agreements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/CreditandOtherDebtAgreementsTables</Role>
      <ShortName>Credit and Other Debt Agreements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/CreditandOtherDebtAgreements</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>9955516 - Disclosure - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/EarningsPerShareTables</Role>
      <ShortName>Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/EarningsPerShare</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>9955517 - Disclosure - Stock Option Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/StockOptionPlansTables</Role>
      <ShortName>Stock Option Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/StockOptionPlans</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>9955518 - Disclosure - Deferred Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/DeferredCompensationTables</Role>
      <ShortName>Deferred Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/DeferredCompensation</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>9955519 - Disclosure - Restricted Stock and Performance Share Awards (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RestrictedStockandPerformanceShareAwardsTables</Role>
      <ShortName>Restricted Stock and Performance Share Awards (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/RestrictedStockandPerformanceShareAwards</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>9955520 - Disclosure - Commitments, Contingencies and Off-Balance Sheet Arrangements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/CommitmentsContingenciesandOffBalanceSheetArrangementsTables</Role>
      <ShortName>Commitments, Contingencies and Off-Balance Sheet Arrangements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/CommitmentsContingenciesandOffBalanceSheetArrangements</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9955521 - Disclosure - Supplemental Disclosures of Cash Flow Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/SupplementalDisclosuresofCashFlowInformationTables</Role>
      <ShortName>Supplemental Disclosures of Cash Flow Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/SupplementalDisclosuresofCashFlowInformation</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9955522 - Disclosure - Accumulated Other Comprehensive Loss (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/AccumulatedOtherComprehensiveLossTables</Role>
      <ShortName>Accumulated Other Comprehensive Loss (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/AccumulatedOtherComprehensiveLoss</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9955523 - Disclosure - Segment Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/SegmentInformationTables</Role>
      <ShortName>Segment Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/SegmentInformation</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9955524 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/SummaryofSignificantAccountingPoliciesAdditionalInformationDetail</Role>
      <ShortName>Summary of Significant Accounting Policies - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9955525 - Disclosure - Business Combinations - Acquisition Method for Recording Business Combinations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/BusinessCombinationsAcquisitionMethodforRecordingBusinessCombinationsDetail</Role>
      <ShortName>Business Combinations - Acquisition Method for Recording Business Combinations (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9955526 - Disclosure - Business Combinations - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/BusinessCombinationsAdditionalInformationDetail</Role>
      <ShortName>Business Combinations - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9955527 - Disclosure - Business Combinations - Summary of Estimated Fair Values of Net Assets Acquired (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/BusinessCombinationsSummaryofEstimatedFairValuesofNetAssetsAcquiredDetail</Role>
      <ShortName>Business Combinations - Summary of Estimated Fair Values of Net Assets Acquired (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9955528 - Disclosure - Business Combinations - Summary of Unaudited Pro Forma Historical Results (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/BusinessCombinationsSummaryofUnauditedProFormaHistoricalResultsDetail</Role>
      <ShortName>Business Combinations - Summary of Unaudited Pro Forma Historical Results (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9955529 - Disclosure - Contracts with Customers - Summary of Unbilled Receivables, Contract Assets and Contract Liabilities from Contracts with Customers (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/ContractswithCustomersSummaryofUnbilledReceivablesContractAssetsandContractLiabilitiesfromContractswithCustomersDetail</Role>
      <ShortName>Contracts with Customers - Summary of Unbilled Receivables, Contract Assets and Contract Liabilities from Contracts with Customers (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9955530 - Disclosure - Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/ContractswithCustomersSummaryofChangesinDeferredRevenueBalancesDetail</Role>
      <ShortName>Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9955531 - Disclosure - Contracts with Customers - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/ContractswithCustomersAdditionalInformationDetail</Role>
