<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.4</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>973</ContextCount>
  <ElementCount>721</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>248</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>20</UnitCount>
  <MyReports>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>100000 - Document - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DocumentCoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>100010 - Statement - Consolidated Statement of Earnings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/StatementConsolidatedStatementOfEarnings</Role>
      <ShortName>Consolidated Statement of Earnings</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>100020 - Statement - Consolidated Statement of Comprehensive Earnings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/StatementConsolidatedStatementOfComprehensiveEarnings</Role>
      <ShortName>Consolidated Statement of Comprehensive Earnings</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>100030 - Statement - Consolidated Balance Sheet</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/StatementConsolidatedBalanceSheet</Role>
      <ShortName>Consolidated Balance Sheet</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>100040 - Statement - Consolidated Balance Sheet (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/StatementConsolidatedBalanceSheetParenthetical</Role>
      <ShortName>Consolidated Balance Sheet (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>100050 - Statement - Consolidated Statement of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/StatementConsolidatedStatementOfCashFlows</Role>
      <ShortName>Consolidated Statement of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>100060 - Statement - Consolidated Statement of Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/StatementConsolidatedStatementOfStockholdersEquity</Role>
      <ShortName>Consolidated Statement of Stockholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>100070 - Statement - Consolidated Statement of Stockholders' Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/StatementConsolidatedStatementOfStockholdersEquityParenthetical</Role>
      <ShortName>Consolidated Statement of Stockholders' Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>100080 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>100090 - Disclosure - Effect of New Accounting Pronouncements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureEffectOfNewAccountingPronouncements</Role>
      <ShortName>Effect of New Accounting Pronouncements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>100100 - Disclosure - Business Combinations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureBusinessCombinations</Role>
      <ShortName>Business Combinations</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>100110 - Disclosure - Contracts with Customers</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureContractsWithCustomers</Role>
      <ShortName>Contracts with Customers</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>100120 - Disclosure - Other Financial Data</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureOtherFinancialData</Role>
      <ShortName>Other Financial Data</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>100130 - Disclosure - Fixed Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureFixedAssets</Role>
      <ShortName>Fixed Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>100140 - Disclosure - Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureIntangibleAssets</Role>
      <ShortName>Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>100150 - Disclosure - Credit and Other Debt Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureCreditAndOtherDebtAgreements</Role>
      <ShortName>Credit and Other Debt Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>100160 - Disclosure - Earnings per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureEarningsPerShare</Role>
      <ShortName>Earnings per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>100170 - Disclosure - Stock Option Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureStockOptionPlans</Role>
      <ShortName>Stock Option Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>100180 - Disclosure - Deferred Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureDeferredCompensation</Role>
      <ShortName>Deferred Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>100190 - Disclosure - Restricted Stock, Performance Share and Cash Awards</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureRestrictedStockPerformanceShareAndCashAwards</Role>
      <ShortName>Restricted Stock, Performance Share and Cash Awards</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>100200 - Disclosure - Retirement Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureRetirementPlans</Role>
      <ShortName>Retirement Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>100210 - Disclosure - Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureInvestments</Role>
      <ShortName>Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>100220 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureLeases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>100230 - Disclosure - Derivatives and Hedging Activity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureDerivativesAndHedgingActivity</Role>
