<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.25.4</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>891</ContextCount>
  <ElementCount>660</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>225</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>20</UnitCount>
  <MyReports>
    <Report instance="ajg-20251231.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-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>0000002 - Document - Audit Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/AuditInformation</Role>
      <ShortName>Audit Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.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>3</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.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>4</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.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>5</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.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>6</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.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>7</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.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>8</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.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>9</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.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>10</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.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>11</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.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>12</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.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>13</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>9952162 - Disclosure - Fixed Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/FixedAssets</Role>
      <ShortName>Fixed Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>9952163 - Disclosure - Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/IntangibleAssets</Role>
      <ShortName>Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>9952164 - 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>16</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>9952165 - 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>17</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>9952166 - 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>18</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>9952167 - Disclosure - Deferred Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/DeferredCompensation</Role>
      <ShortName>Deferred Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>9952168 - Disclosure - Restricted Stock, Performance Share and Cash Awards</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RestrictedStockPerformanceShareandCashAwards</Role>
      <ShortName>Restricted Stock, Performance Share and Cash Awards</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>9952169 - Disclosure - Retirement Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RetirementPlans</Role>
      <ShortName>Retirement Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>9952170 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/Leases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>9952171 - 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>23</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>9952172 - 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>24</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>9952173 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>9952174 - 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>26</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>9952175 - 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>27</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>9952176 - Disclosure - Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/SegmentInformation</Role>
      <ShortName>Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>9952177 - Disclosure - Schedule II. Valuation and Qualifying Accounts</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/ScheduleIIValuationandQualifyingAccounts</Role>
      <ShortName>Schedule II. Valuation and Qualifying Accounts</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.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>30</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.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>31</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>995447 - Disclosure - Insider Trading Policies and Procedures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingPoliciesProc</Role>
      <ShortName>Insider Trading Policies and Procedures</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995550 - Disclosure - Cybersecurity Risk Management and Strategy Disclosure</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/cyd/role/CybersecurityRiskManagementAndStrategyDisclosure</Role>
      <ShortName>Cybersecurity Risk Management and Strategy Disclosure</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.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>34</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9955512 - Disclosure - Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/SummaryofSignificantAccountingPoliciesTables</Role>
      <ShortName>Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/SummaryofSignificantAccountingPolicies</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9955513 - 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>36</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9955514 - 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>37</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9955515 - Disclosure - Fixed Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/FixedAssetsTables</Role>
      <ShortName>Fixed Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/FixedAssets</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9955516 - 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>39</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9955517 - 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>40</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9955518 - 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>41</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9955519 - 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>42</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9955520 - Disclosure - Restricted Stock, Performance Share and Cash Awards (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RestrictedStockPerformanceShareandCashAwardsTables</Role>
      <ShortName>Restricted Stock, Performance Share and Cash Awards (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/RestrictedStockPerformanceShareandCashAwards</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9955521 - Disclosure - Retirement Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RetirementPlansTables</Role>
      <ShortName>Retirement Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/RetirementPlans</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9955522 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/LeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/Leases</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9955523 - Disclosure - Derivatives and Hedging Activity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/DerivativesandHedgingActivityTables</Role>
      <ShortName>Derivatives and Hedging Activity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/DerivativesandHedgingActivity</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9955524 - 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>47</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9955525 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/IncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/role/IncomeTaxes</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>9955526 - 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>49</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>9955527 - 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>50</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>9955528 - 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>51</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>9955529 - 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>52</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>9955530 - Disclosure - Summary of Significant Accounting Policies - Summary of Estimated Useful Life of Fixed Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/SummaryofSignificantAccountingPoliciesSummaryofEstimatedUsefulLifeofFixedAssetsDetail</Role>
      <ShortName>Summary of Significant Accounting Policies - Summary of Estimated Useful Life of Fixed Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>9955531 - 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>54</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>9955532 - 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>55</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>9955533 - 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>56</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>9955534 - 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>57</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>9955535 - 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>58</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>9955536 - 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>59</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>9955537 - 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>60</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>9955538 - 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>61</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>9955539 - Disclosure - Fixed Assets - Summary of Fixed Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/FixedAssetsSummaryofFixedAssetsDetail</Role>
      <ShortName>Fixed Assets - Summary of Fixed Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>9955540 - 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>63</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>9955541 - 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>64</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>9955542 - 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>65</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>9955543 - 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>66</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>9955544 - 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>67</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>9955545 - Disclosure - Credit and Other Debt Agreements - Senior Notes - Additional Information (Detail)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.ajg.com/role/CreditandOtherDebtAgreementsSeniorNotesAdditionalInformationDetail</Role>
      <ShortName>Credit and Other Debt Agreements - Senior Notes - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>9955546 - Disclosure - Credit and Other Debt Agreements - Note Purchase Agreements - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/CreditandOtherDebtAgreementsNotePurchaseAgreementsAdditionalInformationDetail</Role>
      <ShortName>Credit and Other Debt Agreements - Note Purchase Agreements - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>9955547 - Disclosure - Credit and Other Debt Agreements - Credit Agreement - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/CreditandOtherDebtAgreementsCreditAgreementAdditionalInformationDetail</Role>
      <ShortName>Credit and Other Debt Agreements - Credit Agreement - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>9955548 - Disclosure - Credit and Other Debt Agreements - Premium Financing Debt Facility - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/CreditandOtherDebtAgreementsPremiumFinancingDebtFacilityAdditionalInformationDetail</Role>
      <ShortName>Credit and Other Debt Agreements - Premium Financing Debt Facility - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>9955549 - 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>72</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>9955550 - 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>73</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>9955551 - 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>74</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>9955552 - 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>75</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>9955553 - 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>76</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>9955554 - 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>77</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>9955555 - 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>78</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>9955556 - Disclosure - Restricted Stock, Performance Share and Cash Awards - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RestrictedStockPerformanceShareandCashAwardsAdditionalInformationDetail</Role>
