XML 90 R77.htm IDEA: XBRL DOCUMENT
v3.20.2

Supplemental Disclosures of Cash Flow Information - Additional Information (Detail) - Qualified Contributory Savings and Thrift 401(k) Plan [Member] - USD ($) $ in Millions | 9 Months Ended
--- | ---
Sep. 30, 2020 | Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items] |  |
Matching contributions by employer, percentage | 100.00% |
Percentage of eligible compensation for matching contributions by employer | 5.00% |
Matching contributions vesting schedule | 5 years |
Contribution expense to plan | $ 45.9 | $ 43.8

X
- Definition Defined benefit plan percentage of eligible compensation for matching contributions by employer. + References No definition available. + Details
Name: | ajg_DefinedBenefitPlanPercentageOfEligibleCompensationForMatchingContributionsByEmployer
Namespace Prefix: | ajg_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Matching contributions vesting schedule. + References No definition available. + Details
Name: | ajg_MatchingContributionsVestingSchedule
Namespace Prefix: | ajg_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
Name: | us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_RetirementPlanTypeAxis=ajg_QualifiedContributorySavingsAndThriftFourZeroOneKPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |