Retirement Plans - Additional Information (Detail) (USD $)
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1 Months Ended |
3 Months Ended |
12 Months Ended |
Aug. 31, 2014
Employees
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Dec. 31, 2014
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Dec. 31, 2014
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Dec. 31, 2013
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Dec. 31, 2012
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Defined Benefit Plan Disclosure [Line Items] |
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Expected period of return on plan assets, years |
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10 years |
10 years |
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Discretionary contributions by employer |
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$ 6,300,000ajg_DefinedBenefitPlanDiscretionaryContributionsByEmployer |
$ 7,200,000ajg_DefinedBenefitPlanDiscretionaryContributionsByEmployer |
Minimum contribution by employer |
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0ajg_DefinedBenefitPlanMinimumContributionByEmployer |
0ajg_DefinedBenefitPlanMinimumContributionByEmployer |
0ajg_DefinedBenefitPlanMinimumContributionByEmployer |
Aggregate lump sum payout |
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43,300,000us-gaap_DefinedBenefitPlanBenefitsPaid |
54,000,000us-gaap_DefinedBenefitPlanBenefitsPaid |
9,400,000us-gaap_DefinedBenefitPlanBenefitsPaid |
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Number of employees covered in the plan |
2,500us-gaap_DefinedContributionPlanNumberOfEmployeesCovered |
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Defined benefit plan, description |
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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. |
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Defined benefit plan benefit obligation period increase (decrease) |
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(60,000,000)us-gaap_DefinedBenefitPlanBenefitObligationPeriodIncreaseDecrease |
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Defined benefit plan obligation (improvement to pension underfunding) |
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17,000,000us-gaap_DefinedBenefitPlanOtherChanges |
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Non-cash settlement charge, pretax |
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12,000,000ajg_DefinedBenefitPlanSettlementsBenefitObligationBeforeTax |
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Fair value of plan assets |
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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 |
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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 |
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9,000,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost |
3,200,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost |
4,200,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost |
Foreign Retirement Plans [Member] |
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Defined Benefit Plan Disclosure [Line Items] |
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Percentage of eligible compensation for matching contributions by employer |
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5.00%ajg_DefinedBenefitPlanPercentageOfEligibleCompensationForMatchingContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_ForeignPostretirementBenefitPlansDefinedBenefitMember |
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Contribution expense to plan |
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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 |
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100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_ForeignPostretirementBenefitPlansDefinedBenefitMember |
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Additional percentage of eligible compensation for matching contributions by employer |
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5.00%ajg_DefinedBenefitPlanAdditionalPercentageOfEligibleCompensationForMatchingContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_ForeignPostretirementBenefitPlansDefinedBenefitMember |
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Qualified Contributory Savings and Thrift 401(k) Plan [Member] |
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Defined Benefit Plan Disclosure [Line Items] |
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Percentage of eligible compensation for matching contributions by employer |
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5.00%ajg_DefinedBenefitPlanPercentageOfEligibleCompensationForMatchingContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = ajg_QualifiedContributorySavingsAndThriftFourZeroOneKPlanMember |
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Contribution expense to plan |
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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 |
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100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = ajg_QualifiedContributorySavingsAndThriftFourZeroOneKPlanMember |
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Matching contributions vesting schedule |
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5 years |
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Nonqualified Deferred Compensation Plan [Member] |
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Defined Benefit Plan Disclosure [Line Items] |
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Percentage of eligible compensation for matching contributions by employer |
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5.00%ajg_DefinedBenefitPlanPercentageOfEligibleCompensationForMatchingContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = ajg_NonQualifiedDeferredCompensationPlanMember |
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Contribution expense to plan |
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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 |
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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 |
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Retiree Health Plan [Member] |
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Defined Benefit Plan Disclosure [Line Items] |
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Postretirement benefit obligation and unfunded status of plan |
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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 |
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Net periodic postretirement benefit cost |
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$ (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 |