      <ShortName>Restricted Stock, Performance Share and Cash Awards - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>9955557 - Disclosure - Restricted Stock, Performance Share and Cash Awards - Schedule of Restricted Stock Awards Vesting Periods (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RestrictedStockPerformanceShareandCashAwardsScheduleofRestrictedStockAwardsVestingPeriodsDetail</Role>
      <ShortName>Restricted Stock, Performance Share and Cash Awards - Schedule of Restricted Stock Awards Vesting Periods (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>9955558 - Disclosure - Retirement Plans - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RetirementPlansAdditionalInformationDetail</Role>
      <ShortName>Retirement Plans - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>9955559 - Disclosure - Retirement Plans - Reconciliation of Balances of Pension Benefit Obligation and Fair Value of Plan Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RetirementPlansReconciliationofBalancesofPensionBenefitObligationandFairValueofPlanAssetsDetail</Role>
      <ShortName>Retirement Plans - Reconciliation of Balances of Pension Benefit Obligation and Fair Value of Plan Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>9955560 - Disclosure - Retirement Plans - Components of Net Periodic Pension Benefit Cost and Other Changes in Plan Assets and Obligations Recognized in Earnings and Other Comprehensive Earnings (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RetirementPlansComponentsofNetPeriodicPensionBenefitCostandOtherChangesinPlanAssetsandObligationsRecognizedinEarningsandOtherComprehensiveEarningsDetail</Role>
      <ShortName>Retirement Plans - Components of Net Periodic Pension Benefit Cost and Other Changes in Plan Assets and Obligations Recognized in Earnings and Other Comprehensive Earnings (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>9955561 - Disclosure - Retirement Plans - Weighted Average Assumptions of Pension Benefit Obligation and Net Periodic Pension Benefit Cost (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RetirementPlansWeightedAverageAssumptionsofPensionBenefitObligationandNetPeriodicPensionBenefitCostDetail</Role>
      <ShortName>Retirement Plans - Weighted Average Assumptions of Pension Benefit Obligation and Net Periodic Pension Benefit Cost (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>9955562 - Disclosure - Retirement Plans - Schedule of Benefit Payments Expected to be Paid by Plan (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RetirementPlansScheduleofBenefitPaymentsExpectedtobePaidbyPlanDetail</Role>
      <ShortName>Retirement Plans - Schedule of Benefit Payments Expected to be Paid by Plan (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>9955563 - Disclosure - Retirement Plans - Summary of Plan's Weighted Average Asset Allocations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RetirementPlansSummaryofPlansWeightedAverageAssetAllocationsDetail</Role>
      <ShortName>Retirement Plans - Summary of Plan's Weighted Average Asset Allocations (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>9955564 - Disclosure - Retirement Plans - Summary of Plan's Assets Carried at Fair Value (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RetirementPlansSummaryofPlansAssetsCarriedatFairValueDetail</Role>
      <ShortName>Retirement Plans - Summary of Plan's Assets Carried at Fair Value (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>9955565 - Disclosure - Retirement Plans - Reconciliation of Beginning and Ending Balances for Level 3 Assets of Plan Measured at Fair Value (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/RetirementPlansReconciliationofBeginningandEndingBalancesforLevel3AssetsofPlanMeasuredatFairValueDetail</Role>
      <ShortName>Retirement Plans - Reconciliation of Beginning and Ending Balances for Level 3 Assets of Plan Measured at Fair Value (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>9955566 - Disclosure - Leases - Components of Lease Expense (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/LeasesComponentsofLeaseExpenseDetail</Role>
      <ShortName>Leases - Components of Lease Expense (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>9955567 - Disclosure - Leases - Summary of Supplemental Cash Flow Information Related to Leases (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/LeasesSummaryofSupplementalCashFlowInformationRelatedtoLeasesDetail</Role>
      <ShortName>Leases - Summary of Supplemental Cash Flow Information Related to Leases (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>9955568 - Disclosure - Leases - Summary of Supplemental Balance Sheet Information Related to Leases (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/LeasesSummaryofSupplementalBalanceSheetInformationRelatedtoLeasesDetail</Role>
      <ShortName>Leases - Summary of Supplemental Balance Sheet Information Related to Leases (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>9955569 - Disclosure - Leases - Maturities of Operating Lease Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/LeasesMaturitiesofOperatingLeaseLiabilitiesDetail</Role>
      <ShortName>Leases - Maturities of Operating Lease Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>9955570 - Disclosure - Leases - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/LeasesAdditionalInformationDetail</Role>
      <ShortName>Leases - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>9955571 - Disclosure - Derivatives and Hedging Activity - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/DerivativesandHedgingActivityAdditionalInformationDetail</Role>
      <ShortName>Derivatives and Hedging Activity - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>9955572 - Disclosure - Derivatives and Hedging Activity - Summary of Notional and Fair Values of Derivative Instruments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/DerivativesandHedgingActivitySummaryofNotionalandFairValuesofDerivativeInstrumentsDetail</Role>
      <ShortName>Derivatives and Hedging Activity - Summary of Notional and Fair Values of Derivative Instruments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>9955573 - Disclosure - Derivatives and Hedging Activity - Summary of Amounts of Derivative Gains (Losses) Recognized In Accumulated Other Comprehensive Loss (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/DerivativesandHedgingActivitySummaryofAmountsofDerivativeGainsLossesRecognizedInAccumulatedOtherComprehensiveLossDetail</Role>