      <ShortName>Derivatives and Hedging Activity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>100240 - Disclosure - Commitments, Contingencies and Off-Balance Sheet Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureCommitmentsContingenciesAndOffBalanceSheetArrangements</Role>
      <ShortName>Commitments, Contingencies and Off-Balance Sheet Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>100250 - Disclosure - Insurance Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureInsuranceOperations</Role>
      <ShortName>Insurance Operations</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>100260 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureIncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>100270 - Disclosure - Supplemental Disclosures of Cash Flow Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureSupplementalDisclosuresOfCashFlowInformation</Role>
      <ShortName>Supplemental Disclosures of Cash Flow Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>100280 - Disclosure -  Accumulated Other Comprehensive Earnings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureAccumulatedOtherComprehensiveEarnings</Role>
      <ShortName>Accumulated Other Comprehensive Earnings</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>100290 - Disclosure - Quarterly Operating Results (unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureQuarterlyOperatingResultsUnaudited</Role>
      <ShortName>Quarterly Operating Results (unaudited)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>100300 - Disclosure - Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureSegmentInformation</Role>
      <ShortName>Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>100310 - Disclosure - Schedule II. Valuation and Qualifying Accounts</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureScheduleIIValuationAndQualifyingAccounts</Role>
      <ShortName>Schedule II. Valuation and Qualifying Accounts</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>100320 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>100330 - Disclosure - Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesTables</Role>
      <ShortName>Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>100340 - Disclosure - Business Combinations (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureBusinessCombinationsTables</Role>
      <ShortName>Business Combinations (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureBusinessCombinations</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>100350 - Disclosure - Contracts with Customers (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureContractsWithCustomersTables</Role>
      <ShortName>Contracts with Customers (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureContractsWithCustomers</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>100360 - Disclosure - Other Financial Data (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureOtherFinancialDataTables</Role>
      <ShortName>Other Financial Data (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureOtherFinancialData</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>100370 - Disclosure - Fixed Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureFixedAssetsTables</Role>
      <ShortName>Fixed Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureFixedAssets</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>100380 - Disclosure - Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureIntangibleAssetsTables</Role>
      <ShortName>Intangible Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureIntangibleAssets</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>100390 - Disclosure - Credit and Other Debt Agreements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureCreditAndOtherDebtAgreementsTables</Role>
      <ShortName>Credit and Other Debt Agreements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureCreditAndOtherDebtAgreements</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>100400 - Disclosure - Earnings per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureEarningsPerShareTables</Role>
      <ShortName>Earnings per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureEarningsPerShare</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>100410 - Disclosure - Stock Option Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureStockOptionPlansTables</Role>
      <ShortName>Stock Option Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureStockOptionPlans</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>100420 - Disclosure - Restricted Stock, Performance Share and Cash Awards (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureRestrictedStockPerformanceShareAndCashAwardsTables</Role>
      <ShortName>Restricted Stock, Performance Share and Cash Awards (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureRestrictedStockPerformanceShareAndCashAwards</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>100430 - Disclosure - Retirement Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureRetirementPlansTables</Role>
      <ShortName>Retirement Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureRetirementPlans</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>100440 - Disclosure - Investments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureInvestmentsTables</Role>