      <ShortName>Contracts with Customers - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9955532 - Disclosure - Contracts with Customers - Summary of Expected Revenue Related to Performance Obligations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/ContractswithCustomersSummaryofExpectedRevenueRelatedtoPerformanceObligationsDetail</Role>
      <ShortName>Contracts with Customers - Summary of Expected Revenue Related to Performance Obligations (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9955533 - Disclosure - Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/IntangibleAssetsCarryingAmountofGoodwillAllocatedbyDomesticandForeignOperationsDetail</Role>
      <ShortName>Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9955534 - Disclosure - Intangible Assets - Changes in Carrying Amount of Goodwill (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/IntangibleAssetsChangesinCarryingAmountofGoodwillDetail</Role>
      <ShortName>Intangible Assets - Changes in Carrying Amount of Goodwill (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>9955535 - Disclosure - Intangible Assets - Major Classes of Amortizable Intangible Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/IntangibleAssetsMajorClassesofAmortizableIntangibleAssetsDetail</Role>
      <ShortName>Intangible Assets - Major Classes of Amortizable Intangible Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>9955536 - Disclosure - Intangible Assets - Estimated Aggregate Amortization Expense (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/IntangibleAssetsEstimatedAggregateAmortizationExpenseDetail</Role>
      <ShortName>Intangible Assets - Estimated Aggregate Amortization Expense (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>9955537 - Disclosure - Credit and Other Debt Agreements - Summary of Corporate and Other Debt (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/CreditandOtherDebtAgreementsSummaryofCorporateandOtherDebtDetail</Role>
      <ShortName>Credit and Other Debt Agreements - Summary of Corporate and Other Debt (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>9955538 - Disclosure - Credit and Other Debt Agreements - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/CreditandOtherDebtAgreementsAdditionalInformationDetails</Role>
      <ShortName>Credit and Other Debt Agreements - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>9955539 - Disclosure - Earnings Per Share - Computation of Basic and Diluted Net EPS (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/EarningsPerShareComputationofBasicandDilutedNetEPSDetail</Role>
      <ShortName>Earnings Per Share - Computation of Basic and Diluted Net EPS (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>9955540 - Disclosure - Earnings Per Share - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/EarningsPerShareAdditionalInformationDetail</Role>
      <ShortName>Earnings Per Share - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>9955541 - Disclosure - Stock Option Plans - Additional information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/StockOptionPlansAdditionalinformationDetail</Role>
      <ShortName>Stock Option Plans - Additional information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>9955542 - Disclosure - Stock Option Plans - Stock Option Granted and Stock Option Compensation Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/StockOptionPlansStockOptionGrantedandStockOptionCompensationExpenseDetails</Role>
      <ShortName>Stock Option Plans - Stock Option Granted and Stock Option Compensation Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>9955543 - Disclosure - Stock Option Plans - Black-Scholes Option Pricing Model with Weighted Average (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/StockOptionPlansBlackScholesOptionPricingModelwithWeightedAverageDetail</Role>
      <ShortName>Stock Option Plans - Black-Scholes Option Pricing Model with Weighted Average (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>9955544 - Disclosure - Stock Option Plans - Stock Option Activity and Related Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/StockOptionPlansStockOptionActivityandRelatedInformationDetail</Role>
      <ShortName>Stock Option Plans - Stock Option Activity and Related Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>9955545 - Disclosure - Stock Option Plans - Stock Options Outstanding and Exercisable (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/StockOptionPlansStockOptionsOutstandingandExercisableDetail</Role>
      <ShortName>Stock Option Plans - Stock Options Outstanding and Exercisable (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>9955546 - Disclosure - Deferred Compensation - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/DeferredCompensationAdditionalInformationDetail</Role>
      <ShortName>Deferred Compensation - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>9955547 - Disclosure - Deferred Compensation - Deferred Equity Participation Plan and Sub-plan Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/DeferredCompensationDeferredEquityParticipationPlanandSubplanActivityDetails</Role>
      <ShortName>Deferred Compensation - Deferred Equity Participation Plan and Sub-plan Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>9955548 - Disclosure - Deferred Compensation - Deferred Cash Participation Plan Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/DeferredCompensationDeferredCashParticipationPlanActivityDetails</Role>