      <ShortName>Derivatives and Hedging Activity - Summary of Amounts of Derivative Gains (Losses) Recognized In Accumulated Other Comprehensive Loss (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>96</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>9955574 - 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>97</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>9955575 - Disclosure - Commitments, Contingencies and Off-Balance Sheet Arrangements - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/CommitmentsContingenciesandOffBalanceSheetArrangementsAdditionalInformationDetail</Role>
      <ShortName>Commitments, Contingencies and Off-Balance Sheet Arrangements - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>98</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>9955576 - 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>99</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>9955577 - Disclosure - Commitments, Contingencies and Off-Balance Sheet Arrangements - Outstanding Letters of Credit, Financial Guarantees and Funding Commitments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/CommitmentsContingenciesandOffBalanceSheetArrangementsOutstandingLettersofCreditFinancialGuaranteesandFundingCommitmentsDetail</Role>
      <ShortName>Commitments, Contingencies and Off-Balance Sheet Arrangements - Outstanding Letters of Credit, Financial Guarantees and Funding Commitments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>100</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>9955578 - Disclosure - Income Taxes - Components of Earnings Before Income Taxes and Provision for Income Taxes (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/IncomeTaxesComponentsofEarningsBeforeIncomeTaxesandProvisionforIncomeTaxesDetail</Role>
      <ShortName>Income Taxes - Components of Earnings Before Income Taxes and Provision for Income Taxes (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>101</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>9955579 - Disclosure - Income Taxes - Reconciliation of Provision for Income Taxes with Federal Statutory Income Tax Rate - After Adoption of ASU 2023-09 (Detail) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/IncomeTaxesReconciliationofProvisionforIncomeTaxeswithFederalStatutoryIncomeTaxRateAfterAdoptionofASU202309DetailDetails</Role>
      <ShortName>Income Taxes - Reconciliation of Provision for Income Taxes with Federal Statutory Income Tax Rate - After Adoption of ASU 2023-09 (Detail) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>102</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>9955580 - Disclosure - Income Taxes - Reconciliation of Provision for Income Taxes with Federal Statutory Income Tax Rate - Before Adoption of ASU 2023-09 (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/IncomeTaxesReconciliationofProvisionforIncomeTaxeswithFederalStatutoryIncomeTaxRateBeforeAdoptionofASU202309Detail</Role>
      <ShortName>Income Taxes - Reconciliation of Provision for Income Taxes with Federal Statutory Income Tax Rate - Before Adoption of ASU 2023-09 (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>103</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>9955581 - Disclosure - Income Taxes - Gross Unrecognized Tax Benefit (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/IncomeTaxesGrossUnrecognizedTaxBenefitDetail</Role>
      <ShortName>Income Taxes - Gross Unrecognized Tax Benefit (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>104</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>9955582 - Disclosure - Income Taxes - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/IncomeTaxesAdditionalInformationDetail</Role>
      <ShortName>Income Taxes - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>105</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>9955583 - Disclosure - Income Taxes - Deferred Tax Assets and Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/IncomeTaxesDeferredTaxAssetsandLiabilitiesDetail</Role>
      <ShortName>Income Taxes - Deferred Tax Assets and Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>106</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>9955584 - 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>107</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R108.htm</HtmlFileName>
      <LongName>9955585 - 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>108</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R109.htm</HtmlFileName>
      <LongName>9955586 - 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>109</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R110.htm</HtmlFileName>
      <LongName>9955587 - 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>110</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R111.htm</HtmlFileName>
      <LongName>9955588 - Disclosure - Accumulated Other Comprehensive Loss - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/AccumulatedOtherComprehensiveLossAdditionalInformationDetail</Role>
      <ShortName>Accumulated Other Comprehensive Loss - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>111</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R112.htm</HtmlFileName>
      <LongName>9955589 - 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>112</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R113.htm</HtmlFileName>
      <LongName>9955590 - 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>113</Position>
    </Report>
    <Report instance="ajg-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R114.htm</HtmlFileName>
      <LongName>9955591 - Disclosure - Schedule II - Valuation and Qualifying Accounts (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/role/ScheduleIIValuationandQualifyingAccountsDetail</Role>
      <ShortName>Schedule II - Valuation and Qualifying Accounts (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>114</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-K" isDefinitelyFs="true" isUsgaap="true" original="ajg-20251231.htm">ajg-20251231.htm</File>
    <File>ajg-20251231.xsd</File>
    <File>ajg-20251231_cal.xml</File>
    <File>ajg-20251231_def.xml</File>
    <File>ajg-20251231_lab.xml</File>
    <File>ajg-20251231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="1">http://fasb.org/srt/2024</BaseTaxonomy>
    <BaseTaxonomy items="2086">http://fasb.org/us-gaap/2024</BaseTaxonomy>
    <BaseTaxonomy items="16">http://xbrl.sec.gov/cyd/2024</BaseTaxonomy>
    <BaseTaxonomy items="38">http://xbrl.sec.gov/dei/2024</BaseTaxonomy>
    <BaseTaxonomy items="5">http://xbrl.sec.gov/ecd/2024</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