      <ShortName>Investments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureInvestments</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>100450 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureLeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureLeases</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>100460 - Disclosure - Derivatives and Hedging Activity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureDerivativesAndHedgingActivityTables</Role>
      <ShortName>Derivatives and Hedging Activity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureDerivativesAndHedgingActivity</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>100470 - Disclosure - Commitments, Contingencies and Off-Balance Sheet Arrangements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureCommitmentsContingenciesAndOffBalanceSheetArrangementsTables</Role>
      <ShortName>Commitments, Contingencies and Off-Balance Sheet Arrangements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureCommitmentsContingenciesAndOffBalanceSheetArrangements</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>100480 - Disclosure - Insurance Operations (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureInsuranceOperationsTables</Role>
      <ShortName>Insurance Operations (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureInsuranceOperations</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>100490 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureIncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureIncomeTaxes</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>100500 - Disclosure - Supplemental Disclosures of Cash Flow Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureSupplementalDisclosuresOfCashFlowInformationTables</Role>
      <ShortName>Supplemental Disclosures of Cash Flow Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureSupplementalDisclosuresOfCashFlowInformation</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>100510 - Disclosure -  Accumulated Other Comprehensive Earnings (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureAccumulatedOtherComprehensiveEarningsTables</Role>
      <ShortName>Accumulated Other Comprehensive Earnings (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureAccumulatedOtherComprehensiveEarnings</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>100520 - Disclosure - Quarterly Operating Results (unaudited) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureQuarterlyOperatingResultsUnauditedTables</Role>
      <ShortName>Quarterly Operating Results (unaudited) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureQuarterlyOperatingResultsUnaudited</ParentRole>
      <Position>53</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>100530 - Disclosure - Segment Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureSegmentInformationTables</Role>
      <ShortName>Segment Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureSegmentInformation</ParentRole>
      <Position>54</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>100540 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesAdditionalInformationDetail</Role>
      <ShortName>Summary of Significant Accounting Policies - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>100550 - Disclosure - Summary of Significant Accounting Policies - Summary of Estimated Useful Life of Fixed Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesSummaryOfEstimatedUsefulLifeOfFixedAssetsDetail</Role>
      <ShortName>Summary of Significant Accounting Policies - Summary of Estimated Useful Life of Fixed Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>100560 - Disclosure - Effect of New Accounting Pronouncements - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureEffectOfNewAccountingPronouncementsAdditionalInformationDetail</Role>
      <ShortName>Effect of New Accounting Pronouncements - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>100570 - Disclosure - Business Combinations - Acquisition Method for Recording Business Combinations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureBusinessCombinationsAcquisitionMethodForRecordingBusinessCombinationsDetail</Role>
      <ShortName>Business Combinations - Acquisition Method for Recording Business Combinations (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>100580 - Disclosure - Business Combinations - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureBusinessCombinationsAdditionalInformationDetail</Role>
      <ShortName>Business Combinations - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>100590 - Disclosure - Business Combinations - Summary of Estimated Fair Values of Net Assets Acquired (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureBusinessCombinationsSummaryOfEstimatedFairValuesOfNetAssetsAcquiredDetail</Role>
      <ShortName>Business Combinations - Summary of Estimated Fair Values of Net Assets Acquired (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>100600 - Disclosure - Business Combinations - Summary of Unaudited Pro Forma Historical Results (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureBusinessCombinationsSummaryOfUnauditedProFormaHistoricalResultsDetail</Role>
      <ShortName>Business Combinations - Summary of Unaudited Pro Forma Historical Results (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>100610 - 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/20201231/taxonomy/role/DisclosureContractsWithCustomersSummaryOfUnbilledReceivablesContractAssetsAndContractLiabilitiesFromContractsWithCustomersDetail</Role>
      <ShortName>Contracts with Customers - Summary of Unbilled Receivables, Contract Assets and Contract Liabilities from Contracts with Customers (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>100620 - Disclosure - Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureContractsWithCustomersSummaryOfChangesInDeferredRevenueBalancesDetail</Role>
      <ShortName>Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>100630 - Disclosure - Contracts with Customers - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureContractsWithCustomersAdditionalInformationDetail</Role>
      <ShortName>Contracts with Customers - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>100640 - Disclosure - Contracts with Customers - Summary of Expected Revenue Related to Performance Obligations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureContractsWithCustomersSummaryOfExpectedRevenueRelatedToPerformanceObligationsDetail</Role>
      <ShortName>Contracts with Customers - Summary of Expected Revenue Related to Performance Obligations (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>100650 - Disclosure - Contracts with Customers - Summary of Expected Revenue Related to Performance Obligations 1 (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureContractsWithCustomersSummaryOfExpectedRevenueRelatedToPerformanceObligations1Detail</Role>
      <ShortName>Contracts with Customers - Summary of Expected Revenue Related to Performance Obligations 1 (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>100660 - Disclosure - Other Financial Data - Summary of Major Classes of Other Current Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureOtherFinancialDataSummaryOfMajorClassesOfOtherCurrentAssetsDetail</Role>
      <ShortName>Other Financial Data - Summary of Major Classes of Other Current Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>100670 - Disclosure - Fixed Assets - Summary of Fixed Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureFixedAssetsSummaryOfFixedAssetsDetail</Role>
      <ShortName>Fixed Assets - Summary of Fixed Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>100680 - Disclosure - Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureIntangibleAssetsCarryingAmountOfGoodwillAllocatedByDomesticAndForeignOperationsDetail</Role>
      <ShortName>Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>100690 - Disclosure - Intangible Assets - Changes in Carrying Amount of Goodwill (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureIntangibleAssetsChangesInCarryingAmountOfGoodwillDetail</Role>
      <ShortName>Intangible Assets - Changes in Carrying Amount of Goodwill (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>100700 - Disclosure - Intangible Assets - Major Classes of Amortizable Intangible Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureIntangibleAssetsMajorClassesOfAmortizableIntangibleAssetsDetail</Role>
      <ShortName>Intangible Assets - Major Classes of Amortizable Intangible Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>100710 - Disclosure - Intangible Assets - Estimated Aggregate Amortization Expense (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureIntangibleAssetsEstimatedAggregateAmortizationExpenseDetail</Role>
      <ShortName>Intangible Assets - Estimated Aggregate Amortization Expense (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>100720 - Disclosure - Credit and Other Debt Agreements - Summary of Corporate and Other Debt (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureCreditAndOtherDebtAgreementsSummaryOfCorporateAndOtherDebtDetail</Role>
      <ShortName>Credit and Other Debt Agreements - Summary of Corporate and Other Debt (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>100730 - Disclosure - Credit and Other Debt Agreements - Summary of Corporate and Other Debt (Parenthetical) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureCreditAndOtherDebtAgreementsSummaryOfCorporateAndOtherDebtParentheticalDetail</Role>
      <ShortName>Credit and Other Debt Agreements - Summary of Corporate and Other Debt (Parenthetical) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>100740 - Disclosure - Credit and Other Debt Agreements - Note Purchase Agreements - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureCreditAndOtherDebtAgreementsNotePurchaseAgreementsAdditionalInformationDetail</Role>
      <ShortName>Credit and Other Debt Agreements - Note Purchase Agreements - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>100750 - Disclosure - Credit and Other Debt Agreements - Credit Agreement - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureCreditAndOtherDebtAgreementsCreditAgreementAdditionalInformationDetail</Role>
      <ShortName>Credit and Other Debt Agreements - Credit Agreement - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>100760 - Disclosure - Earnings Per Share - Computation of Basic and Diluted Net EPS (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureEarningsPerShareComputationOfBasicAndDilutedNetEPSDetail</Role>
