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Retirement Plans - Additional Information (Detail) (USD $)
1 Months Ended 3 Months Ended 12 Months Ended
Aug. 31, 2014
Employees
Dec. 31, 2014
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]          
Expected period of return on plan assets, years     10 years 10 years  
Discretionary contributions by employer       $ 6,300,000ajg_DefinedBenefitPlanDiscretionaryContributionsByEmployer $ 7,200,000ajg_DefinedBenefitPlanDiscretionaryContributionsByEmployer
Minimum contribution by employer     0ajg_DefinedBenefitPlanMinimumContributionByEmployer 0ajg_DefinedBenefitPlanMinimumContributionByEmployer 0ajg_DefinedBenefitPlanMinimumContributionByEmployer
Aggregate lump sum payout   43,300,000us-gaap_DefinedBenefitPlanBenefitsPaid 54,000,000us-gaap_DefinedBenefitPlanBenefitsPaid 9,400,000us-gaap_DefinedBenefitPlanBenefitsPaid  
Number of employees covered in the plan 2,500us-gaap_DefinedContributionPlanNumberOfEmployeesCovered        
Defined benefit plan, description     Eligible participants had from September 12, 2014 to November 30, 2014 to accept the offer, and the lump-sum payments were made in November and December of 2014, and the accelerated reduced annuity payments began as of December 1, 2014.    
Defined benefit plan benefit obligation period increase (decrease)     (60,000,000)us-gaap_DefinedBenefitPlanBenefitObligationPeriodIncreaseDecrease    
Defined benefit plan obligation (improvement to pension underfunding)     17,000,000us-gaap_DefinedBenefitPlanOtherChanges    
Non-cash settlement charge, pretax   12,000,000ajg_DefinedBenefitPlanSettlementsBenefitObligationBeforeTax      
Fair value of plan assets   217,200,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 217,200,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 254,900,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 227,400,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Postretirement benefit obligation and unfunded status of plan   272,000,000us-gaap_DefinedBenefitPlanBenefitObligation 272,000,000us-gaap_DefinedBenefitPlanBenefitObligation 272,500,000us-gaap_DefinedBenefitPlanBenefitObligation 292,000,000us-gaap_DefinedBenefitPlanBenefitObligation
Net periodic postretirement benefit cost     9,000,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost 3,200,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost 4,200,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
Foreign Retirement Plans [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Percentage of eligible compensation for matching contributions by employer     5.00%ajg_DefinedBenefitPlanPercentageOfEligibleCompensationForMatchingContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPostretirementBenefitPlansDefinedBenefitMember
   
Contribution expense to plan     29,700,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPostretirementBenefitPlansDefinedBenefitMember
18,100,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPostretirementBenefitPlansDefinedBenefitMember
16,000,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPostretirementBenefitPlansDefinedBenefitMember
Matching contributions by employer, percentage     100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPostretirementBenefitPlansDefinedBenefitMember
   
Additional percentage of eligible compensation for matching contributions by employer     5.00%ajg_DefinedBenefitPlanAdditionalPercentageOfEligibleCompensationForMatchingContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPostretirementBenefitPlansDefinedBenefitMember
   
Qualified Contributory Savings and Thrift 401(k) Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Percentage of eligible compensation for matching contributions by employer     5.00%ajg_DefinedBenefitPlanPercentageOfEligibleCompensationForMatchingContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_QualifiedContributorySavingsAndThriftFourZeroOneKPlanMember
   
Contribution expense to plan     38,000,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_QualifiedContributorySavingsAndThriftFourZeroOneKPlanMember
36,800,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_QualifiedContributorySavingsAndThriftFourZeroOneKPlanMember
33,000,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_QualifiedContributorySavingsAndThriftFourZeroOneKPlanMember
Matching contributions by employer, percentage     100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_QualifiedContributorySavingsAndThriftFourZeroOneKPlanMember
   
Matching contributions vesting schedule     5 years    
Nonqualified Deferred Compensation Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Percentage of eligible compensation for matching contributions by employer     5.00%ajg_DefinedBenefitPlanPercentageOfEligibleCompensationForMatchingContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_NonQualifiedDeferredCompensationPlanMember
   
Contribution expense to plan     3,700,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_NonQualifiedDeferredCompensationPlanMember
2,800,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_NonQualifiedDeferredCompensationPlanMember
2,500,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_NonQualifiedDeferredCompensationPlanMember
Fair value of plan assets   177,500,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_NonQualifiedDeferredCompensationPlanMember
177,500,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_NonQualifiedDeferredCompensationPlanMember
148,200,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_NonQualifiedDeferredCompensationPlanMember
 
Retiree Health Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Postretirement benefit obligation and unfunded status of plan   4,100,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_RetireeHealthPlanMember
4,100,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_RetireeHealthPlanMember
3,100,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_RetireeHealthPlanMember
 
Net periodic postretirement benefit cost     $ (500,000)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_RetireeHealthPlanMember
$ (500,000)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_RetireeHealthPlanMember
$ (100,000)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= ajg_RetireeHealthPlanMember