      <ShortName>Deferred Compensation - Deferred Cash Participation Plan Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>9955549 - Disclosure - Restricted Stock and Performance Share Awards - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RestrictedStockandPerformanceShareAwardsAdditionalInformationDetail</Role>
      <ShortName>Restricted Stock and Performance Share Awards - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>9955550 - Disclosure - Restricted Stock and Performance Share Awards - Restricted Stock Units Activity under the LTIP (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RestrictedStockandPerformanceShareAwardsRestrictedStockUnitsActivityundertheLTIPDetails</Role>
      <ShortName>Restricted Stock and Performance Share Awards - Restricted Stock Units Activity under the LTIP (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>9955551 - Disclosure - Restricted Stock and Performance Share Awards - Performance Share Award Activity under the LTIP (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RestrictedStockandPerformanceShareAwardsPerformanceShareAwardActivityundertheLTIPDetails</Role>
      <ShortName>Restricted Stock and Performance Share Awards - Performance Share Award Activity under the LTIP (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>9955552 - Disclosure - Derivatives and Hedging Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/DerivativesandHedgingActivityDetails</Role>
      <ShortName>Derivatives and Hedging Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.ajg.com/role/DerivativesandHedgingActivity</ParentRole>
      <Position>66</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>9955553 - Disclosure - Commitments, Contingencies and Off-Balance Sheet Arrangements - Contractual Obligations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/CommitmentsContingenciesandOffBalanceSheetArrangementsContractualObligationsDetail</Role>
      <ShortName>Commitments, Contingencies and Off-Balance Sheet Arrangements - Contractual Obligations (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>9955554 - Disclosure - Commitments, Contingencies and Off-Balance Sheet Arrangements - Off-Balance Sheet Commitments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/CommitmentsContingenciesandOffBalanceSheetArrangementsOffBalanceSheetCommitmentsDetail</Role>
      <ShortName>Commitments, Contingencies and Off-Balance Sheet Arrangements - Off-Balance Sheet Commitments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>9955555 - Disclosure - Supplemental Disclosures of Cash Flow Information - Cash Flow (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/SupplementalDisclosuresofCashFlowInformationCashFlowDetail</Role>
      <ShortName>Supplemental Disclosures of Cash Flow Information - Cash Flow (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.ajg.com/role/SupplementalDisclosuresofCashFlowInformationTables</ParentRole>
      <Position>69</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>9955556 - Disclosure - Supplemental Disclosures of Cash Flow Information - Cash Equivalents, Restricted Cash and Fiduciary Cash (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/SupplementalDisclosuresofCashFlowInformationCashEquivalentsRestrictedCashandFiduciaryCashDetail</Role>
      <ShortName>Supplemental Disclosures of Cash Flow Information - Cash Equivalents, Restricted Cash and Fiduciary Cash (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>9955557 - Disclosure - Supplemental Disclosures of Cash Flow Information - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/SupplementalDisclosuresofCashFlowInformationAdditionalInformationDetail</Role>
      <ShortName>Supplemental Disclosures of Cash Flow Information - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>9955558 - Disclosure - Accumulated Other Comprehensive Loss - Schedule of Accumulated Other Comprehensive Loss Attributable to Controlling Interests (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/AccumulatedOtherComprehensiveLossScheduleofAccumulatedOtherComprehensiveLossAttributabletoControllingInterestsDetail</Role>
      <ShortName>Accumulated Other Comprehensive Loss - Schedule of Accumulated Other Comprehensive Loss Attributable to Controlling Interests (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>9955559 - Disclosure - Segment Information - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/SegmentInformationAdditionalInformationDetail</Role>
      <ShortName>Segment Information - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="ajg-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>9955560 - Disclosure - Segment Information - Schedule of Segment Reporting Information by Segment (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/SegmentInformationScheduleofSegmentReportingInformationbySegmentDetail</Role>
      <ShortName>Segment Information - Schedule of Segment Reporting Information by Segment (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</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="ajg-20260331.htm">ajg-20260331.htm</File>
    <File>ajg-20260331.xsd</File>
    <File>ajg-20260331_cal.xml</File>
    <File>ajg-20260331_def.xml</File>
    <File>ajg-20260331_lab.xml</File>
    <File>ajg-20260331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="991">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>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