      <ShortName>Earnings Per Share - Computation of Basic and Diluted Net EPS (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>100770 - Disclosure - Earnings Per Share - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureEarningsPerShareAdditionalInformationDetail</Role>
      <ShortName>Earnings Per Share - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>100780 - Disclosure - Stock Option Plans - Additional information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureStockOptionPlansAdditionalInformationDetail</Role>
      <ShortName>Stock Option Plans - Additional information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>100790 - Disclosure - Stock Option Plans - Black-Scholes Option Pricing Model with Weighted Average (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureStockOptionPlansBlackScholesOptionPricingModelWithWeightedAverageDetail</Role>
      <ShortName>Stock Option Plans - Black-Scholes Option Pricing Model with Weighted Average (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>100800 - Disclosure - Stock Option Plans - Stock Option Activity and Related Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureStockOptionPlansStockOptionActivityAndRelatedInformationDetail</Role>
      <ShortName>Stock Option Plans - Stock Option Activity and Related Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>100810 - Disclosure - Stock Option Plans - Stock Options Outstanding and Exercisable (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureStockOptionPlansStockOptionsOutstandingAndExercisableDetail</Role>
      <ShortName>Stock Option Plans - Stock Options Outstanding and Exercisable (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>100820 - Disclosure - Deferred Compensation - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureDeferredCompensationAdditionalInformationDetail</Role>
      <ShortName>Deferred Compensation - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>100830 - Disclosure - Restricted Stock, Performance Share and Cash Awards - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureRestrictedStockPerformanceShareAndCashAwardsAdditionalInformationDetail</Role>
      <ShortName>Restricted Stock, Performance Share and Cash Awards - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>100840 - 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/20201231/taxonomy/role/DisclosureRestrictedStockPerformanceShareAndCashAwardsScheduleOfRestrictedStockAwardsVestingPeriodsDetail</Role>
      <ShortName>Restricted Stock, Performance Share and Cash Awards - Schedule of Restricted Stock Awards Vesting Periods (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>100850 - 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/20201231/taxonomy/role/DisclosureRetirementPlansReconciliationOfBalancesOfPensionBenefitObligationAndFairValueOfPlanAssetsDetail</Role>
      <ShortName>Retirement Plans - Reconciliation of Balances of Pension Benefit Obligation and Fair Value of Plan Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>100860 - 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/20201231/taxonomy/role/DisclosureRetirementPlansComponentsOfNetPeriodicPensionBenefitCostAndOtherChangesInPlanAssetsAndObligationsRecognizedInEarningsAndOtherComprehensiveEarningsDetail</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>87</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>100870 - 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/20201231/taxonomy/role/DisclosureRetirementPlansWeightedAverageAssumptionsOfPensionBenefitObligationAndNetPeriodicPensionBenefitCostDetail</Role>
      <ShortName>Retirement Plans - Weighted Average Assumptions of Pension Benefit Obligation and Net Periodic Pension Benefit Cost (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>100880 - Disclosure - Retirement Plans - Schedule of Benefit Payments Expected to be Paid by Plan (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureRetirementPlansScheduleOfBenefitPaymentsExpectedToBePaidByPlanDetail</Role>
      <ShortName>Retirement Plans - Schedule of Benefit Payments Expected to be Paid by Plan (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>100890 - Disclosure - Retirement Plans - Summary of Plan's Weighted Average Asset Allocations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureRetirementPlansSummaryOfPlanSWeightedAverageAssetAllocationsDetail</Role>
      <ShortName>Retirement Plans - Summary of Plan's Weighted Average Asset Allocations (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>100900 - Disclosure - Retirement Plans - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureRetirementPlansAdditionalInformationDetail</Role>
      <ShortName>Retirement Plans - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>100910 - Disclosure - Retirement Plans - Summary of Plan's Assets Carried at Fair Value (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureRetirementPlansSummaryOfPlanSAssetsCarriedAtFairValueDetail</Role>
      <ShortName>Retirement Plans - Summary of Plan's Assets Carried at Fair Value (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>100920 - 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/20201231/taxonomy/role/DisclosureRetirementPlansReconciliationOfBeginningAndEndingBalancesForLevel3AssetsOfPlanMeasuredAtFairValueDetail</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>93</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>100930 - Disclosure - Investments - Investments Reported in Other Current and Non-Current Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureInvestmentsInvestmentsReportedInOtherCurrentAndNonCurrentAssetsDetail</Role>
      <ShortName>Investments - Investments Reported in Other Current and Non-Current Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>100940 - Disclosure - Investments - Investments Reported in Other Current and Non-Current Assets (Parenthetical) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureInvestmentsInvestmentsReportedInOtherCurrentAndNonCurrentAssetsParentheticalDetail</Role>
      <ShortName>Investments - Investments Reported in Other Current and Non-Current Assets (Parenthetical) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>100950 - Disclosure - Investments - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureInvestmentsAdditionalInformationDetail</Role>
      <ShortName>Investments - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>96</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>100960 - Disclosure - Investments - Clean Coal Investments - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureInvestmentsCleanCoalInvestmentsAdditionalInformationDetail</Role>
      <ShortName>Investments - Clean Coal Investments - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>97</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>100970 - Disclosure - Investments - Other Investments - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureInvestmentsOtherInvestmentsAdditionalInformationDetail</Role>
      <ShortName>Investments - Other Investments - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>98</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>100980 - Disclosure - Leases - Components of Lease Expense (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureLeasesComponentsOfLeaseExpenseDetail</Role>
      <ShortName>Leases - Components of Lease Expense (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>99</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>100990 - Disclosure - Leases - Summary of Supplemental Cash Flow Information Related to Leases (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureLeasesSummaryOfSupplementalCashFlowInformationRelatedToLeasesDetail</Role>
      <ShortName>Leases - Summary of Supplemental Cash Flow Information Related to Leases (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>100</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>101000 - Disclosure - Leases - Summary of Supplemental Balance Sheet Information Related to Leases (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureLeasesSummaryOfSupplementalBalanceSheetInformationRelatedToLeasesDetail</Role>
      <ShortName>Leases - Summary of Supplemental Balance Sheet Information Related to Leases (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>101</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>101010 - Disclosure - Leases - Maturities of Operating Lease Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureLeasesMaturitiesOfOperatingLeaseLiabilitiesDetail</Role>
      <ShortName>Leases - Maturities of Operating Lease Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>102</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>101030 - Disclosure - Leases - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureLeasesAdditionalInformationDetail</Role>
      <ShortName>Leases - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>103</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>101040 - Disclosure - Derivatives and Hedging Activity - Summary of Notional and Fair Values of Derivative Instruments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureDerivativesAndHedgingActivitySummaryOfNotionalAndFairValuesOfDerivativeInstrumentsDetail</Role>
      <ShortName>Derivatives and Hedging Activity - Summary of Notional and Fair Values of Derivative Instruments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>104</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>101050 - Disclosure - Derivatives and Hedging Activity - Summary of Notional and Fair Values of Derivative Instruments (Parenthetical) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureDerivativesAndHedgingActivitySummaryOfNotionalAndFairValuesOfDerivativeInstrumentsParentheticalDetail</Role>
      <ShortName>Derivatives and Hedging Activity - Summary of Notional and Fair Values of Derivative Instruments (Parenthetical) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>105</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>101060 - 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/20201231/taxonomy/role/DisclosureDerivativesAndHedgingActivitySummaryOfAmountsOfDerivativeGainsLossesRecognizedInAccumulatedOtherComprehensiveLossDetail</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>106</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>101070 - Disclosure - Derivatives and Hedging Activity - Summary of Amounts of Derivative Gains (Losses) Recognized In Accumulated Other Comprehensive Loss (Parenthetical) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureDerivativesAndHedgingActivitySummaryOfAmountsOfDerivativeGainsLossesRecognizedInAccumulatedOtherComprehensiveLossParentheticalDetail</Role>
      <ShortName>Derivatives and Hedging Activity - Summary of Amounts of Derivative Gains (Losses) Recognized In Accumulated Other Comprehensive Loss (Parenthetical) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>107</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R108.htm</HtmlFileName>
      <LongName>101080 - Disclosure - Derivatives and Hedging Activity - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureDerivativesAndHedgingActivityAdditionalInformationDetail</Role>
      <ShortName>Derivatives and Hedging Activity - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>108</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R109.htm</HtmlFileName>
      <LongName>101090 - Disclosure - Commitments, Contingencies and Off-Balance Sheet Arrangements - Contractual Obligations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureCommitmentsContingenciesAndOffBalanceSheetArrangementsContractualObligationsDetail</Role>
      <ShortName>Commitments, Contingencies and Off-Balance Sheet Arrangements - Contractual Obligations (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>109</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R110.htm</HtmlFileName>
      <LongName>101100 - Disclosure - Commitments, Contingencies and Off-Balance Sheet Arrangements - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureCommitmentsContingenciesAndOffBalanceSheetArrangementsAdditionalInformationDetail</Role>
      <ShortName>Commitments, Contingencies and Off-Balance Sheet Arrangements - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>110</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R111.htm</HtmlFileName>
      <LongName>101110 - Disclosure - Commitments, Contingencies and Off-Balance Sheet Arrangements - Off-Balance Sheet Commitments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureCommitmentsContingenciesAndOffBalanceSheetArrangementsOffBalanceSheetCommitmentsDetail</Role>
      <ShortName>Commitments, Contingencies and Off-Balance Sheet Arrangements - Off-Balance Sheet Commitments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>111</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R112.htm</HtmlFileName>
      <LongName>101120 - 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/20201231/taxonomy/role/DisclosureCommitmentsContingenciesAndOffBalanceSheetArrangementsOutstandingLettersOfCreditFinancialGuaranteesAndFundingCommitmentsDetail</Role>
      <ShortName>Commitments, Contingencies and Off-Balance Sheet Arrangements - Outstanding Letters of Credit, Financial Guarantees and Funding Commitments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>112</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R113.htm</HtmlFileName>
      <LongName>101130 - Disclosure - Insurance Operations - Summary of Reconciliations of Direct to Net premiums on Written and Earned Basis Related to Wholly Owned Underwriting Enterprise Subsidiary (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureInsuranceOperationsSummaryOfReconciliationsOfDirectToNetPremiumsOnWrittenAndEarnedBasisRelatedToWhollyOwnedUnderwritingEnterpriseSubsidiaryDetail</Role>
      <ShortName>Insurance Operations - Summary of Reconciliations of Direct to Net premiums on Written and Earned Basis Related to Wholly Owned Underwriting Enterprise Subsidiary (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>113</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R114.htm</HtmlFileName>
      <LongName>101140 - Disclosure - Insurance Operations - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureInsuranceOperationsAdditionalInformationDetail</Role>
      <ShortName>Insurance Operations - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>114</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R115.htm</HtmlFileName>
      <LongName>101150 - Disclosure - Income Taxes - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureIncomeTaxesAdditionalInformationDetail</Role>
      <ShortName>Income Taxes - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>115</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R116.htm</HtmlFileName>
      <LongName>101160 - Disclosure - Income Taxes - Components of Earnings Before Income Taxes and Provision for Income Taxes (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureIncomeTaxesComponentsOfEarningsBeforeIncomeTaxesAndProvisionForIncomeTaxesDetail</Role>
      <ShortName>Income Taxes - Components of Earnings Before Income Taxes and Provision for Income Taxes (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>116</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R117.htm</HtmlFileName>
      <LongName>101170 - Disclosure - Income Taxes - Reconciliation of Provision for Income Taxes with Federal Statutory Income Tax Rate (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureIncomeTaxesReconciliationOfProvisionForIncomeTaxesWithFederalStatutoryIncomeTaxRateDetail</Role>
      <ShortName>Income Taxes - Reconciliation of Provision for Income Taxes with Federal Statutory Income Tax Rate (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>117</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R118.htm</HtmlFileName>
      <LongName>101180 - Disclosure - Income Taxes - Gross Unrecognized Tax Benefit (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureIncomeTaxesGrossUnrecognizedTaxBenefitDetail</Role>
      <ShortName>Income Taxes - Gross Unrecognized Tax Benefit (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>118</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R119.htm</HtmlFileName>
      <LongName>101190 - Disclosure - Income Taxes - Deferred Tax Assets and Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureIncomeTaxesDeferredTaxAssetsAndLiabilitiesDetail</Role>
      <ShortName>Income Taxes - Deferred Tax Assets and Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>119</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R120.htm</HtmlFileName>
      <LongName>101200 - Disclosure - Supplemental Disclosures of Cash Flow Information - Cash Flow (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureSupplementalDisclosuresOfCashFlowInformationCashFlowDetail</Role>
      <ShortName>Supplemental Disclosures of Cash Flow Information - Cash Flow (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureSupplementalDisclosuresOfCashFlowInformationTables</ParentRole>
      <Position>120</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R121.htm</HtmlFileName>
      <LongName>101210 - Disclosure - Supplemental Disclosures of Cash Flow Information - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureSupplementalDisclosuresOfCashFlowInformationAdditionalInformationDetail</Role>
      <ShortName>Supplemental Disclosures of Cash Flow Information - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>121</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R122.htm</HtmlFileName>
      <LongName>101220 - Disclosure - Supplemental Disclosures of Cash Flow Information - Cash Equivalents And Restricted Cash (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureSupplementalDisclosuresOfCashFlowInformationCashEquivalentsAndRestrictedCashDetail</Role>
      <ShortName>Supplemental Disclosures of Cash Flow Information - Cash Equivalents And Restricted Cash (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>122</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R123.htm</HtmlFileName>
      <LongName>101230 - Disclosure - Accumulated Other Comprehensive Earnings - Schedule of Accumulated Other Comprehensive Earnings (Loss) Attributable to Controlling Interests (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureAccumulatedOtherComprehensiveEarningsScheduleOfAccumulatedOtherComprehensiveEarningsLossAttributableToControllingInterestsDetail</Role>
      <ShortName>Accumulated Other Comprehensive Earnings - Schedule of Accumulated Other Comprehensive Earnings (Loss) Attributable to Controlling Interests (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>123</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R124.htm</HtmlFileName>
      <LongName>101240 - Disclosure - Accumulated Other Comprehensive Earnings - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureAccumulatedOtherComprehensiveEarningsAdditionalInformationDetail</Role>
      <ShortName>Accumulated Other Comprehensive Earnings - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>124</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R125.htm</HtmlFileName>
      <LongName>101250 - Disclosure - Quarterly Operating Results (unaudited) - Quarterly Operating Results (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureQuarterlyOperatingResultsUnauditedQuarterlyOperatingResultsDetail</Role>
      <ShortName>Quarterly Operating Results (unaudited) - Quarterly Operating Results (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.ajg.com/20201231/taxonomy/role/DisclosureQuarterlyOperatingResultsUnauditedTables</ParentRole>
      <Position>125</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R126.htm</HtmlFileName>
      <LongName>101260 - Disclosure - Segment Information - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureSegmentInformationAdditionalInformationDetail</Role>
      <ShortName>Segment Information - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>126</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R127.htm</HtmlFileName>
      <LongName>101270 - Disclosure - Segment Information - Schedule of Segment Reporting Information by Segment (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureSegmentInformationScheduleOfSegmentReportingInformationBySegmentDetail</Role>
      <ShortName>Segment Information - Schedule of Segment Reporting Information by Segment (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>127</Position>
    </Report>
    <Report instance="ajg-10k_20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R128.htm</HtmlFileName>
      <LongName>101280 - Disclosure - Schedule II - Valuation and Qualifying Accounts (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ajg.com/20201231/taxonomy/role/DisclosureScheduleIIValuationAndQualifyingAccountsDetail</Role>
      <ShortName>Schedule II - Valuation and Qualifying Accounts (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>128</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-K" original="ajg-10k_20201231.htm">ajg-10k_20201231.htm</File>
    <File>ajg-20201231.xsd</File>
    <File>ajg-20201231_cal.xml</File>
    <File>ajg-20201231_def.xml</File>
    <File>ajg-20201231_lab.xml</File>
    <File>ajg-20201231_pre.xml</File>
    <File>ajg-ex1015_15.htm</File>
    <File>ajg-ex211_173.htm</File>
    <File>ajg-ex231_6.htm</File>
    <File>ajg-ex241_12.htm</File>
    <File>ajg-ex311_11.htm</File>
    <File>ajg-ex312_9.htm</File>
    <File>ajg-ex321_8.htm</File>
    <File>ajg-ex322_10.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/country/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2020-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
